32: Plant Reproduction

Question 1

What is the androecium?

Answer 1

The sum of all the stamens in a flower.

Question 2

What are the antipodals?

Answer 2

The three cells away from the micropyle.

Question 3

What is the exine?

Answer 3

The outermost covering of pollen.

Question 4

What is a gametophyte?

Answer 4

The multicellular stage of the plant that gives rise to haploid gametes or spores.

Question 5

What is the gynoecium?

Answer 5

The sum of all the carpels in a flower.

Question 6

What is the intine?

Answer 6

The inner lining of the pollen.

Question 7

What is megagametogenesis?

Answer 7

The second phase of female gametophyte development, during which the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, AKA the megagametophyte or embryo sac.

Question 8

What is a megasporangium?

Answer 8

Tissue found in the ovary that gives rise to the female gamete or egg.

Question 9

What is megasporogenesis?

Answer 9

The first phase of female gametophyte development, during which a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives.

Question 10

What is a megasporophyll?

Answer 10

A bract (a type of modified leaf) on the central axis of a female gametophyte.

Question 11

What is the micropyle?

Answer 11

The opening on the ovule sac through which the pollen tube can gain entry.

Question 12

What is a microsporangium?

Answer 12

Tissue that gives rise to the microspores or the pollen grain.

Question 13

What is a microsporophyll?

Answer 13

The central axis of a male cone on which bracts (a type of modified leaf) are attached.

Question 14

What is the perianth?

Answer 14

The part of the flower consisting of the calyx and/or corolla; forms the outer envelope of the flower.

Question 15

What are polar nuclei?

Answer 15

They are found in the ovule sac; fusion with one sperm cell forms the endosperm.

Question 16

What is a sporophyte?

Answer 16

The multicellular diploid stage in plants that is formed after the fusion of male and female gametes.

Question 17

What is a synergid?

Answer 17

A type of cell found in the ovule sac that secretes chemicals to guide the pollen tube towards the egg.

Question 18

What is the term for a flower that has all four whorls?

Answer 18

If all four whorls (the calyx, corolla, androecium, and gynoecium) are present, the flower is described as complete. If any of the four parts is missing, the flower is known as incomplete.

Question 19

What are the terms for a flower that contains both an androecium and a gynoecium?

Answer 19

Perfect, androgynous, or hermaphrodites.

Question 20

What are the types of imperfect flowers?

Answer 20

Staminate flowers contain only an androecium, and carpellate flowers contain only a gynoecium.

Question 21

What are some examples of monoecious plants?

Answer 21

Corn and peas.

Question 22

What are some examples of dioecious plants?

Answer 22

C. papaya and Cannabis.

Question 23

What are the orientation-dependent terms for an ovary?

Answer 23

The ovary, which may contain one or multiple ovules, may be placed above other flower parts, which is referred to as superior; or, it may be placed below the other flower parts, referred to as inferior.

Question 24

Where does pollen develop?

Answer 24

The male gametophyte (the pollen grain) develops and reaches maturity in an immature anther. In a plant’s reproductive organs, development of pollen takes place in a structure known as the microsporangium. The microsporangia, which are usually bi-lobed, are pollen sacs in which the microspores develop into pollen grains. These are found in the anther, which is at the end of the stamen.

Question 25

How are microspores developed?

Answer 25

Within the microsporangium, the microspore mother cell divides by meiosis to give rise to the four microspores, each of which will ultimately form a pollen grain.

Question 26

What is the tapetum?

Answer 26

An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall.

Question 27

What cells do mature pollen grains contain?

Answer 27

A generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes (sperm cells).

Question 28

How is pollen released?

Answer 28

Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.

Question 29

What are the coverings of pollen grains?

Answer 29

Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine (thinner, inner layer).

Question 30

How are pollen grains protected?

Answer 30

The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.

Question 31

What are the components of an embryo sac and how do they develop?

Answer 31

Two of the nuclei–the polar nuclei–move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.

Question 32

What is the role of the integument?

Answer 32

A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.

Question 33

Where are sporophytes and gametophytes found in conifers?

