Leaves have a large surface area and are specially adapted for carrying out photosynthesis in most plants. The flat leaf blade optimizes the available sunlight in the immediate area.

Leaf characteristics vary greatly from plant to plant and are useful in classification and identification. Leaves are simple or compound. See glossary page 5.

Simple leaves have a single blade. See glossary page 5.

Compound leaves have two or more leaflets or pinnae. See glossary page 5.

Leaflet – secondary leaf of a compound leaf. See glossary page 5.

Apex – the terminal point of the leaf. See glossary page 8.

Base –the point of the leaf that joins onto the stem or petiole. See glossary page 8.

Blade – The surface area of a leaf. See glossary page 5.

Margin – edge of a leaf. See glossary page 7.

Midrib/Main vein – the most prominent central vein in a leaf. See glossary page 5.

Lateral veins – secondary veins in a leaf. See glossary page 5.

Rachis – an extension of the petiole bearing leaflets. See glossary page 5.

Stipules  – leaf-like appendages at the base of the petiole of some leaves. They may protect the young leaf and may be modified into spines  or tendrils . See glossary page 5.

Leaf Positions on the stems

Opposite – two leaves (a pair) attached per node, usually opposite each other. See glossary page 9.

Alternate – one leaf attached per node, usually staggered (spiral) along stem. See glossary page 9.

Whorled – three or more leaves attached per node, usually equally spaced around the node. See glossary page 9.
Leaf attachment

petiolate – the blade is attached to the stem by a petiole. See glossary page 5.

Petiolule – the leaflet stalk. See glossary page 5.

Petiole – the leaf stalk which connects the blade to stem or to the rachis. See glossary page 5.

Sessile – the blade is attached directly to the stem without a petiole.

Leaf venation – the arrangement of the lateral and tertiary veins in relation to the mid vein. See glossary page 5.

Mid vein – The main vein or vascular bundles that extend from the petiole or stem to the apex of the leaf. See glossary page 8.

Pinnate venation – feather-like, net venation with lateral veins extending from a central midrib. See glossary page 8.

Palmate venation – finger-like, net venation with several major veins diverging from the union of the petiole and the leaf blade. See glossary page 8.

Parallel venation – principal veins parallel to the axis of the leaf. See glossary page 8.

Penniveined – Run at a forward angle from the mid vein almost to the margins. See glossary page 8.

Leaf tissues:

Cross Section of Angiosperm Leaf

Cross Section of Gymnosperm Leaf

Leaf Tissue Functions

Epidermis – the external layer of cells covering the leaf surface is one cell thick. The cells have 3 main functions including the prevention of water loss, regulate gasses into and out of the leaf and allow sunlight to penetrate the outer layer.

Stomata (stoma singular) – is an aperture (the stomatal pore) in the epidermis and are surrounded by two elongated guard cells. The pore is bordered by two specialized parenchyma  cells known as guard cells . These two cells are responsible for regulating the size of the opening. The function of the stomatis the exchange of gasses – the intake of carbon dioxide enters the plant through these openings and is used by the chloroplasts in the mesophyll cells during respiration . As Oxygen is produced as a by product of photosynthesis it diffuses out to the atmosphere through these same openings. Water vapour is released into the atmosphere through these pores during transpiration .

Mesophyll – These cells make up the internal leaf tissue and include the palisade parenchyma and spongy parenchyma cells.

Palisade parenchyma – Are the upper one or two or at times more layers of cylindrical cells below the epidermis. They contain 70 to 80% of the chloroplasts found in the leaves. The palisade cells contain the largest number of chloroplasts  per cell, making them the primary site for accumulating the sun’s energy to carry out photosynthesis and converting the light energy to the chemical carbohydrate energy.

Spongy mesophyll – Are the lower layers of irregularly shaped cells. These cells also perform photosynthesis. It is the much larger surface areas with air spaces and ducts between that allow for the better exchange of gasses as most of the stomata occur in the lower epidermis of the leaf surface.

Vascular bundles – Are the visible network of veins in each leaf. They connect through to the petiole and the vascular tissue of the stem. The vascular bundles are made up of the microscopic cells conducting mineralized water into the leaf through the xylem and removing photosynthetic products out of the leaf through the phloem cells.

 
The Leaf Canopy

As a plant grows it increases its total leaf area available to harness the sun’s energy and to capitalize on photosynthesis, provided its leaves are exposed to the sun.

The arrangement of foliage in plants is known as leaf mosaic. This pattern in plants places the maximum number of leaves in a way that each leaf is exposed to the amount of sun that most benefits the plant.

Photosynthesis in leaves exposed to full sun is usually at its maximum output. Light saturation means these leaves would waste a lot of the energy they absorb. To increase the overall efficiency of photosynthesis, a plant often angles its upper leaves to the sun, cutting down the amount of light falling on them and allowing more light to penetrate on the lower leaves. Eucalyptus trees especially those in arid areas or exposed to coast glare usually have their leaves hanging vertically down to capture morning or afternoon sunlight with minimal exposure to hot, drying heat during the worst part of the day

  
Shade Plants vs Sun Plants

Those plants that have adapted for living in full sunlight are known as sun plants. Species adapted to low-light habitats are better known as shade plants. The leaves of those plants that grow in shade are usually larger, softer and much thinner than plants that grow in the sun. Shade loving plants have less cell material to maintain and contain fewer mitochondria per square millimetre, they respire more slowly than plants growing in the sun and they require less photosynthesis to equal the leaf’s respiration rate.

