R2101 2.5 Describe the structure and state the functions of leaves Flashcards
What is the main function of the leaf?
To carry out photosynthesis
What is the lamina?
- This is the leaf blade.
- Broad and flat to collect sunlight
(A leaf consists of the leaf blade and stalk [pertiole])
What is the petiole?
- This is the leaf stalk
- Turns to follow the sun and hold the leaf in best position to catch maximum light
- Sessile leaves lack a petiole
- Peltate leaves attach to the centre of the lamina not the base
What is a Sessile leaf
These leaves lack a petiole
What is a peltate leaf
Petiole attaches to the centre of the lamina e.g Nasturtium
What is a petiolate leaf
A leaf with a stalk or petiole
What is the midrib?
Central main vein carrying the vascular bundle from which others branch (dicots)
What are stipules?
Leafy structures at base of, or attached to, the petiole e.g. Rosa spp
What is a simple leaf?
One leaf blade only. A continuous leaf blade with an axillary bud at the base of the petiole
What is a compound leaf?
- A leaf with several leaflets either pinnate or palmate
- The only axillary bud is at the base of the main leaf stalk where is attaches to the stem
Leaf vein arrangment monocots v dicot
- Parallel veins are a feature of monocots
- Dicots have a wide variety of veinations
Leaf arrangement examples for identification
- Salix alba has leaves alternately along the stem
- Cornus alba leaves are opposite each other
- Lilium leaves are attached in a whorl around the stem
Basic division of leaves
- Simple or compound
- Margin
- Base
- Tip
Margins of leaves
Can be desribed in many ways:
1. Unbroken or smooth
2. Serrate
3. Undulate
4. Lobed
Main types of leaves (4)
(not margins of leaves or shapes of leaves)
- Deciduous leaves - fall off in autumn and replaced in spring
- Evergreen leaves - fall and are replaced a few at at time
- Foliage leaves - green leaves that surround flowers known as bracts
- Scale leaves - brown papery leaves that occur on rhizomes and buds for protection
What is phyllotaxy?
Making sure one leaf does not sit in the shadow of another
4 types of phyllotaxy
- Alternate - one leaf at each node
- Opposite - two leaves per node
- Whorled - three or more leaves per node
- Decussate - opposite pairs, each at right angles to the pair below
How to tell difference between simple and compound leaves
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
Leaf structure: conifers (2)
- Needle like leaves (e.g. Pines)
- Flat and overlapping scale leaves (e.g. Thuja)
Leaf structure: Dicotyledons (4)
- A broad blade or blades
- A leaf stalk
- Usually a stipule at base
- Net veins (reticulate)
Leaf structure: Monocotyledons (4)
- Relatively narrow leaf blade
- Usually a sheath surrounding the stem (clum)
- Always parallel veined
- Never have stipules
Leaf colour (5)
- Do not need to be green to photosynthesise
- Pale variegations often indicate chlorophyll is reduced or absent in some parts of the leaf
- Leaves in shade are often a deeper green than those in sun - a concentration of chlorophyll
- Most purple leaves need sun to keep full colour e.g. Cotinus
- Young leaves have brighter colours (gold, red, pink) which tend to fade to a greener tone as they mature e.g. Acer negundo ‘Flamingo’
Why Have Leaf modifications for survival (4)
- As a storage organ to help plant over-winter
- To help lift plant above competition for light
- To withstand harsh and difficult environments
- To defend the plants against damage and browsing animal
Leaf shape adapations (2)
(holes and divisions)
- Divided leaves offer less resistance to wind and rain
- Swiss cheese plant Monstera deliciosa has huge holes in leaves to help water run off and ensure it reaches the roots
Hair adaptations (5)
- Epidermal hairs - reduce water loss by increasing the layer of still air across the leaf surface e.g. Stachys byzantina
- Glandular hairs - understand of leaves that secrete strong aromatic oils to deter insects and animals e.g. Lavandula sp.
- Insect barriers - Some leaves have hairs whcih are a barrier to insects - they are on the underside of leaves e.g. Tilia euchlora
- Poison and irritants - hairs that help sting and defend e.g. stinging nettle Urtica species
- Insect traps e.g. venus fly trap Dionaea muscipula
Leaf Climbing adaptations (3)
- Leaf petiole curling around the stems of other plants or structures to support the climber e.g. Clematis ssp.
- Leaf tendrils e.g. pea Pisum sativum
- Leaf apex tendrils - the tip of the leaf is modified into a tendril e.g. Gloriosa nepenthes
Protection adaptations (3)
- *Berberis *species have leaf spines
- In many desert species leaves become spines which also help prevent water loss and photosynthesis is taken over by the stem.
- Some leaves have highly serrated spiny margins for additional protection from grazing e.g. Ilex aquifolium
Leaf Water storage adaptations (3)
- Thickened leaves act as water storage organs, in succulents e.g.* Sedium acre
- Rosettes of leaves can help conserve moisture
- Cacti such as Opuntia tuna have leaves modified as spines
Functions of Bracts (3)
- Modifed leaves acting as false flowers to attract pollinating insects
- Can be coloured
- e.g. *Hydrangea *
Scale leaves
- Protect the bulbs from extremes of climate or pests such as eelworms or mice
What is a bulb?
- Specialised food store evolved from the leaves
- 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
- The scale leaves are densely packed together around the terminal bud
- Bulbs have both adventitious and contractile root
- e.g. Tulipa species
What is a phyllode
- Modified petioles or leaf stems which replace and carry out the functions of leaves
- Can become flattened and widened, while the leaf itself becomes reduced or vanishes altogether
- e.g. Opuntia
Role of cuticle (2)
- Protective waxy coating that stops leaf drying out
- Keeps out some pathogens
Role of epidermis (3)
- A single layer of cells covering the whole surface
- A thin transparent layer permitting the transmission of light to lower leaf tissues
- Has no chloroplasts
Role of stomata [sing. stoma] (2)
- Openings which allow exit and entry of gases and water vapour
- Open and closed by the water pressure in guard cells which surround them
Mechanism of the stoma (2)
- When the guard cells fill with fluid (become turgid) the stomata open allowing moisture to escape
- The stomata close as the guard cells lose fluid (become flaccid) conserving mositure inside the leaf
Role of palisade mesophyll (3)
- Cell which contain the most chloroplasts
- Pointing downwards beneath the epidermis
- Chloroplasts can move around in the cells to maxmise their exposure to sunlight
Role of spongy mesophyll (3)
- Loose packed cells with lots of inter-cellular spaces to allow free movement of gases and water vapour (two-way diffusion)
- Stomata open into the spaces
- Below the palisade mesophyll
