R2101 2.5 Describe the structure and state the functions of leaves

Question 1

What is the main function of the leaf?

Answer 1

To carry out photosynthesis

Question 2

What is the lamina?

Answer 2

1. This is the leaf blade.
2. Broad and flat to collect sunlight

(A leaf consists of the leaf blade and stalk [pertiole])

Question 3

What is the petiole?

Answer 3

1. This is the leaf stalk
2. Turns to follow the sun and hold the leaf in best position to catch maximum light
3. Sessile leaves lack a petiole
4. Peltate leaves attach to the centre of the lamina not the base

Question 4

What is a Sessile leaf

Answer 4

These leaves lack a petiole

Question 5

What is a peltate leaf

Answer 5

Petiole attaches to the centre of the lamina e.g Nasturtium

Question 6

What is a petiolate leaf

Answer 6

A leaf with a stalk or petiole

Question 7

What is the midrib?

Answer 7

Central main vein carrying the vascular bundle from which others branch (dicots)

Question 8

What are stipules?

Answer 8

Leafy structures at base of, or attached to, the petiole e.g. Rosa spp

Question 9

What is a simple leaf?

Answer 9

One leaf blade only. A continuous leaf blade with an axillary bud at the base of the petiole

Question 10

What is a compound leaf?

Answer 10

1. A leaf with several leaflets either pinnate or palmate
2. The only axillary bud is at the base of the main leaf stalk where is attaches to the stem

Question 11

Leaf vein arrangment monocots v dicot

Answer 11

1. Parallel veins are a feature of monocots
2. Dicots have a wide variety of veinations

Question 12

Leaf arrangement examples for identification

Answer 12

1. Salix alba has leaves alternately along the stem
2. Cornus alba leaves are opposite each other
3. Lilium leaves are attached in a whorl around the stem

Question 13

Basic division of leaves

Answer 13

1. Simple or compound
2. Margin
3. Base
4. Tip

Question 14

Margins of leaves

Answer 14

Can be desribed in many ways:
1. Unbroken or smooth
2. Serrate
3. Undulate
4. Lobed

Question 15

Main types of leaves (4)
(not margins of leaves or shapes of leaves)

Answer 15

1. Deciduous leaves - fall off in autumn and replaced in spring
2. Evergreen leaves - fall and are replaced a few at at time
3. Foliage leaves - green leaves that surround flowers known as bracts
4. Scale leaves - brown papery leaves that occur on rhizomes and buds for protection

Question 16

What is phyllotaxy?

Answer 16

Making sure one leaf does not sit in the shadow of another

Question 17

4 types of phyllotaxy

Answer 17

1. Alternate - one leaf at each node
2. Opposite - two leaves per node
3. Whorled - three or more leaves per node
4. Decussate - opposite pairs, each at right angles to the pair below

Question 18

How to tell difference between simple and compound leaves

Answer 18

If there is a bud it is a leaf
If there is no bud, but there is one at the base of the stalk (rachis) these are leafets and part of a compound leaf

Question 19

Leaf structure: conifers (2)

Answer 19

1. Needle like leaves (e.g. Pines)
2. Flat and overlapping scale leaves (e.g. Thuja)

Question 20

Leaf structure: Dicotyledons (4)

Answer 20

1. A broad blade or blades
2. A leaf stalk
3. Usually a stipule at base
4. Net veins (reticulate)

Question 21

Leaf structure: Monocotyledons (4)

Answer 21

1. Relatively narrow leaf blade
2. Usually a sheath surrounding the stem (clum)
3. Always parallel veined
4. Never have stipules

Question 22

Leaf colour (5)

Answer 22

1. Do not need to be green to photosynthesise
2. Pale variegations often indicate chlorophyll is reduced or absent in some parts of the leaf
3. Leaves in shade are often a deeper green than those in sun - a concentration of chlorophyll
4. Most purple leaves need sun to keep full colour e.g. Cotinus
5. Young leaves have brighter colours (gold, red, pink) which tend to fade to a greener tone as they mature e.g. Acer negundo ‘Flamingo’

Question 23

Why Have Leaf modifications for survival (4)

