Leaves demonstrate the greatest morphological variability of any plant organ. They are also known for their plasticity, that is, difference in leaf form within a single species. Except in the earliest vascular plants, modern Equisetum, Psilotum, and Ephedra, and a few specialized angiosperms such as stem-succulent cacti, which have highly reduced leaves or are leafless, leaves function as the primary photosynthetic units in plants. Cotyledons, or seed leaves, represent the first leaves produced during embryonic development and they function as storage organs, providing food for the developing plant until the first true leaves appear and begin to photosynthesize. In many plants that live in arid environments, leaves may be fleshy and function in water storage. Modified leaves, which may be non-photosynthetic, are important as protective structures, for example, bud scales, and as parts of reproductive structures, such as flower petals and sepals or floral bracts, where they may be brightly colored and serve as signals for pollinators. Although leaf form is highly variable, most consist of a flattened blade borne on a narrow, elongate axis—the petiole. At the point of attachment to the stem (the axil of the leaf), many leaves have an abscission layer, which functions in separating the leaf from the stem and preventing water loss at the same time.
Morphologically, leaves are classified as simple or compound; compound leaves are made up of leaflets. If the leaflets are attached at a single point, the leaf is palmately compound, for example, Sagenopteris, a leaf type found in the Mesozoic Caytoniales (Chapter 15). If the leaflets are attached along the petiole, the leaf is pinnately compound. This leaf morphology is very common in ferns, where multiple levels of leaflets may occur (Chapter 11). In a pinnately compound leaf, the term petiole on stipe used below the level of leaflet attachment. The region where the leaflets are attached is called the rachis (pl. rachides). The morphology of fossil angiosperm leaves, including overall shape and features of the margin, are widely used by paleobotanists to reconstruct paleoclimates using leaf physiognomy (see Chapter 1).
Leaf venation is highly variable among vascular plants and has evolved differently in different groups of plants at various points in geologic time. The non-seed plants tend to have venation patterns in which the vascular bundles dichotomize to fill the leaf, and veins often end at the leaf margin (open venation). Seed plants have more complex venation. If they exhibit dichotomous venation, the veins may also anastomose (join together) and then dichotomize again, forming enclosed meshes. This type of venation is seen in the Permian pteridosperm, Glossopteris (Chapter 14) and several other fossil seed plants. Veins often do not end at the leaf margin but bend back to fuse with other veins (closed venation). Some fossil gymnosperms exhibit relatively complex venation with multiple orders of veins (primary, secondary, tertiary, etc.), but the flowering plants have evolved the most diverse and complex pattern of venation. They include multiple, distinct orders of venation, and the ultimate veinlets enclose small patches of the leaf, termed areoles. Doyle and Hickey (1976) were able to trace the progressive changes in venation patterns in some of the earliest angiosperm leaf fossils and demonstrated a progressive increase in the organization of veins and the number of orders of veins (see Chapter 22) in successively younger rocks. Venation patterns in fossil leaves are an important systematic character, especially when coupled with leaf morphology and epidermal anatomy.
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