30: Plant Form and Physiology

Question 1

What characteristics do all plants have in common?

Answer 1

They all share a common structure: a plant body consisting of stems, roots, and leaves. They all transport water, minerals, and sugars produced through photosynthesis through the plant body in a similar manner. All plant species also respond to environmental factors, such as light, gravity, competition, temperature, and predation.

Question 2

What is an apical meristem?

Answer 2

Meristematic tissue located at the tips of stems and roots; enables a plant to extend in length.

Question 3

What is dermal tissue?

Answer 3

Protective plant tissue covering the outermost part of the plant; controls gas exchange.

Question 4

What is ground tissue?

Answer 4

Plant tissue involved in photosynthesis; provides support, and stores water and sugars.

Question 5

What is intercalary meristem?

Answer 5

Meristematic tissue located at nodes and the bases of leaf blades; found only in monocots.

Question 6

What is lateral meristem?

Answer 6

Meristematic tissue that enables a plant to increase in thickness or girth.

Question 7

What is meristematic tissue?

Answer 7

Tissue containing cells that constantly divide; contributes to plant growth.

Question 8

What is the meristem?

Answer 8

Plant region of continuous growth.

Question 9

What is permanent tissue?

Answer 9

Plant tissue composed of cells that are no longer actively dividing.

Question 10

What is the root system?

Answer 10

Belowground portion of the plant that supports the plant and absorbs water and minerals.

Question 11

What is the shoot system?

Answer 11

Aboveground portion of the plant; consists of non-reproductive plant parts, such as leaves and stems, and reproductive parts, such as flowers and fruits.

Question 12

What is a vascular bundle?

Answer 12

Strands of stem tissue made up of xylem and phloem.

Question 13

What is a vascular stele?

Answer 13

Strands of root tissue made up of xylem and phloem.

Question 14

What is vascular tissue?

Answer 14

Tissue made up of xylem and phloem that transports food and water throughout the plant.

Question 15

What are the two distinct organ systems in vascular plants?

Answer 15

The shoot system and the root system.

Question 16

What are the general types of plant tissue regarding cell differentiation?

Answer 16

Meristematic tissue and permanent (non-meristematic) tissue.

Question 17

What are the different types of meristematic tissue?

Answer 17

Apical meristems, lateral meristems, and intercalary meristems.

Question 18

Which type of meristematic tissue causes lawn grasses to grow?

Answer 18

Intercalary meristem tissue enables the monocot leaf blade to increase in length from the leaf base, which allows lawn grass leaves to elongate even after repeated mowing.

Question 19

What are the main types of cells that meristematic cells differentiate into?

Answer 19

Dermal, vascular, and ground tissue.

Question 20

What are the types of secondary tissues?

Answer 20

Simple (composed of similar cell types) or complex (composed of different cell types).

Question 21

What are the different cell types in xylem tissue?

Answer 21

Vessel elements and tracheids (both of which conduct water), and xylem parenchyma.

Question 22

What are the different cell types in phloem tissue?

Answer 22

Sieve cells (which conduct photosynthates), companion cells, phloem parenchyma, and phloem fibers.

Question 23

Are conducting cells alive or dead at maturity?

Answer 23

Unlike xylem conducting cells, phloem conducting cells are alive at maturity (whereas xylem cells are dead at functional maturity).

Question 24

What is an apical bud?

Answer 24

A bud formed at the tip of the shoot.

Question 25

What is an axillary bud?

Answer 25

A bud located in the axil: the stem area where the petiole connects to the stem.

Question 26

What is bark?

Answer 26

Tough, waterproof, outer epidermal layer of cork cells.

Question 27

What is a bulb?

Answer 27

A modified underground stem that consists of a large bud surrounded by numerous leaf scales.

Question 28

What is a collenchyma cell?

Answer 28

An elongated plant cell with unevenly thickened walls; provides structural support to the stem and leaves.

Question 29

What is a companion cell?

Answer 29

A phloem cell that is connected to sieve-tube cells; has large amounts of ribosomes and mitochondria.

Question 30

What is a corm?

Answer 30

A rounded, fleshy underground stem that contains stored food.

Question 31

What is cortex?

Answer 31

Ground tissue found between the vascular tissue and the epidermis in a stem or root.

Question 32

What is the epidermis?

Answer 32

Single layer of cells found in plant dermal tissue; covers and protects underlying tissue.

Question 33

What are guard cells?

Answer 33

Paired cells on either side of a stoma that control stomatal opening and thereby regulate the movement of gases and water vapor.

Question 34

What is the internode?

Answer 34

The region between nodes on a stem.

Question 35

What is a lenticel?

Answer 35

An opening on the surface of mature woody stems that facilitates gas exchange.

Question 36

What is a node?

Answer 36

A point along the stem at which leaves, flowers, or aerial roots originate.

Question 37

What is a parenchyma cell?

Answer 37

The most common type of plant cell; found in the stem, root, leaf, and in fruit pulp; site of photosynthesis and starch storage.

Question 38

What is the periderm?

Answer 38

The outermost covering of woody stems; consists of the cork cambium, cork cells, and the phelloderm.

Question 39

What is pith?

Answer 39

Ground tissue found towards the interior of the vascular tissue in a stem or root.

Question 40

What is primary growth?