Answer 23

1. As a storage organ to help plant over-winter
2. To help lift plant above competition for light
3. To withstand harsh and difficult environments
4. To defend the plants against damage and browsing animal

Question 24

Leaf shape adapations (2)
(holes and divisions)

Answer 24

1. Divided leaves offer less resistance to wind and rain
2. Swiss cheese plant Monstera deliciosa has huge holes in leaves to help water run off and ensure it reaches the roots

Question 25

Hair adaptations (5)

Answer 25

1. Epidermal hairs - reduce water loss by increasing the layer of still air across the leaf surface e.g. Stachys byzantina
2. Glandular hairs - understand of leaves that secrete strong aromatic oils to deter insects and animals e.g. Lavandula sp.
3. Insect barriers - Some leaves have hairs whcih are a barrier to insects - they are on the underside of leaves e.g. Tilia euchlora
4. Poison and irritants - hairs that help sting and defend e.g. stinging nettle Urtica species
5. Insect traps e.g. venus fly trap Dionaea muscipula

Question 26

Leaf Climbing adaptations (3)

Answer 26

1. Leaf petiole curling around the stems of other plants or structures to support the climber e.g. Clematis ssp.
2. Leaf tendrils e.g. pea Pisum sativum
3. Leaf apex tendrils - the tip of the leaf is modified into a tendril e.g. Gloriosa nepenthes

Question 27

Protection adaptations (3)

Answer 27

1. *Berberis *species have leaf spines
2. In many desert species leaves become spines which also help prevent water loss and photosynthesis is taken over by the stem.
3. Some leaves have highly serrated spiny margins for additional protection from grazing e.g. Ilex aquifolium

Question 28

Leaf Water storage adaptations (3)

Answer 28

1. Thickened leaves act as water storage organs, in succulents e.g.* Sedium acre
2. Rosettes of leaves can help conserve moisture
3. Cacti such as Opuntia tuna have leaves modified as spines

Question 29

Functions of Bracts (3)

Answer 29

1. Modifed leaves acting as false flowers to attract pollinating insects
2. Can be coloured
3. e.g. *Hydrangea *

Question 30

Scale leaves

Answer 30

1. Protect the bulbs from extremes of climate or pests such as eelworms or mice

Question 31

What is a bulb?

Answer 31

1. Specialised food store evolved from the leaves
2. Papery scale leaves enclose the succulent light coloured scale leaves which contain all the food and moisture for the bulbs emergence making this a perennating organ
3. The scale leaves are densely packed together around the terminal bud
4. Bulbs have both adventitious and contractile root
5. e.g. Tulipa species

Question 32

What is a phyllode

Answer 32

1. Modified petioles or leaf stems which replace and carry out the functions of leaves
2. Can become flattened and widened, while the leaf itself becomes reduced or vanishes altogether
3. e.g. Opuntia

Question 33

Role of cuticle (2)

Answer 33

1. Protective waxy coating that stops leaf drying out
2. Keeps out some pathogens

Question 34

Role of epidermis (3)

Answer 34

1. A single layer of cells covering the whole surface
2. A thin transparent layer permitting the transmission of light to lower leaf tissues
3. Has no chloroplasts

Question 35

Role of stomata [sing. stoma] (2)

Answer 35

1. Openings which allow exit and entry of gases and water vapour
2. Open and closed by the water pressure in guard cells which surround them

Question 36

Mechanism of the stoma (2)

Answer 36

1. When the guard cells fill with fluid (become turgid) the stomata open allowing moisture to escape
2. The stomata close as the guard cells lose fluid (become flaccid) conserving mositure inside the leaf

Question 37

Role of palisade mesophyll (3)

Answer 37

1. Cell which contain the most chloroplasts
2. Pointing downwards beneath the epidermis
3. Chloroplasts can move around in the cells to maxmise their exposure to sunlight

Question 38

Role of spongy mesophyll (3)

Answer 38

1. Loose packed cells with lots of inter-cellular spaces to allow free movement of gases and water vapour (two-way diffusion)
2. Stomata open into the spaces
3. Below the palisade mesophyll