As a result leaves are more efficient in harvesting sunlight at lower light levels having  a different type of chloroplasts that utilize a greater spectrum of the sun’s light energy usually in the higher wave lengths. As a result the leaves are often deeper in colour tending to be almost black in some leaves of tropical shade plants and sub polar plants. Those leaves exposed to stronger sunlight and thus stronger light saturations point have a maximum rate of photosynthesis. To sum it up without enough sunlight, a plant cannot photosynthesize glucose quickly, even if there is an optimum supply of water and Carbon Dioxide for the specific plant requirements. Increasing the light intensity will boost the speed of photosynthesis and the health of the plant up to a certain light intensity.

Leaf Drop – Abscission

In Autumn we notice the profusion of leaves falling to the ground but if we look around we will see leaves falling to the ground throughout the year. Leaves are discarded for a reason whether it be the weather, drought, heat flood or the removal of toxins plants have a reason for discarding old leaves. If leaves fell without adequate preparation there would be an open wound at the site where each leaf petiole broke off an opening for disease and insects to enter. At the end of their life, the plant extracts all the nutrients and the leaves have begun to dry up, the plant actively severs the leaves by destroying the cell walls across a section of the leaf stalk. This area is called the abscission zone. The growth hormone auxin plays a role in the process.

Once this layer of cells is destroyed, a corky later begins to form between the base of the leaf stalk and the branch, protecting living tissues. When this layer is complete dry, the only thing attaching the leaf petiole to the limb is the vascular tissue and these brittle cells break very easily. This leaves the shield-shaped leaf scars on the twig where leaves have dropped. The dots inside the scar are the remnants of the vascular bundles. 

The various colours are created because of the removal or extraction of nutrients and moisture during the break down of the chloroplasts, the green colour. They are then re transported back along the phloem that allows the carotenoid pigments remaining to display their red, orange or red colouring. This is more evident in exotic plants growing on better quality soils than native plants that are growing on poorer soils where the leaves are retained for longer periods. Here all the nutrients even the carotenoids are removed prior to leaf excision.

Many plants shed their leaves when the respiration rate exceeds the rate of photosynthesis which often occurs closer to the Polar Regions and at higher altitudes during winter. Broad leaf plants may even shed their leaves as a result of cold weather thus preventing broken limbs due to the excess weight of snow and ice or as an alternative to producing anti freeze. Another reason closer to home is when evaporation rate exceeds the uptake of ground moisture at the end of the dry season.

This leaves the question of evergreen trees as well as gymnosperms which hold their leaves for longer periods. The reasons maybe complex or simple and include such things as when the respiration rate exceeds the rate of photosynthesis in shaded areas of the plant the leaves become superfluous and are discarded.  

Recently it has been mentioned that it is the plants excretory mechanism. We know that all living things must undergo excretion of toxic build ups. It is one of the essential properties of all life but why use the leaves? The leaves in tropical plants have been known to contain higher levels of heavy metals and other toxins which the plants do not use and that carotenoids are a valuable antioxidant in cleansing the body. Higher order plants also have flowers which contain a high proportion of carotenoid, pigments in the petals, sepals and stamens often with no or little in the way of chloroplasts. These are usually discarded immediately after or shortly after anthesis so wouldn’t this function be far less extravagant and far more efficient than that of the leaves in times of high photosynthesis and flowering? Leaf abscission under these circumstances is far less efficient for the plant as an excretory organ than the modified sexual organs which are discarded following their useful period through which metabolic wastes pertaining to the metabolic processes of sexual reproduction in angiosperms can be utilized?

Remember

1. Simple leaves have a single blade while compound leaves have 2 or more blades. 
 
2. The petiole connects the leaf blade and vascular system to the stems vascular system. 
 
3. The secondary leaf of a compound leaf is known as a leaflet while on a fern they are known as pinnae
 
4. Plants have 3 leaf arrangements off the stem, opposite, alternate or whorls where there are three or more leaves attached at a    node. 
 
5. A sessile leaf is attached directly to the stem without a petiole. 
 
6. Feather like veins extending from a central midrib is known as Pinnate venation.
 
7. Palisade parenchyma cells are specialized cells for carrying out photosynthesis and contain 70% to 80% of the leaf chloroplasts. 
 
8. The Vascular Bundles make up the visible veins of a leaf. 
 
9. The leaf mosaic reduces the amount of shading of leaves below the outer canopy. 
 
10. Plant species that have adapted to low light habitats are known as shade plants


11. The abscission zone is the area where cell walls are destroyed to facilitate leaf drop. 
 
12. The edge of a leaf is called its margin
 
13. The leaflets of a compound leaf are attached to the rachis by a petiolule
 
14. A heart shaped leaf has a reniform or cordate shape. 
 
15. The margin of a Grevillea sinuata leaf is known as a cleft leaf
 
16. Stomata are open when guard cells are turgid due to water uptake and potassium influx and are closed when the stomata are flaccid
 
17. The spongy mesophyll is the inner leaf zone where the cells have large surface areas and large air spaces between.
 
18. Mutual shading is usually unavoidable when total leaf area exceeds the area of land to which the plant is growing. 
 
19. A leaf is a liability to a plant when its photosynthesis rate falls below its respiration rate. 
 
20. Stipules are leaf like appendages at the base of the petiole of some leaves.

Further Comments from Members:

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