A leaf is an organ of a vascular plant and is the principal lateral appendage of the stem. The leaves and stem together form the shoot. Foliage is a mass noun that refers to leaves collectively.
Typically a leaf is a thin, dorsiventrally flattened organ, borne above ground and specialized for photosynthesis. Most leaves have distinctive upper (adaxial) and lower (abaxial) surfaces that differ in colour, hairiness, the number of stomata (pores that intake and output gases) and other features.
The primary site of photosynthesis in most leaves (palisade mesophyll) almost always occurs on the upper side of the blade or lamina of the leaf but in some species, including the mature foliage of Eucalyptus palisade occurs on both sides and the leaves are said to be isobilateral.
Many types of leaves are adapted in ways almost unrecognisable in those terms: some are not flat (for example many succulent leaves and the needles of conifers), some are not above ground (such as bulb scales), and some are without major photosynthetic function (consider for example cataphylls, and spines).
Furthermore, several kinds of leaf-like structures found in vascular plants are not totally homologous with them. Examples include phyllodes, cladodes, and phylloclades that differ from leaves in their structure and origin. Conversely, many structures of non-vascular plants, and even of some lichens, which are not plants at all (in the sense of being members of the kingdom Plantae), do look and function much like leaves.
Login
653 Views Share:
