Leaf Morphology and Anatomy
Angiosperm leaves display much morphological and anatomical diversity. Mature leaves of monocots are typically narrow and consist of a linear lamina with parallel venation and a leaf base that ensheathes the stem. This contrasts with the typical leaf of eudicots and magnoliids, which has a well-defined petiole and elliptical blade (lamina) with reticulate venation. However, exceptions and transitional forms are common; for example, leaves of some monocots (e.g. Dioscorea and Smilax) are petiolate and net-veined, and leaves of some eudicots (e.g. some Apiaceae) are linear. Some species possess compound leaves in which individual leaflets are borne either on a central stem-like axis (pinnate leaves; e.g. tomato, Solanum lycopersicum) or radiate from a single point at the distal end of the petiole (palmate leaves; e.g. Arisaema).
Some species that grow in dry (xeric) or seasonally dry habitats, or otherwise nutrient-deficient habitats, possess specialized xeromorphic features, including sunken stomata to minimize water loss and well-developed sclerenchyma to provide mechanical support and minimize tissue collapse. Other xeromorphic features include the presence of a hypodermis or thick epidermis and thick cuticle which diminish the intensity of light that reaches photosynthetic tissue. Well-developed palisade tissue is also sometimes correlated with high light intensity. Some xeromorphic species possess thick, sometimes even succulent, leaves; others have terete (centric or cylindrical) leaves, or hairy leaves, or even folded (plicate) or rolled leaves (Fig. 4.1).