Answer 33

In conifers such as pines, the green leafy part of the plant is the sporophyte, and the cones contain the male and female gametophytes.

Question 34

What prevents self-pollination in gymnosperms?

Answer 34

The female cones are larger than the male cones and positioned towards the top of the tree; the small, male cones are located in the lower region of the tree. Because pollen is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.

Question 35

How does pollen develop in gymnosperms?

Answer 35

Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus, and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on the female cone.

Question 36

How does the ovule develop in gymnosperms?

Answer 36

Within the megasporangium, the megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.

Question 37

How does fertilization occur in gymnosperms?

Answer 37

Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative cell migrates towards the female gametophyte through the micropyle. It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After fertilization, the diploid zygote is formed, which divides by mitosis to form the embryo.

Question 38

How do seeds develop in gymnosperms?

Answer 38

The scales of the cones are closed during the development of the seed. The seed is covered by a seed coat, which is derived from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of seed; no fruit formation takes place because gymnosperm seeds have no covering.

Question 39

How does the location of female gametophytes differ between angiosperms and gymnosperms?

Answer 39

In angiosperms, the female gametophyte exists in an enclosed structure–the ovule–which is within the ovary; in gymnosperms, the female gametophyte is present on exposed bracts of the female cone.

Question 40

How does fertilization differ between angiosperms and gymnosperms?

Answer 40

Double fertilization is a key event in the lifecycle of angiosperms, but is completely absent in gymnosperms.

Question 41

How does the location of gametophytes differ between angiosperms and gymnosperms?

Answer 41

The male and female gametophyte structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are part of the flower.

Question 42

How does pollination differ between angiosperms and gymnosperms?

Answer 42

Wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination is more common.

Question 43

What is accessory fruit?

Answer 43

Fruit derived from tissues other than the ovary.

Question 44

What is aggregate fruit?

Answer 44

Fruit that develops from multiple carpels in the same flower.

Question 45

What is the aleurone?

Answer 45

The single layer of cells just inside the seed coat that secretes enzymes upon germination.

Question 46

What is the coleoptile?

Answer 46

The covering of the shoot tip, found in germinating monocot seeds.

Question 47

What is the coleorhiza?

Answer 47

The covering of the root tip, found in germinating monocot seeds.

Question 48

What is the cotyledon?

Answer 48

The fleshy part of the seed that provides nutrition to the embryo.

Question 49

What is cross-pollination?

Answer 49

Transfer of pollen from the anther of one flower to the stigma of a different flower.

Question 50

What is dormancy?

Answer 50

A period of no growth and very slow metabolic processes.

Question 51

What is double fertilization?

Answer 51

Two fertilization events in angiosperms; one sperm fuses with the egg, forming the zygote, whereas the other sperm fuses with the polar nuclei, forming endosperm.

Question 52

What is the endocarp?

Answer 52

The innermost part of the fruit.

Question 53

What is the endosperm?

Answer 53

The triploid structure resulting from fusion of a sperm with polar nuclei, which serves as a nutritive tissue for the embryo.

Question 54

What is an endospermic dicot?

Answer 54

A dicot that stores food reserves in the endosperm.

Question 55

What is the exocarp?

Answer 55

The outermost covering of a fruit.

Question 56

What is the epicotyl?

Answer 56

The embryonic shoot above the cotyledons.

Question 57

What is gravitropism?

Answer 57

Response of a plant growth in the same direction as gravity.

Question 58

What is the hypocotyl?

Answer 58

The part of the embryonic axis below the cotyledons.

Question 59

What is the mesocarp?

Answer 59

The middle part of a fruit.

Question 60

What is multiple fruit?

Answer 60

Fruit that develops from multiple flowers on an inflorescence.

Question 61

What is a nectar guide?

Answer 61

A pigment pattern on a flower that guides an insect to the nectaries.

Question 62

What is a non-endospermic dicot?

Answer 62

A dicot that stores food reserves in the developing cotyledon.

Question 63

What is the pericarp?

Answer 63

A collective term describing the exocarp, mesocarp, and endocarp; the structure that encloses the seed and is a part of the fruit.

