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Types of leaves
There are many kinds of plant leaves. The most common and conspicuous leaves are referred to as foliage and are the primary location of photosynthesis. However, there are many other types of modified leaves:
- Scale leaves (cataphylls) are found on rhizomes and buds, which they enclose and protect.
- Seed leaves (cotyledons) are found on embryonic plants. They store food for the developing seedling.
- Spines and tendrils, such as those found on barberry and pea plants, protect a plant or help support its stems.
- Storage leaves, such as those on bulbous plants and succulents, store food.
- Bracts often are brightly colored. For example, the showy structures on dogwoods and poinsettias are bracts, not petals.
Venation
The vascular bundles of xylem and phloem extend from the stem, through the petiole, and into the leaf blade as veins. The term venation refers to how veins are distributed in the blade. There are two principal types of venation: parallel-veined and net-veined (Figure 13).
In parallel-veined leaves, numerous veins run essentially parallel to each other and are connected laterally by minute, straight veinlets. Parallel-veined leaves occur most often on monocotyledonous plants. The most common type of parallel veining is found in plants of the grass family, whose veins run from the leaf's base to its apex. Another type of parallel venation is found in plants such as banana, calla, and pickerelweed, whose veins run laterally from the midrib.
In net-veined leaves (also called reticulate-veined), veins branch from the main rib or ribs and subdivide into finer veinlets. These veinlets then unite in a complicated network. This system of enmeshed veins makes the leaf more resistant to tearing than does a parallel vein structure. Net-veined leaves occur on dicotyledonous plants.
Net venation may be either pinnate or palmate. In pinnate venation, the veins extend laterally from the midrib to the edge (e.g., apples, cherries, and peaches). In palmate venation, the principal veins extend outward, like the ribs of a fan, from the base of the leaf blade (e.g., grapes and maples).
