Stems Module 5

Question 1

Features about Stems

Answer 1

usually grow upwards, but can have prostrate (flat) habit
usually above ground, but can also have underground stems (rhizomes, potatoes)
usually new growth at the tips (apex)
consists of nodes, internodes, apical meristems and buds

Question 2

Functions of the Stem

Answer 2

structural support and connecting organs
conducting water and nutrients from roots
spreads out leaf area vertically and horizontally

Question 3

Leaf index of 3

Answer 3

the most efficient way for photosynthesis to occur, the top layer gets the most, middle layer gets second most, bottom layer gets what remains, if the plant grows more the bottom layer dies out

Question 4

Organization of the Shoot

Answer 4

Leaf primordia (brand new baby leaves)
Apical Meristem
Region of cell elongation
Dermal tissue
Axillary bud
Vascular tissue

Question 5

Region of Cell Division

Answer 5

shoot apex
outer portion produces leaves
central portion provides structural support ( new stems) and vascular tissue

Question 6

Regions of cell elongation and cell differentiation

Answer 6

causes the increase in plant height
as the stem elongates the internodes elongate and the leaves separate
internodes can elongate in response to competition

Question 7

Divot vs Monocot Stems

Answer 7

Dicot is uniform and circular, has phloem, vascular cambium, and xylem
Monocot has scattered vascular bundles has xylem and phloem

Question 8

Primary roots vs primary shoots (5)

Answer 8

root apex is covered by a root cap whereas the stem apex is covered by primordia leaves
arrangement of vascular bundles is different
roots have pericycle present while stems do not
endodermis present in all roots not in shoots
lateral branches come from pericycle in roots vs buds on shoots

Question 9

Dicot Stem Structure

Answer 9

nodes and internodes
axillary buds at nodes
nodes can regulate plant functions

Question 10

Monocot Stem Structures

Answer 10

modes are initially compressed in young grasses
as grasses become older, the internodes elongate and the growing point is pushed upwards
in grasses the growing point is underground for several weeks until the internodes begin to lengthen; while underground it is protected from light frost and wheel traffic
the nodes are already a part of the plant to begin with

Question 11

Lateral branching in Dicot Stems

Answer 11

almost always found in the axil of leaves
usually one branch per axil
branching enables a plant to respond to good growing conditions (nutrients, more space)
many plants can branch to fill in gaps
plasticity, ability to flex to conditions

Question 12

Lateral Branching in Monocot Stems

Answer 12

branching in monocots called tillering
tillers occur at the base (crown) of grass stems
roots that do not arise from the seed are called adventitious roots
very important for grain yield in cereal crops, turfgrass, and ornamental grasses

Question 13

Bud Dormancy, 4 factors related to dormancy

Answer 13

keeps branches from forming
Apical dominance - apex exerts control over later buds, can be overcome by removing
Tillering - monocots show less apical dominance compared to dicots
Pruning - pruning back to encourage branching, must be sure that there are buds to grow out or else no branching, pinch flowers off of plants
Competition - bud growth may not occur due to lack of resources, may affect size of branch

Question 14

Modifications of stems (7)

Answer 14

Stolons (runners)
Rhizomes
Tubers
Corms
Climbing Modifications (eg tendrils)
Thorns
Other (such as thickened stem for water, cactus)

Question 15

Stolons (runners)

Answer 15

strawberries
roots develop at the node points above ground
stolons emerge from the crown
stem bends over under its weight and forms roots and stems where it has contact with the soil (purslane)

Question 16

Rhizomes

Answer 16

Asparagus, irises, quackgrass
underground stems with scale like leaves and axillary buds
roots and shoots develop at each node or at the apical bud
difficult to control weeds often have rhizomes - rhizomes are spread by tillage, there is rapid growth from underground rhizomes in the spring, if mowed or pulled the plant will grow back from axillary buds pn the rhizome

Question 17

Tubers (enlarged rhizomes)

Answer 17

potatoes
food accumulation at the tips of the underground rhizomes
internodes swell to become tubers
“eyes” are nodes with the eye being the axillary bud
when planting potatoes always ensure that there is at least one eye for new growth
weeds that have tubers are often difficult to control eg yellow nutsedge

Question 18

Corms (short and thickened underground stem)

Answer 18

gladiolus, crocus
function as food storage
composed entirely of solid tissue covered by leaves

Question 19

Climbing Modifications (tendrils)

Answer 19

grapes, ivy
twining action of stem to find support
ivy produces adventitious roots at each node for adhering to walls (form of non root tissue)
weeds with a twining habit can make harvest difficult eg wild cucumber

Question 20

Thorns

Answer 20

orange, rose
thorns form directly on the stem
modified leaf or part of the stem

Question 21

Other modifications of Stems

Answer 21

thickened stem for water storage (cactus)
stem becomes main organ for photosynthesis (cactus)

Question 22

What is Secondary Growth in stems (Dicots only)

Answer 22

an increase in diameter of the stem
it is a mechanism whereby the plant can increase its own mechanical support as it increases in height and can slowly ensure a steady supply of new conducting tissues

Question 23

Where does secondary growth occur?

Answer 23

vascular cambium
cork cambium

Question 24

Growth of the vascular cambium

Answer 24

occurs in 3 directions
cells on the inside become xylem
cells on the outside become phloem
sideways-add more cells to the meristem
cork cambium divides the wood and bark

Question 25

What happens if the tree is girdled?

Answer 25

it dies, cutting the transport of water and nutrients

Question 26

Cork Cambium

Answer 26

cork replaces the epidermis on herbaceous stems
porous to allow gas exchange
suberized (waxy layers) to reduce water loss
bark is the tissue from the vascular cambium outward (includes phloem)

Question 27

No secondary growth in monocots

Answer 27

no woody monocots because no cambium present
root xylem and phloem tend to alternate, so there is no way to form vascular cambium
stem vascular bundles tend to be scattered, so there is no way to form vascular cambium
monocots have a form of second growth called anomalous thickening (bamboo)

Question 28

Woody Stem Structures

Answer 28

Consist of bark and wood

Question 29

Bark

Answer 29

the outer portion of the stem
consists of epidermis, cortex, phloem, vascular cambium
dicots only

Question 30

Wood

Answer 30

inner portion of the stem
consists of secondary xylem, primary xylem, and pith

Question 31

What are the sections of Wood?

Answer 31

Heartwood - central portion, often clogged with waste and is darker, vascular rays transport waste to the heartwood, dead
Sapwood - outer portion, younger secondary xylem, cells still carrying water and nutrients
Annual rings - in spring moisture is usually more limiting so new xylem are produced, rings don’t always coincide with the year (wet summers, may have large rings)