Question 64

What is a plumule?

Answer 64

A shoot that develops from the germinating seed.

Question 65

What is pollination?

Answer 65

Transfer of pollen to the stigma.

Question 66

What is the radicle?

Answer 66

The original root that develops from the germinating seed.

Question 67

What is scarification?

Answer 67

Mechanical or chemical processes to soften the seed coat.

Question 68

What is a scutellum?

Answer 68

A type of cotyledon found in monocots, as in grass seeds.

Question 69

What is self-pollination?

Answer 69

Transfer of pollen from the anther to the stigma of the same flower.

Question 70

What is simple fruit?

Answer 70

Fruit that develops from a single carpel or fused carpels.

Question 71

What is the suspensor?

Answer 71

The part of the growing embryo that makes connection with the maternal tissues.

Question 72

What is the tegmen?

Answer 72

The inner layer of the seed coat.

Question 73

What is the testa?

Answer 73

The outer layer of the seed coat.

Question 74

What is vernalization?

Answer 74

Exposure to cold required by some seeds before they can germinate.

Question 75

How is corn an example of selective breeding?

Answer 75

Today’s corn is a result of years of breeding that started with its ancestor, teosinte. The teosinte that the ancient Mayans originally began cultivating had tiny seeds–vastly different from today’s relatively giant ears of corn. Though these two plants appear to be entirely different, the genetic difference between them is miniscule.

Question 76

Why does self-pollination occur?

Answer 76

Self-pollination occurs in flowers where the stamen and carpel mature at the same time, and are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.

Question 77

What are some ways that plants avoid self-pollination?

Answer 77

In some species, the pollen and the ovary mature at different times. By the time pollen matures and has been shed, the stigma of the flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. The primrose is one such flower. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower has anthers positioned at the pollen tube’s halfway point, and the thrum-eyed flower’s stigma is likewise located at the halfway point. Insects easily cross-pollinate while seeking the nectar at the bottom of the pollen tube. This phenomenon is known as heterostyly. Many plants, such as cucumber, have male and female flowers located on different parts of the plant, making self-pollination difficult. In other species, the male and female flowers are borne on different plants (dioecious).

Question 78

How do incompatibility genes prevent pollen germination?

Answer 78

Incompatibility genes, which prevent pollen from germinating or growing into the stigma of a flower, have been discovered in many angiosperm species. If plants do not have compatible genes, the pollen tube stops growing. Self-incompatibility is controlled by the S (sterility) locus. Pollen tubes have to grow through the tissue of the stigma and style before they can enter the ovule. The carpel is selective in the type of pollen it allows to grow inside. The interaction is primarily between the pollen and the stigma epidermal cells. In some plants, like cabbage, the pollen is rejected at the surface of the stigma, and the unwanted pollen does not germinate. In other plants, pollen tube germination is arrested after growing one-third the length of the style, leading to pollen tube death. Pollen tube death is due either to apoptosis or to degradation of pollen tube RNA. The degradation results from the activity of a ribonuclease encoded by the S locus. The ribonuclease is secreted from the cells of the style in the extracellular matrix, which lies alongside the growing pollen tube.

Question 79

What are some characteristics of bee pollinators?

Answer 79

Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees. The most common species of bees are bumblebees and honeybees. Since bees cannot see the color red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees collect energy-rich pollen or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored, have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A nectar guide includes regions on the flower petals that are visible only to bees, and not to humans; it helps to guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks to the bees’ fuzzy hair, and when the bee visits another flower, some of the pollen is transferred to the second flower.

Question 80

What are some characteristics of fly pollinators?

Answer 80

Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which produce nectar, usually have dull colors, such as brown or purple. The nectar provides energy, whereas the pollen provides protein.

Question 81

What are some examples of flowers that use fly pollination?

Answer 81

The corpse flower or voodoo lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapelia, Rafflesia).

Question 82

Other than bees, what other stinging insect pollinates flowers?

Answer 82

Wasps are also important insect pollinators, and pollinate many species of figs.