Answer 40

Growth resulting in an increase in length of the stem and the root; caused by cell division in the shoot or root apical meristem.

Question 41

What is a rhizome?

Answer 41

A modified underground stem that grows horizontally to the soil surface and has nodes and internodes.

Question 42

What is a runner?

Answer 42

A stolon that runs above the ground and produces new clone plants at nodes.

Question 43

What is a sclerenchyma cell?

Answer 43

A plant cell that has thick secondary walls and provides structural support; usually dead at maturity.

Question 44

What is secondary growth?

Answer 44

Growth resulting in an increase in thickness or girth; caused by the lateral meristem and cork cambium.

Question 45

What is a sieve-tube cell?

Answer 45

A phloem cell arranged end-to-end to form a sieve tube that transports organic substances such as sugars and amino acids.

Question 46

What is a stolon?

Answer 46

A modified stem that runs parallel to the ground and can give rise to new plants at the nodes.

Question 47

What is a tendril?

Answer 47

A modified stem consisting of slender, twining strands used for support or climbing.

Question 48

What is a thorn?

Answer 48

A modified stem branch appearing as a sharp outgrowth that protects the plant.

Question 49

What is a tracheid?

Answer 49

A xylem cell with thick secondary walls that are lignified and that helps transport water.

Question 50

What is a trichome?

Answer 50

A hair-like structure on the epidermal surface.

Question 51

What is a tuber?

Answer 51

A modified underground stem adapted for starch storage; has many adventitious buds.

Question 52

What is a vessel element?

Answer 52

A xylem cell that is shorter than a tracheid and has thinner walls.

Question 53

What are some characteristics of stems?

Answer 53

Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to provide support to the plant, holding leaves, flowers, and buds; in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.

Question 54

What is a petiole?

Answer 54

A stalk that extends from the stem to the base of the leaf.

Question 55

What types of cells are stem and other plant organs made of?

Answer 55

The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells.

Question 56

What is the role of parenchyma cells?

Answer 56

Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.

Question 57

Where are collenchyma cells found?

Answer 57

Collenchyma cells are alive at maturity and are usually found below the epidermis.

Question 58

What is an example of collenchyma cells?

Answer 58

The “strings” of a celery stalk.

Question 59

What are the types of sclerenchyma cells?

Answer 59

Fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin. Fibers are long, slender cells; sclereids are smaller-sized.

Question 60

What are some examples of sclerenchyma cells?

Answer 60

Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope.

Question 61

How prevalent are epidermal cells in the epidermis?

Answer 61

Epidermal cells are the most numerous and least differentiated of the cells in the epidermis.

Question 62

What is the purpose of trichomes?

Answer 62

Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds that defend the leaves against predation by herbivores. They can also increase solar reflectance, and reduce the rate of transpiration by blocking air flow across the leaf surface.

Question 63

How are vascular bundles arranged in a stem?

Answer 63

When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue.

Question 64

Which xylem cells are alive at maturity?

Answer 64

Tracheids and vessel elements are dead at maturity, only xylem parenchyma is live.

Question 65

How does water move through tracheids?

Answer 65

Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent.

Question 66

How does water move through vessel elements?

Answer 66

Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.

Question 67

What is the role of the different types of cells in phloem tissue?

Answer 67

A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.

Question 68

What is ground tissue made of?

Answer 68

Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.

Question 69

In which types of plants does secondary growth occur?

Answer 69

Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots.

Question 70

What is determinate and indeterminate growth?

Answer 70

Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.

Question 71

What are some characteristics of apical dominance?

Answer 71

The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Question 72

What are lateral meristems made of?

Answer 72

Lateral meristems include the vascular cambium and, in woody plants, the cork cambium.

Question 73

What are some characteristics of vascular cambium?

Answer 73

The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside.

Question 74

What causes secondary growth in lateral meristems?

Answer 74

The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

Question 75

What are some characteristics of cork cambium?

Answer 75

In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere. This supplies oxygen to the living and metabolically active cells of the cortex, xylem and phloem.

Question 76

How are annual rings formed?

Answer 76

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem. An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.

Question 77

What are some examples of shoots from rhizomes?

Answer 77

Vertical shoots may arise from the buds on the rhizomes of some plants, such as ginger (Zingiber officinale) and ferns.

Question 78

What is an example of a corm?

Answer 78

Gladiolus, or the carrion flower (Amorphophallus titanum).

Question 79

What is an example of a runner?

Answer 79

Strawberry (Fragaria ananassa).

Question 80

What is an example of a tuber?

Answer 80

Potato (Solanum tuberosum).

Question 81

What is an example of a bulb?

Answer 81

Iris, or the red onion (Allium).

Question 82

What is an example of a stolon?

Answer 82

Rhodes grass (Chloris gayana).

Question 83

What are examples of plants that produce tendrils?

Answer 83

Vines (such as the buckwheat vine, Brunnichia ovata) and pumpkins.

Question 84

What are examples of plants that produce thorns?

Answer 84

Roses, Osage oranges, and devil’s walking stick.

Question 85

What is an adventitious root?

Answer 85

An aboveground root that arises from a plant part other than the radicle of the plant embryo.

Question 86

What is a Casparian strip?

