R2101 2.4 Describe the structure and state the functions of stems Flashcards
Main functions of a stem (5)
- Support for plant
- Spaces out leaves for air and sunlight to maximise photosynthesis
- Holds flowers above ground to help with pollination
- If green, helps with photosynthesis
- Is the link between roots (where food is stored and minerals and water are taken up) and the leaves (where water is lost and food manufactured)
What are the spaces between nodes called?
Internodes
What is a shoot?
Collective name for the stem and all its attachments
What is a bud? What do they consist of (2)?
A bud is a compact undeveloped shoot. Consists of:
1. Crowded overlapping immature leaves. The outside has thicker leaves that form bud scale which protect from drying and damage.
2. Minute buds - precursors
What are the two positionally type of buds?
- Apical (or teminal) at the tip of a shoot
- Axillary - off to the side
What is a leaf scar and why is it useful?
A leaf scars shows where a leaf or fruit has been attached
It is useful in identifying some plants in winter with bare stems. e.g. Aesculus (horse chesnut) has very distinct leaf scars
Four main areas of tissue in the stem?
- Epidermis
- Cortex
- Vascular tissue
- Pith
What are lenticals? (4)
- Blister like breaks in the surface of the skin
- Is a pathway through which gases can diffuse to the living cells of the bark
- Replace stomata when the epidermis gives way to a waterproof and gas tight bark layer
- Shape and form can be distinctive in indentification of some species e.g. Prunus serrula
What are growth rings? What do they show?
- On the surface of the stem show where one years growth has ended and the next years begins.
- The growth ring is the bud scale scar where the terminal bud scales were attached to the shoot before the next year’s growth started. This can be helpful in pruning.
Epidermis (2)
- Produces waterproof waxy layer made of cutin on its surface called the cuticle
- Pores called stomata punctuate its surface. Guard cells control the opeing and closing of these pores
Cortex (3)
- Beneath the epidermis and largely made up of parenchyma
- Help maintain plant shape
- Inside the cortex are a ring of vascular bundles in dicots and the vascular bundles are scattered throughout the stem in moncots
Pith
Central zone of the stem, mostly parenchyma cells. May sometimes breakdown to give a hollow stems.
Vascular bundle - Xylem
- Contains long wide open-ended cells with thick lignified walls to withstand high pressure of water
Vascular bundle - phloem
- Long tube like cells called sieve tubes
- Transport food
- Have cellulose cell walls - not lignified
- Do not contain nucelus
- End walls only partially broken down to produce sieve plates
- Companion cells regulate the flow of liquids through the sieve tube
Vascular bundle - vascular cambium (dicots)
- This is a lateral meristem
- Dividing cells producing more xylem and phloem tissue to increase the girth of the stem of woody plants as it grows - secondary thickening
Vascular bundle - support (dicots)
Arranged in ring which gives strength and support to the stem in the much the same way steel rods do in reinforced concrete
Role of apical bud
- Where growth is inititated
- Also give rise to small leaves (bud scales) to protect the meristem
Role of apical bud
- Where growth is inititated
- Also give rise to small leaves (bud scales) to protect the meristem
- Deep inside the bud is a tiny mass of cells - each with a conspicuous nucleus but no vacuole. These make up the apical meristem, and this is where cells divide to produce new tissues.
Role of axilliary buds
- Have their own apical meristems which often give rise to side branches
- Located lower down the stem in the angle of the leaf
What are twining stems
- Wind around structures for support
- Can be either clockwise or anitclockwise (e.g. Wisteria floribunda)
- Can become large and woody. Then are called lianes
What are thornes?
1.Modified branches growing from axillary buds
2. Discourages herbivores
3. e.g. Crataegus ssp
What are prickles?
- Specialised outgrowths of stem epidermis
- Easily rubbed off
- Both protect the plant and help it scramble over other vegetation
- e.g. Roses
What are rhizomes?
- Organ of perennation
- Stem grow horizontally underneath and sometime just above the ground
- Nodes and internodes can be clearly seen along the stem
- e.g. Iris germanica
- Have thickend contractile roots which help pull them to an appropriate level in the soil
What are corms?
- Organ of perennation
- Compressed underground shoots in which stem is swollen with strach
- Have dry scale leaves on their outer surface
- Each year a new corm forms on top of an old one
- Have thickend contractile roots which help pull them to an appropriate level in the soil
- e.g. Crocus
What are stem tubers?
- Organ of perennation
- Formed at the end of rhizomes
- Distinguised from root tubers by having vestigial nodes and axillary buds which grow into shoots
- Can turn green exposed to sunlight (roots cannot)
- e,g, Solanum tuberosum
What are stolon?
- A lateral stem that grows horizontally over the surface of the soil
- Will produce adventitious roots at some of its nodes
- It produces adventitious buds and subsquently new plants along its length
- e,g, Fragaria
- The stolon feeds the new plant until it becomes self-sufficient, then dies back
Difference between stolon and a runner?
Runner only puts down roots at its tip
What is a sucker?
- Upright shoots arising from the root (Not a stolon)
How stems use hairs?
- Help them climb to spread seeds widely
- e.g. Galium aparine
Examples of where buds help identification (3)
- *Fraxinus excelsior *has black buds
- Fagus sylvatica has long and pointed buds
- Magnolia buds are hairy