Question 83

What are some characteristics of butterfly pollinators?

Answer 83

Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to make access to nectar easier. The pollen is picked up and carried on the butterfly’s limbs.

Question 84

What are some characteristics of moth pollinators?

Answer 84

Moths pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white and are flat, enabling the moths to land.

Question 85

What are some characteristics of the symbiotic relationship between the yucca plant and the yucca moth?

Answer 85

The shape of the flower and moth have adapted in such a way as to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and developing seeds.

Question 86

Which moth and plant have a similar relationship to the yucca moth and plant?

Answer 86

The corn earworm moth and Gaura plant have a similar relationship.

Question 87

What are some characteristics of bat pollinators?

Answer 87

In the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning glory. The flowers are usually large and white or pale-colored; thus, they can be distinguished from the dark surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar. They are naturally large and wide-mouthed to accommodate the head of a bat. As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.

Question 88

What are some characteristics of bird pollinators?

Answer 88

Many species of small birds, such as the hummingbird and sun birds, are pollinators for plants such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in such a way as to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower typically has a curved, tubular shape, which allows access for the bird’s beak. Brightly colored, odorless flowers that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited on the bird’s head and neck and is then transferred to the next flower it visits.

Question 89

How can the range of plants be determined by bird pollination?

Answer 89

Botanists have been known to determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from the same site.

Question 90

What are some characteristics of pollination by wind?

Answer 90

Most species of conifers, and many angiosperms, such as grasses, maples, and oaks, are pollinated by wind. Pine cones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green, small, may have small or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated flowers, flowers adapted to pollination by wind do not produce nectar or scent. In wind-pollinated species, the microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it. The flowers usually emerge early in the spring, before the leaves, so that the leaves do not block the movement of the wind. The pollen is deposited on the exposed feather stigma of the flower.

Question 91

What are some characteristics of pollination by water?

Answer 91

Some weeds, such as Australian sea grass and pond weeds, are pollinated by water. The pollen floats on water, and when it comes into contact with the flower, it is deposited inside the flower.

Question 92

How abundant are orchids?

Answer 92

Orchids are highly valued flowers, with many rare varieties. They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of orchids have been identified.

Question 93

How is food deception used for pollination?

Answer 93

Some species of orchid have evolved a pollination method known as food deception, in which bright colors and perfumes are offered, but no nectar. Anacamptis morio, commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent–which usually indicates food for a bee–and in the process, picks up the pollen to be transported to another flower.

Question 94

How is sexual deception used for pollination?

Answer 94

Some orchids use sexual deception for pollination. Chiloglottis trapeziformis emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and in the process, transfers pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in a different sexual deception strategy to attract wasps. The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, and in the process, picks up pollen, which it then transfers to the next counterfeit mate.

Question 95

During double fertilization, how does the pollen tube grow through the style?

Answer 95

The germination of the pollen tube requires water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen tube’s growth is supported by the tissues of the style. In the meantime, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac, and it enters the ovule sac through the micropyle. After fertilization is complete, no other sperm can enter.

Question 96

How does cell division occur after double fertilization?

Answer 96

After fertilization, the zygote divides to form two cells: the upper cell, or terminal cell, and the lower, or basal, cell. The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo.

Question 97

How does the embryo in a seed develop in dicots?

Answer 97

In dicots (eudicots), the developing embryo has a heart shape, due to the presence of the two rudimentary cotyledons. In non-endospermic dicots, such as Capsella bursa, the endosperm develops initially, but is then digested, and the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they run out of room inside the developing seed, and are forced to bend. Ultimately, the embryo and cotyledons fill the seed, and the seed is ready for dispersal.

Question 98

How does the embryo develop after seed maturity?

Answer 98

Embryonic development is suspended after some time, and growth is resumed only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.

Question 99

How does the embryo access food reserves in monocots?

Answer 99

In monocots, such as corn and wheat, the single cotyledon is called a scutellum; the scutellum is connected directly to the embryo via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins, and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.

Question 100

How does the embryo access food reserves in endospermic dicots?

