A flattened structure that develops from a superficial group of tissues, the leaf buttress, on the side of the stem apex. Each leaf has a lateral bud in its axil. Leaves are arranged in a definite pattern (see phyllotaxis) and usually show limited growth. Each consists of a broad flat lamina (leaf blade) and a leaf base, which attaches the leaf to the stem; a leaf stalk (petiole) may also be present. The leaves of bryophytes are simple appendages, which are not homologous with the leaves of vascular plants as they develop on the gametophyte generation.
Leaves show considerable variation in size, shape, arrangement of veins, type of attachment to the stem, and texture. They may be simple or divided into leaflets, i.e. compound (see illustration). Types of leaf include: cotyledons (seed leaves); scale leaves, which lack chlorophyll and develop on rhizomes or protect the inner leaves of a bud; foliage leaves, which are the main organs for photosynthesis and transpiration; and bracts and floral leaves, such as sepals, petals, stamens, and carpels, which are specialized for reproduction.
Leaves may be modified for special purposes. For example the leaf bases of bulbs are swollen with food to survive the winter. In some plants leaves are reduced to spines for protection and their photosynthetic function is carried out by another organ, such as a cladode.
Transverse section through a leaf blade
Simple leaves Compound leaves
Subjects: Biological Sciences.