Answer 86

A waxy coating that forces water to cross endodermal plasma membranes before entering the vascular cylinder, instead of moving between endodermal cells.

Question 87

What is the endodermis?

Answer 87

The layer of cells in the root that forms a selective barrier between the ground tissue and the vascular tissue, allowing water and minerals to enter the root while excluding toxins and pathogens.

Question 88

What is a fibrous root system?

Answer 88

A type of root system in which the roots arise from the base of the stem in a cluster, forming a dense network of roots; found in monocots.

Question 89

What is a pericycle?

Answer 89

The outer boundary of the stele from which lateral roots can arise.

Question 90

What is a root cap?

Answer 90

The protective cells covering the tip of the growing root.

Question 91

What is a root hair?

Answer 91

A hair-like structure that is an extension of epidermal cells; increases the root surface area and aids in absorption of water and minerals.

Question 92

What is a stele?

Answer 92

The inner portion of the root containing the vascular tissue; surrounded by the endodermis.

Question 93

What is a tap root system?

Answer 93

A type of root system with a main root that grows vertically with few lateral roots; found in dicots.

Question 94

What are the functions of roots?

Answer 94

The roots of seed plants have three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis. Some roots are modified to absorb moisture and exchange gases.

Question 95

What are the types of root systems?

Answer 95

Dicots have a tap root system, and monocots have a fibrous root system.

Question 96

What is an example of a plant with a tap root system?

Answer 96

Dandelions, whose tap roots usually break off when trying to pull these weeds, and they can regrow another shoot from the remaining root.

Question 97

How close to the surface are the different root systems?

Answer 97

A tap root system penetrates deep into the soil, while a fibrous root system is located closer to the soil surface, which helps to prevent soil erosion. Some plants have a combination of tap roots and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water are likely to have shallower root systems.

Question 98

What are some examples of plants with a fibrous root system?

Answer 98

Lawn grasses, wheat, rice, and corn.

Question 99

How does root growth begin?

Answer 99

Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system.

Question 100

How is the tip of the root protected?

Answer 100

The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through soil.

Question 101

What are the zones of the root tip?

Answer 101

The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation. The zone of cell division is closest to the root tip; it is made up of the actively dividing cells of the root meristem. The zone of elongation is where the newly formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three zones are in the first centimeter or so of the root tip.

Question 102

What is the epidermis of the root?

Answer 102

The root has an outer layer of cells called the epidermis, which surrounds areas of ground tissue and vascular tissue. The epidermis provides protection and helps in absorption. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals.

Question 103

How is the ground tissue arranged in roots?

Answer 103

Inside the root, the ground tissue forms two regions: the cortex and the pith. Compared to stems, roots have lots of cortex and little pith. Both regions include cells that store photosynthetic products. The cortex is between the epidermis and the vascular tissue, whereas the pith lies between the vascular tissue and the center of the root.

Question 104

How is the vascular tissue arranged in roots?

Answer 104

The vascular tissue in the root is arranged in the inner portion of the root, which is called the stele. A layer of cells known as the endodermis separates the stele from the ground tissue in the outer portion of the root. The endodermis is exclusive to roots, and serves as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root’s vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.

Question 105

What are some examples of root modifications?

Answer 105

Some roots are bulbous and store starch. Aerial roots and prop roots are two forms of aboveground roots that provide additional support to anchor the plant.

Question 106

What are some examples of roots modified for food storage?

Answer 106

Tap roots, such as carrots, turnips, and beets.

Question 107

What are epiphytic roots?

Answer 107

Epiphytic roots enable a plant to grow on another plant.

Question 108

What are some examples of epiphytic roots?

Answer 108

The epiphytic roots of orchids develop a spongy tissue to absorb moisture. The banyan tree (Ficus sp., AKA the strangler fig) begins as an epiphyte, germinating in the branches of a host tree; aerial roots develop from the branches and eventually reach the ground, providing additional support (which also eventually strangles the host tree). In screwpine (Pandanus sp.), a palm-like tree that grows in sandy tropical soils, aboveground prop roots develop from the nodes to provide additional support.

Question 109

What is a compound leaf?

Answer 109

A leaf in which the leaf blade is subdivided to form leaflets, all attached to the midrib.

Question 110

What is a cuticle?

Answer 110

A waxy protective layer on the leaf surface.

Question 111

What is a lamina?

Answer 111

A leaf blade.

Question 112

What is a palmately compound leaf?

Answer 112

A leaf type with leaflets that emerge from a point, resembling the palm of a hand.

Question 113

What is phyllotaxy?

Answer 113

The arrangement of leaves on a stem.

Question 114

What is a pinnately compound leaf?

Answer 114

A leaf type with a divided leaf blade consisting of leaflets arranged on both sides of the midrib.

Question 115

What does it mean to be sessile?

Answer 115

A leaf without a petiole that is attached directly to the plant stem.

Question 116

What is a simple leaf?

Answer 116

A leaf type in which the lamina is completely undivided or merely lobed.

Question 117

What is a stipule?

Answer 117

A small green structure found on either side of the leaf stalk or petiole.

Question 118

What is a venation?

Answer 118

The pattern of veins in a leaf; may be parallel (as in monocots), reticulate (as in dicots), or dichotomous (as in Gingko biloba).

Question 119

What does it mean to be whorled?