Answer 100

In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive organs to take up the enzymatically released food reserves, much like in monocots (which, by definition, also have endospermic seeds).

Question 101

What are some examples of endospermic dicots?

Answer 101

Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper (Capsicum annuum).

Question 102

How does the embryo access food reserves in non-endospermic dicots?

Answer 102

In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage.

Question 103

What are some examples of non-endospermic dicots?

Answer 103

The two halves of a peanut seed (Arachis hypogaea) and the split peas (Pisum sativum) of split pea soup are individual cotyledons loaded with food reserves.

Question 104

What are some characteristics of the seed coat?

Answer 104

The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat known as the testa and inner coat known as the tegmen.

Question 105

What are the parts of the embryonic axis?

Answer 105

The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the cotyledons”). The embryonic axis terminates in a radicle (the embryonic root), which is the region from which the root will develop. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant. In monocots, the hypocotyl does not show above ground because monocots do not exhibit stem elongation. The part of the embryonic axis that projects above the cotyledons is known as the epicotyl. The plumule is composed of the epicotyl, young leaves, and the shoot apical meristem.

Question 106

How does germination occur in dicot seeds?

Answer 106

Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards. This shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate. During this time, the radicle is also growing and producing the primary root. As it grows downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system.

Question 107

How does germination occur in monocot seeds?

Answer 107

In monocot seeds, the testa and tegmen of the seed coat are fused. As the seed germinates, the primary root emerges, protected by the root-tip covering: the coleorhiza. Next, the primary shoot emerges, protected by the coleoptile: the covering of the shoot tip. Upon exposure to light (when the plumule has exited the soil and the protective coleoptile is no longer needed), elongation of the coleoptile ceases and the leaves expand and unfold. At the other end of the embryonic axis, the primary root soon dies, while other, adventitious roots emerge from the base of the stem. This gives the monocot a fibrous root system.

Question 108

How does dormancy increase seed viability?

Answer 108

Dormancy helps keep seeds viable during unfavorable conditions. Upon a return to favorable conditions, seed germination takes place. Dormancy may last for months, years, or even centuries.

Question 109

What are some examples of favorable conditions?

Answer 109

Favorable conditions could be as diverse as moisture, light, cold, fire, or chemical treatments. After heavy rains, many new seedlings emerge. Forest fires also lead to the emergence of new seedlings.

Question 110

How are temperature treatments used in seed germination?

Answer 110

Some seeds require vernalization (cold treatment) before they can germinate. This guarantees that seeds produced by plants in temperate climates will not germinate until the spring. Plants growing in hot climates may have seeds that need a heat treatment in order to germinate, to avoid germination in the hot, dry summers.

Question 111

How does the seed coat affect germination?

Answer 111

In many seeds, the presence of a thick seed coat retards the ability to germinate. Scarification, which includes mechanical or chemical processes to soften the seed coat, is often employed before germination. Presoaking in hot water, or passing through an acid environment, such as an animal’s digestive tract, may also be employed.

Question 112

How does seed size affect germination?

Answer 112

Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have enough food reserves to germinate deep below ground, and still extend their epicotyl all the way to the soil surface. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath the surface, the developing seedling would not have enough food reserves to reach the sunlight.

Question 113

Where do fruits develop?

Answer 113

After fertilization, the ovary of the flower usually develops into the fruit. In most cases, flowers in which fertilization has taken place will develop into fruits, and flowers in which fertilization has not taken place will not. Some fruits develop from the ovary and are known as true fruits, whereas others develop from other parts of the female gametophyte and are known as accessory fruits.

Question 114

What are some examples of fruit?

Answer 114

The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull of corn (the tough, thin part that gets stuck in your teeth when you eat popcorn).

Question 115

How may fruits be classified?

Answer 115

Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin.

Question 116

What are some examples of simple fruits?

Answer 116

Nuts and beans.

Question 117

What is an example of an aggregate fruit?

Answer 117

Raspberries.

Question 118

What is an example of a multiple fruit?

Answer 118

Pineapples, where the flowers fuse together to form the fruit.

