The Leaf

The leaf is a lateral outgrowth of the stem, arising at a node, generally flattened and green, and specialised chiefly for photosynthesis and transpiration. A typical leaf has three parts: a leaf base that attaches it to the stem (in some plants, such as members of family Leguminosae, the leaf base is swollen into a pulvinus); a slender petiole, or leaf stalk, that holds the blade out towards the light; and the lamina, or leaf blade itself, the expanded green part where photosynthesis chiefly occurs, with a prominent midrib and branching veins. Every leaf, whatever its shape, bears a single axillary bud in the angle between the petiole and the stem.

Venation. The arrangement of veins and veinlets within the lamina is called venation, and it follows one of two broad patterns. In reticulate venation, veins branch repeatedly and interconnect to form an irregular net-like pattern across the whole lamina -- this is the typical pattern of dicotyledonous leaves, seen clearly in the leaves of mango and China rose. In parallel venation, veins run alongside one another from the base of the leaf to its tip (or radiate out from a central point) without forming a connecting net -- this is the typical pattern of monocotyledonous leaves, seen in grasses, banana and canna.

Simple and compound leaves. A simple leaf has a single, undivided lamina; even where its margin is deeply cut or lobed, the incisions never extend all the way down to the midrib. A compound leaf, by contrast, has its lamina divided completely, right down to the midrib or to the petiole itself, into a number of separate leaflets. The decisive test for telling a compound leaf's leaflet apart from a genuinely separate small simple leaf is the axillary bud: only the whole compound leaf bears an axillary bud at its base, where the petiole meets the stem -- individual leaflets never have their own axillary buds. Compound leaves are further divided into two types by how their leaflets are arranged: in a pinnately compound leaf, as in neem, the leaflets are arranged along both sides of a common central axis called the rachis (itself equivalent to the midrib of a simple leaf), like the structure of a feather; in a palmately compound leaf, as in silk cotton, all the leaflets radiate outward from a single common point at the tip of the petiole, like the fingers of an outstretched hand. …