Answer 119

The pattern of leaf arrangement in which three or more leaves are connected at a node.

Question 120

What gives a leaf its color?

Answer 120

Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll.

Question 121

What factors influence a leaf’s size and shape?

Answer 121

The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical rainforests have larger surface areas than those of plants growing in deserts or very cold conditions, which are likely to have a smaller surface area to minimize water loss.

Question 122

What is the structure of a typical leaf?

Answer 122

Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant stem are called sessile leaves. Small green appendages usually found at the base of the petiole are known as stipules. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin.

Question 123

What are the different venation patterns?

Answer 123

Monocots have parallel venation; the veins run in straight lines across the length of the leaf without converging at a point. In dicots, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. Ginkgo biloba has a dichotomous venation where the veins fork.

Question 124

How are leaves arranged on a stem?

Answer 124

The number and placement of a plant’s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are said to be either alternate - meaning the leaves alternate on each side of the stem in a flat plane - or spiral, meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled.

Question 125

What are the different leaf forms?

Answer 125

Leaves may be simple or compound. In a simple leaf, the blade is either completely undivided or it has lobes, but the separation does not reach the midrib. In a compound leaf, the leaf blade is completely divided, forming leaflets. Each leaflet may have its own stalk, but is attached to the rachis. A palmately compound leaf resembles the palm of a hand, with leaflets radiating outwards from one point. Pinnately compound leaves take their name from their feather-like appearance; the leaflets are arranged along the midrib.

Question 126

What are examples of simple leaves?

Answer 126

The banana leaf (Musa sp.), which is completely undivided, and the maple leaf, which is lobed.

Question 127

What are some examples of palmately compound leaves?

Answer 127

Poison ivy, the buckeye tree, and the familiar houseplant Schefflera sp. (common name “umbrella plant”), or the horse chestnut (Aesculus hippocastanum).

Question 128

What are some examples of pinnately compound leaves?

Answer 128

Rose leaves (Rosa sp.), hickory (including the scrub hickory, Carya floridana), pecan, ash, or walnut trees.

Question 129

What is an example of double compound leaves?

Answer 129

The honey locust.

Question 130

What are some characteristics of the epidermis of leaves?

Answer 130

The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata, openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing.

Question 131

How thick is the epidermis?

Answer 131

The epidermis is usually one cell layer thick; however in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration.

Question 132

What is the purpose of the cuticle?

Answer 132

A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface.

Question 133

How is the mesophyll arranged?

Answer 133

Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or “middle leaf”. The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma. The palisade parenchyma (also called the palisade mesophyll) has column-shaped, tightly-packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely-arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.

Question 134

How is vascular tissue in leaves arranged?

Answer 134

Like the stem, the leaf contains vascular bundles composed of xylem and phloem. The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.

Question 135

What are some leaf adaptations found in cold climates?

Answer 135

Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area: two attributes that aid in reducing water loss.

Question 136

What are some leaf adaptations found in hot climates?

Answer 136

In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water.

Question 137

What are some leaf adaptations in aquatic climates?

Answer 137

Many aquatic plants have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels water.

Question 138

What are some plant adaptations found in rainforests?

Answer 138

In tropical rainforests, light is often scarce, since many trees and plants grow close together and block much of the sunlight from reaching the forest floor. Many tropical plant species have exceptionally broad leaves to maximize the capture of sunlight. Other species are epiphytes: plants that grow on other plants that serve as a physical support. Such plants are able to grow high up in the canopy atop the branches of other trees, where sunlight is more plentiful. Epiphytes live on rain and minerals collected in the branches and leaves of the supporting plant. Bromeliads (members of the pineapple family, such as Spanish moss), ferns, and orchids are examples of tropical epiphytes. Many epiphytes have specialized tissues that enable them to efficiently capture and store water.

Question 139

What are some plant adaptations found in nutrient-poor environments?

Answer 139

Some plants have special adaptations that help them to survive in nutrient-poor environments. Carnivorous plants, such as the Venus flytrap and the pitcher plant, grow in bogs where the soil is low in nitrogen. In these plants, leaves are modified to capture insects. The insect-capturing leaves may have evolved to provide these plants with a supplementary source of nitrogen.

Question 140

How do Venus flytraps capture insects?

Answer 140

The Venus flytrap has modified leaves that can capture insects. When an insect touches the trigger hairs inside the leaf, the trap suddenly closes.

Question 141

How do pitcher plants capture insects?

Answer 141

The opening of the pitcher plant is lined with a slippery wax. Insects crawling on the lip slip and fall into a pool of water in the bottom of the pitcher, where they are digested by bacteria. The plant then absorbs the smaller molecules.

Question 142

What are some plant adaptations found in wetlands?

Answer 142

Many swamp plants have adaptations that enable them to thrive in wet areas, where their roots grow submerged underwater. In these aquatic areas, the soil is unstable and little oxygen is available to reach the roots. Trees such as mangroves (Rhizophora sp.) growing in coastal waters produce aboveground roots that help support the tree. Some species of mangroves, as well as cypress trees, have pneumatophores: upward-growing roots containing pores and pockets of tissue specialized for gas exchange. Wild rice is an aquatic plant with large air spaces in the root cortex. The air-filled tissue - called aerenchyma - provides a path for oxygen to diffuse down to the root tips, which are embedded in oxygen-poor bottom sediments.