Question 119

What is another term for accessory fruit?

Answer 119

False fruit.

Question 120

What are some examples of accessory fruits?

Answer 120

Strawberries, which are derived from the receptacle, and apples and pears, which are derived from the hypanthium.

Question 121

What are the parts of a fruit?

Answer 121

Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of the fruit), and the endocarp (the inner part of the fruit). Together, all three are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the endocarp is the edible part. In many fruits, two or all three of the layers are fused, and are indistinguishable at maturity. Fruits can be dry or fleshy.

Question 122

What are dehiscent and indehiscent fruits?

Answer 122

Dehiscent fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release their seeds.

Question 123

How are fruits adapted for seed dispersal?

Answer 123

Some fruits have built-in mechanisms so they can disperse by themselves, whereas others require the help of agents like wind, water, and animals. Modifications in seed structure, composition, and size help in dispersal.

Question 124

How are some fruits adapted for dispersal by wind?

Answer 124

Wind-dispersed fruit are lightweight and may have wing-like appendages that allow them to be carried by the wind. Some have a parachute-like structure to keep them afloat. Some fruits–for example, the dandelion–have hairy, weightless structures that are suited to dispersal by wind.

Question 125

How are some fruits adapted for dispersal by water?

Answer 125

Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are well known for their ability to float on water to reach land where they can germinate. Similarly, willow and silver birches produce lightweight fruit that can float on water.

Question 126

How are some fruits adapted for dispersal by animals?

Answer 126

Animals eat fruits, and the seeds that are not digested are excreted in their droppings some distance away. Some animals, like squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash of fruit, and if conditions are favorable, the seeds germinate. Some fruits, like the cocklebur, have hooks or sticky structures that stick to an animal’s coat and are then transported to another place. Humans play a big role in dispersing seeds when they carry fruits to new places and throw away the inedible part that contains the seeds.

Question 127

What is apomixis?

Answer 127

The process by which seeds are produced without fertilization of sperm and egg.

Question 128

What is cutting?

Answer 128

The method of asexual reproduction where a portion of the stem that contains nodes and internodes is placed in moist soil and allowed to root.

Question 129

What is grafting?

Answer 129

The method of asexual reproduction where the stem from one plant species is spliced to a different plant.

Question 130

What is layering?

Answer 130

The method of propagating plants by bending a stem under the soil.

Question 131

What is micropropagation?

Answer 131

Propagation of desirable plants from a plant part; carried out in a laboratory.

Question 132

What are monocarpic plants?

Answer 132

Plants that flower once in their lifetime.

Question 133

What are polycarpic plants?

Answer 133

Plants that flower several times in their lifetime.

Question 134

What is a scion?

Answer 134

The part of a plant that is grafted onto the root stock of another plant.

Question 135

What is senescence?

Answer 135

The process that describes aging in plant tissues.

Question 136

Why does asexual reproduction occur in plants?

Answer 136

Many plants are able to propagate themselves using asexual reproduction. This method does not require the investment to produce a flower, attract pollinators, or find a means of dispersal. Traditionally, these plants survive well under stable environmental conditions when compared with plants produced from sexual reproduction because they carry genes identical to those of their parents.

Question 137

What are some different types of root modifications?

Answer 137

The corm is used by gladiolus and garlic. Bulbs, such as a scaly bulb in lilies and a tunicate bulb in daffodils, are other common examples. A potato is a stem tuber, while parsnip propagates from a taproot. Ginger and iris produce rhizomes, while ivy uses an adventitious root, and the strawberry plant has a stolon, which is also called a runner.

Question 138

How does apomixis occur?

Answer 138

Either the ovule or part of the ovary, which is diploid in nature, gives rise to a new seed.

Question 139

What are some advantages of asexual reproduction?

Answer 139

An advantage of asexual reproduction is that the resulting plant will reach maturity faster. Since the new plant is arising from an adult plant or plant parts, it will also be sturdier than a seedling.

Question 140

What are some natural methods of asexual reproduction?

