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Leaf structure and function
The leaf lamina is a sandwich of photosynthetic parenchyma between two layers of epidermis. Stomata are often confined to the lower epidermis, and the parenchyma is differentiated into palisade cells and spongey mesophyll. The extra thickness of sun leaves is made up of extra layers of palisade cells by comparison with shade leaves of the same plant.

The mesophyll is honeycombed with air space, allowing access of CO2 to the individual cells. Vascular bundles also run through it, in close parallel lines in many monocots or branching extensively in dicots. Individual mesophyll cells are never far from a xylem vessel for water supply or a sieve tube for export of sugar.

Leaves have a number of anatomical and physiological adaptations to conserve water. For example pine needles have a much reduced surface area by comparison with lilac, and a thicker cuticle. The stomata in pine are sunk below the leaf surface; this creates a pocket of high humidity with little air circulation. A similar effect is achieved in plants with densely hairy leaves.

Transverse section of Lilac (Syringa) leaf

T.S. of pine needle

Leaves, like roots and stems can be modified for special functions.

In the pea family (Fabaceae) leaflets are often modified as tendrils to provide support for these trailing plants. Plant breeders took advantage of this tendency to produce the "leafless pea" in which all of the leaflets are tendrils and the stipules are enlarged to provide much of the photosynthetic capacity of the plant

In a number of plant families leaves have become digestive traps for unwary insects. In a sense they have taken on the nutrient absorbing functions of roots. This is the pitcher plant, Nepenthes

See Photosynthesis and transpiration page for QUIZ

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Copyright © Michael Knee,
The Ohio State University
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