Question 143

What is a megapascal (MPa)?

Answer 143

Pressure units that measure water potential.

Question 144

What is a sink?

Answer 144

The growing parts of a plant, such as roots and young leaves, which require photosynthate.

Question 145

What is a source?

Answer 145

An organ that produces photosynthate for a plant.

Question 146

What is translocation?

Answer 146

Mass transport of photosynthates from source to sink in vascular plants.

Question 147

What is transpiration?

Answer 147

Loss of water vapor to the atmosphere through stomata.

Question 148

What is water potential (Ψ_w)?

Answer 148

The potential energy of a water solution per unit volume in relation to pure water at atmospheric pressure and ambient temperature.

Question 149

What influences how water and nutrients are transported in plants?

Answer 149

Water potential, evapotranspiration, and stomatal regulation.

Question 150

How is water potential denoted and expressed?

Answer 150

Water potential is denoted by the Greek letter Ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ_w^{pure H2O}) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψ_w^{pure H2O}.

Question 151

What is the equation for water potential of a system?

Answer 151

The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:

ψ_system = ψ_total = ψ_s + ψ_p + ψ_g + ψ_m

where ψ_s, ψ_p, ψ_g, and ψ_m refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (ψ^soil), root water (ψ^root), stem water (ψ^stem), leaf water (ψ^leaf) or water in the atmosphere (ψ^atmosphere): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0).

Question 152

What is the solute potential of plant cells?

Answer 152

Solute potential (Ψ_s), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψ_w) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen binds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψ_s decreases with increasing solution concentration. Because Ψ_s is one of the four components of Ψ_system or Ψ_total, a decrease in Ψ_s will cause a decrease in Ψ_total. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content. Because of this difference in water potential, water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.

Question 153

What is pressure potential?

Answer 153

Pressure potential (Ψ_p), also called turgor potential, may be positive or negative. Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, positive Ψ_p (compression) increases Ψ_total, and a negative Ψ_p (tension) decreases Ψ_total. Positive pressure inside cells is contained by the cell wall, producing turgor pressure.

Question 154

What is the pressure potential of plants?

Answer 154

Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψ_p of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb/in² MPa^-1 = 210 lb/in²). As a comparison, most automobile tires are kept at a pressure of 30–34 psi.

Question 155

What is an example of turgor pressure in plants?

Answer 155

An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered. Water is lost from the leaves via transpiration (approaching Ψ_p = 0 MPa at the wilting point) and restored by uptake via the roots.

Question 156

How do plants control pressure potential?

Answer 156

A plant can manipulate Ψ_p via its ability to manipulate Ψ_s and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψ_s will decline, Ψ_total will decline, and the ΔΨ between the cell and the surrounding tissue will increase, water will move into the cell by osmosis, and Ψ_p will increase. Ψ_p is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψ_p and Ψ_total of the leaf and increasing the ΔΨ between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.

Question 157

How does gravity potential affect plants?

Answer 157

Gravity potential (Ψ_g) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (Ψ_total). The taller the plant, the taller the water column, and the more influential Ψ_g becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m^–1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψ_g.

Question 158

How does matric potential affect plants?

Answer 158

Matric potential (Ψ_m) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψ_m is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψ_m, the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψ_m is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψ_m cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.

Question 159

What determines how water moves in a plant?

Answer 159

Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ.

Question 160

How does transpiration result in the transport of water through plants?

Answer 160

Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension.

Question 161

How is water movement in plants affected when transpiration stops?

Answer 161

At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion-tension theory of sap ascent.

Question 162

How does water movement occur within xylem tissue?

Answer 162

Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

Question 163

Where does the energy for transpiration come from?

Answer 163

Transpiration is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.

Question 164

How much water is lost through transpiration?

Answer 164

The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90% of the water taken up by roots may be lost through transpiration.

Question 165

How do plants regulate transpiration?

Answer 165

Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.

Question 166

How are plants adapted to regulate transpiration?

Answer 166

Plants have evolved over time to adapt to their local environment and reduce transpiration. Desert plants (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.

Question 167

What other adaptations are found to reduce transpiration?

Answer 167

Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.

Question 168

How is energy produced and stored in plants?

Answer 168

Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly-developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.

Question 169

What are the sources and destinations of photosynthates?

Answer 169

Structures that produce photosynthates for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.

Question 170

Where do photosynthates flow during translocation?

Answer 170

The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.

Question 171

How are photosynthates moved from sources into phloem cells?

Answer 171

Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H⁺ symporter.

Question 172

How are sieve-tube elements organized to facilitate translocation?

Answer 172

Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs.

Question 173

How are photosynthates translocated from the phloem to sinks?

Answer 173

Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30% sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψ_s, which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink. Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.

Question 174

What is abscisic acid (ABA)?

Answer 174

A plant hormone that induces dormancy in seeds and other organs.

Question 175

What is abscission?

Answer 175

A physiological process that leads to the fall of a plant organ (such as leaf or petal drop).

Question 176

What is auxin?

Answer 176

A plant hormone that influences cell elongation (in phototropism), gravitropism, apical dominance and root growth.

Question 177

What is a chromophore?