Answer 140

Many plants–like ginger, onion, gladioli, and dahlia–continue to grow from buds that are present on the surface of the stem. In some plants, such as the sweet potato, adventitious roots or runners can give rise to new plants. In Bryophyllum and kalanchoe, the leaves have small buds on their margins. When these are detached from the plant, they grow into independent plants; or, they may start growing into independent plants if the leaf touches the soil.

Question 141

What are some artificial methods of asexual reproduction in plants?

Answer 141

Grafting, cutting, layering, and micropropagation.

Question 142

How is grafting performed?

Answer 142

In grafting, two plant species are used; part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than a right angle), placed in close contact with each other, and are then held together. Matching up these two surfaces as closely as possible is extremely important because these will be holding the plant together. The vascular systems of the two plants grow and fuse, forming a graft. After a period of time, the scion starts producing shoots, and eventually starts bearing flowers and fruits.

Question 143

Where is grafting used?

Answer 143

Grafting has long been used to produce novel varieties of roses, citrus species, and other plants. Grafting is widely used in viticulture (grape growing) and the citrus industry. Scions capable of producing a particular fruit variety are grated onto root stock with specific resistance to disease.

Question 144

How is cutting performed?

Answer 144

Plants such as coleus and money plant are propagated through stem cuttings, where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start producing a root even when placed only in water. For example, leaves of the African violet will root if kept in water undisturbed for several weeks.

Question 145

How is layering performed?

Answer 145

Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can be bent easily without any injury are preferred. Jasmine and bougainvillea (paper flower) can be propagated this way. In some plants, a modified form of layering known as air layering is employed. A portion of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some gardeners also apply rooting hormone. After some time, roots will appear, and this portion of the plant can be removed and transplanted into a separate pot.

Question 146

Where is micropropagation used?

Answer 146

Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a single plant in a short time under laboratory conditions. This method allows propagation of rare, endangered species that may be difficult to grow under natural conditions, are economically important, or are in demand as disease-free plants.

Question 147

How is micropropagation performed?

Answer 147

To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used. The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated mass known as callus, from which individual plantlets begin to grow after a period of time. These can be separated and are first grown under greenhouse conditions before they are moved to field conditions.

Question 148

What is the difference between life span and life cycle?

Answer 148

The length of time from the beginning of development to the death of a plant is called its life span. The life cycle, on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of the mature plant.

Question 149

How long are plant life spans?

Answer 149

Some plants, such as annuals, only need a few weeks to grow, produce seeds and die. Other plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented age of 4500 years.

Question 150

How do the cells of a plant grow over its life span?

Answer 150

Some parts of a plant, such as regions containing meristematic tissue, continue to grow, and others undergo apoptosis. The cork found on stems, and the water-conducting tissue of the xylem, for example, are composed of dead cells.

Question 151

How are plants categorized by seasonal lifecycles?

Answer 151

Plant species that complete their lifecycle in one season are known as annuals, an example of which is Arabidopsis, or mouse-ear cress. Biennials such as carrots complete their lifecycle in two seasons. In a biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its reproductive phase. Commercial growers harvest the carrot roots after the first year of growth, and do not allow the plants to flower. Perennials, such as the magnolia, complete their lifecycle in two years or more.

Question 152

What are some characteristics of monocarpic plants?

Answer 152

Monocarpic plants flower only once in their lifetime; examples include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as 120 years in some bamboo species), these plants may reproduce asexually and accumulate a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon after flowering, these plants die.

Question 153

What are some characteristics of polycarpic plants?

Answer 153

Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this means, the plant does not require all its nutrients to be channeled towards flowering each year.

Question 154

What factors influence plant survival?

Answer 154

Genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue such as cork. Individual parts of plants, such as flowers and leaves, have different rates of survival.

Question 155

Why does leaf fall occur?

Answer 155

In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency, due to shading by upper leaves, or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling.

Question 156

How does senescence occur?

Answer 156

Senescence is marked by several complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues.

Question 157

What role do hormones play in senescence?

Answer 157

The complex pathways of nutrient recycling within a plant are not well understood. Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic acid causes premature onset of senescence.