Answer 177

A molecule that absorbs light.

Question 178

What is a cryptochrome?

Answer 178

A protein that absorbs light in the blue and ultraviolet regions of the light spectrum.

Question 179

What is cytokinin?

Answer 179

A plant hormone that promotes cell division.

Question 180

What is ethylene?

Answer 180

A volatile plant hormone that is associated with fruit ripening, flower wilting, and leaf fall.

Question 181

What is gibberellin (GA)?

Answer 181

A plant hormone that stimulates shoot elongation, seed germination, and the maturation and dropping of fruits and flowers.

Question 182

What are jasmonates?

Answer 182

A small family of compounds derived from the fatty acid linoleic acid.

Question 183

What is negative gravitropism?

Answer 183

Growth away from Earth’s gravity.

Question 184

What is oligosaccharin?

Answer 184

A hormone important in plant defenses against bacterial and fungal infections.

Question 185

What is photomorphogenesis?

Answer 185

Growth and development of plants in response to light.

Question 186

What is photoperiodism?

Answer 186

Occurrence of plant processes, such as germination and flowering, according to the time of year.

Question 187

What is phototropin?

Answer 187

A blue-light receptor that promotes phototropism, stomatal opening and closing, and other responses that promote photosynthesis.

Question 188

What is phototropism?

Answer 188

Directional bending of a plant toward a light source.

Question 189

What is a phytochrome?

Answer 189

A plant pigment protein that exists in two reversible forms (Pr and Pfr) and mediates morphologic changes in response to red light.

Question 190

What is positive gravitropism?

Answer 190

Growth toward Earth’s gravitational center.

Question 191

What is a statolith?

Answer 191

A plant organelle that contains heavy starch granules. AKA amyloplast.

Question 192

What is strigolactone?

Answer 192

A hormone that promotes seed germination in some species and inhibits lateral apical development in the absence of auxins.

Question 193

What is thigmomorphogenesis?

Answer 193

Developmental response to touch.

Question 194

What does it mean to be thigmonastic?

Answer 194

Directional growth of a plant independent of the direction in which contact is applied.

Question 195

What is thigmotropism?

Answer 195

Directional growth of a plant in response to constant contact.

Question 196

How do plants respond to environmental factors?

Answer 196

Plants have sophisticated systems to detect and respond to light, gravity, temperature, and physical touch. Receptors sense environmental factors and relay the information to effector systems - often through intermediate chemical messengers - to bring about plant responses.

Question 197

What are some different ways that plants respond to light?

Answer 197

Photomorphogenesis is the growth and development of plants in response to light. It allows plants to optimize their use of light and space. Photoperiodism is the ability to use light to track time. Plants can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Phototropism is a directional response that allows plants to grow towards, or even away from, light.

Question 198

Which molecules in plants are used to sense light?

Answer 198

The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors, which are comprised of a protein covalently bonded to a light-absorbing pigment called a chromophore. Together, the two are called a chromoprotein.

Question 199

How does light wavelength influence plant development?

Answer 199

The red/far-red and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue-green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.

Question 200

How is the phytochrome system composed?

Answer 200

The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of the protein; therefore, exposure to red light yields physiological activity. Exposure to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system.

Question 201

How does the phytochrome system toggle biological activity?

Answer 201

The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark, or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression.

Question 202

How does the phytochrome system influence plant development?

Answer 202

Once the phytochrome system evolved, plants adapted it to serve a variety of needs. Unfiltered, full sunlight contains much more red light than far-red light. Because chlorophyll absorbs strongly in the red region of the visible spectrum, but not in the far-red region, any plant in the shade of another plant on the forest floor will be exposed to red-depleted, far-red-enriched light. The preponderance of far-red light converts phytochrome in the shaded leaves to the Pr (inactive) form, slowing growth. The nearest non-shaded (or even less-shaded) areas on the forest floor have more red light; leaves exposed to these areas sense the red light, which activates the Pfr form and induces growth. In short, plant shoots use the phytochrome system to grow away from shade and towards light. Because competition for light is so fierce in a dense plant community, the evolutionary advantages to the phytochrome system are obvious.

Question 203

How is the phytochrome system used in seeds?

Answer 203

In seeds, the phytochrome system is not used to determine direction and quality of light (shaded versus unshaded). Instead, it is used merely to determine if there is any light at all. This is especially important in species with very small seeds, such as lettuce. Because of their size, lettuce seeds have few food reserves. Their seedlings cannot grow for long before they run out of fuel. If they germinated even a centimeter under the soil surface, the seedling would never make it to the sunlight and would die. In the dark, phytochrome is in the Pr (inactive form) and the seed will not germinate; it will only germinate if exposed to light at the surface of the soil. Upon exposure to light, Pr is converted to Pfr and germination proceeds.

Question 204

How is the phytochrome system used to detect season change?

Answer 204

Plants use the phytochrome system to sense the change of seasons. Photoperiodism is a biological response to the timing and duration of day and night. It controls flowering, setting of winter buds, and vegetative growth. Detection of seasonal changes is crucial to plant survival. Although temperature and light intensity influence plant growth, they are not reliable indicators of season because they may vary from one year to the next. Day length is a better indicator of the time of year.

Question 205

How is the phytochrome system used to detect changes in the length of daylight?

Answer 205

Unfiltered sunlight is rich in red light but deficient in far-red light. Therefore, at dawn, all the phytochrome molecules in a leaf quickly convert to the active Pfr form, and remain in that form until sunset. In the dark, the Pfr form takes hours to slowly revert back to the Pr form. If the night is long (as in winter), all of the Pfr form reverts. If the night is short (as in summer), a considerable amount of Pfr may remain at sunrise. By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the day/night cycle. In addition, leaves retain the information for several days, allowing a comparison between the length of the previous night and the preceding several nights. Shorter nights indicate springtime to the plant; when the nights become longer, autumn is approaching. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly. Short-day (long-night) plants use this information to flower in the late summer and early fall, when nights exceed a critical length (often eight or fewer hours). Long-day (short-night) plants flower during the spring, when darkness is less than a critical length (often eight to fifteen hours). Not all plants use the phytochrome system in this way. Flowering in day-neutral plants is not regulated by daylength.

Question 206

How has the creation of the horticulturist careers influenced plant production?

Answer 206

The word “horticulturist” comes form the Latin words for garden (hortus) and culture (cultura). This career has been revolutionized by progress made in the understanding of plant responses to environmental stimuli. Growers of crops, fruit, vegetables, and flowers were previously constrained by having to time their sowing and harvesting according to the season. Now, horticulturists can manipulate plants to increase leaf, flower, or fruit production by understanding how environmental factors affect plant growth and development.

Question 207

What are some aspects of greenhouse management that affect plant growth?

Answer 207

Greenhouse management is an essential component of a horticulturist’s education. To lengthen the night, plants are covered with a blackout shade cloth. Long-day plants are irradiated with red light in winter to promote early flowering. For example, fluorescent (cool white) light high in blue wavelengths encourages leafy growth and is excellent for starting seedlings. Incandescent lamps (standard light bulbs) are rich in red light, and promote flowering in some plants. The timing of fruit ripening can be increased or delayed by applying plant hormones. Recently, considerable progress has been made in the development of plant breeds that are suited to different climates and resistant to pests and transportation damage. Both crop yield and quality have increased as a result of practical applications of the knowledge of plant responses to external stimuli and hormones.

Question 208

Where are horticulturists employed and what education do they undergo?

Answer 208

Horticulturists find employment in private and governmental laboratories, greenhouses, botanical gardens, and in production or research fields. They improve crops by applying their knowledge of genetics and plant physiology. To prepare for a horticulture career, students take classes in botany, plant physiology, plant pathology, landscape design, and plant breeding. To complement these traditional courses, horticulture majors add studies in economics, business, computer science, and communications.

Question 209

How does blue light influence plant growth?

Answer 209

Phototropism - the directional bending of a plant toward or away from a light source - is a response to blue wavelengths of light. Positive phototropism is growth towards a light source, while negative phototropism (also called skototropism) is growth away from light.

Question 210

How are phototropins organized?

Answer 210

Phototropins are protein-based receptors responsible for mediating the phototropic response. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore. In phototropins, the chromophore is a covalently-bound molecule of flavin; hence, phototropins belong to a class of proteins called flavoproteins.

Question 211

In addition to phototropism, what behavior do phototropins influence?

Answer 211

Other responses under the control of phototropins are leaf opening and closing, chloroplast movement, and the opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the longest and is the best understood.

Question 212

How and when was phototropism first described?

Answer 212

In their 1880 treatise The Power of Movements in Plants, Charles Darwin and his son Francis first described phototropism as the bending of seedlings toward light. Darwin observed that light was perceived by the tip of the plant (the apical meristem), but that the response (bending) took place in a different part of the plant. They concluded that the signal had to travel from the apical meristem to the base of the plant.

Question 213

Which experiment demonstrated that chemical signals result in phototropism?

Answer 213

In 1913, Peter Boysen-Jensen demonstrated that a chemical signal produced in the plant tip was responsible for the bending at the base. He cut off the tip of a seedling, covered the cut section with a layer of gelatin, and then replaced the tip. The seedling bent toward the light when illuminated. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend. A refinement of the experiment showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the illuminated side, the plant did bend towards the light. Therefore, the chemical signal was a growth stimulant because the phototropic response involved faster cell elongation on the shaded side than on the illuminated side. We now know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem. Most activation occurs on the lit side, causing the plant hormone indole acetic acid (IAA) to accumulate on the shaded side. Stem cells elongate under the influence of IAA.

Question 214

How does blue light influence the daily plant activity cycle?

Answer 214

Cryptochromes are another class of blue-light absorbing photoreceptors that also contain a flavin-based chromophore. Cryptochromes set the plant’s 24-hour activity cycle, also known as its circadian rhythm, using blue light cues. There is some evidence that cryptochromes work together with phototropins to mediate the phototropic response.

Question 215

How does gravity affect plant growth?

Answer 215

Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, and roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism.

Question 216

How do plastids respond to gravity in plants?

Answer 216

Amyloplasts (also known as statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.

Question 217

How do amyloplasts cause gravitropic responses?

Answer 217

The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER), causing the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone IAA to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root, while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion, causing the shoot to grow up. After the shoot or root begins to grow vertically, the amyloplasts return to their normal position. Other hypotheses - involving the entire cell in the gravitropism effect - have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.

Question 218

How do hormones affect plant growth?

Answer 218

A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Potentially every cell in a plant can produce plant hormones. They can act in their cell of origin or be transported to other portions of the plant body, with many plant responses involving the synergistic or antagonistic interaction of two or more hormones. In contrast, animal hormones are produced in specific glands and transported to a distant site for action, and they act alone.

Question 219

What are the major plant hormones?

Answer 219

Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant hormones are traditionally described: auxins (particularly IAA), cytokinins, gibberellins, ethylene, and abscisic acid. In addition, other nutrients and environmental conditions can be characterized as growth factors.

Question 220

How do auxins affect plant growth?

Answer 220

The term auxin is derived from the Greek word auxein, which means “to grow”. Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue, and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance - the inhibition of lateral bud formation - is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses.

Question 221

What are some examples of commercial uses of auxins?

Answer 221

Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.

Question 222

How do cytokinins affect plant growth?

Answer 222

The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to be cytokinin, a hormone that promotes cytokinesis. Almost 200 naturally occurring or synthetic cytokinins are known to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds, and cytokinins that promote bushier growth.

Question 223

How do gibberellins affect plant growth?

Answer 223

Gibberellins (GAs) are a group of about 125 closely related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems, young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.

Question 224

What are some of the effects of gibberellins?

Answer 224

GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. Abscisic acid is a strong antagonist of GA action. Other effects of GAs include gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the instance of mildew infection.

Question 225

How does abscisic acid affect plant growth?

Answer 225

The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping of cotton bolls. However, more recent studies indicate that ABA plays only a minor role in the abscission process. ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Its activity counters many of the growth-promoting effects of GAs and auxins. ABA inhibits stem elongation and induces dormancy in lateral buds.

Question 226

What are the effects of abscisic acid in plants?

Answer 226

ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Plants adapted to temperate climates requires a long period of cold temperature before seeds germinate. This mechanism protects young plants from sprouting too early during unseasonably warm weather in winter. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss in winter buds.

Question 227

How does ethylene affect plant growth?

Answer 227

Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas (C₂H₄). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps.

Question 228

What are some of the effects of ethylene?

Answer 228

Aging tissues (especially senescing leaves) and nodes of stems produce ethylene. The best-known effect of the hormone, however, is the promotion of fruit ripening. Ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as avocadoes, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting of bulbs and potatoes.

Question 229

What are some commercial applications of ethylene?

Answer 229

Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application of the gas. Horticulturists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation.

Question 230

Are other hormones involved in plant development?

Answer 230

Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones.

Question 231

What are the effects of jasmonates?

Answer 231

Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded by a predator, resulting in an increase in toxic secondary metabolites. They contribute to the production of volatile compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest.

Question 232

What are the effects of oligosaccharins?

Answer 232

Oligosaccharins play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury, and can also be transported to other tissues.

Question 233

What are the effects of strigolactones?

Answer 233

Strigolactones promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi.

Question 234

What are the effects of brassinosteroids?

Answer 234

Brassinosteroids are important to many developmental and physiological processes. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect.

Question 235

In general, what plant activity is influenced by hormones?

Answer 235

Apical dominance, seed germination, gravitropism, and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited by steroids.

Question 236

What are some examples of plant responses to touch and wind?

Answer 236

The shoot of a pea plant winding around a trellis, and trees that grow on an angle in response to strong prevailing winds.

Question 237

What is an example of thigmotropism?

Answer 237

The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning “touch”, and tropism implying “direction”. Tendrils are one example of this. The meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand. Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.

Question 238

What is an example of a thigmonastic response?

Answer 238

A thigmonastic response is a touch response independent of the direction of the stimulus. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.

Question 239

What is an example of thigmomorphogenesis?

Answer 239

Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers hypothesize that mechanical strain induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis.

Question 240

What are the two types of enemies of plants?

Answer 240

Plants face two types of enemies: herbivores and pathogens. Herbivores both large and small use plants as food, and actively chew them. Pathogens are agents of disease. These infectious microorganisms, such as fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers.

Question 241

How do plant exteriors provide a defense mechanism for plants?

Answer 241

The first line of defense in plants is an intact and impermeable barrier. Bark and the waxy cuticle can protect against predators. Other adaptations against herbivory include thorns, which are modified branches, and spines, which are modified leaves. They discourage animals by causing physical damage and inducing rashes and allergic reactions. A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes.

Question 242

How do plants use secondary metabolites as defense mechanisms?

Answer 242

Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development. Many metabolites are toxic, and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some compounds become toxic after ingestion; for instance, gycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore.

Question 243

How does mechanical damage activate defense mechanisms?

Answer 243

Mechanical wounding and predator attacks activate defense and protection mechanisms both in the damaged tissue and at sites farther from the injury location. Some defense reactions occur within minutes: others over several hours. The infected and surrounding cells may die, thereby stopping the spread of infection.

Question 244

How is long-distance signaling used as defense mechanisms?

Answer 244

Long-distance signaling elicits a systematic response aimed at deterring the predator. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages in or on another insect, and eventually kill their host. The plant may activate abscission of injured tissue if it is damaged beyond repair.