Leaf parts and venation

A leaf is a flattened outgrowth on the side of the stem. It forms at a node and has a bud in its axil, and that axillary bud later grows into a branch.

Leaves come from the shoot apical meristem and appear in acropetal order. They are the chief vegetative organs of photosynthesis.

A typical leaf has three parts: leaf base, petiole and lamina. The leaf base joins the leaf to the stem and may carry two small leaf-like stipules.

In monocots the leaf base broadens into a sheath that wraps the stem partly or fully. In some legumes the leaf base is swollen; this is the pulvinus.

The petiole holds the blade up to the light. A long, thin, flexible petiole lets the blade flutter in the wind, which cools the leaf and brings fresh air to its surface.

The lamina is the green, expanded blade with veins and veinlets; the prominent middle vein is the midrib. Veins keep the blade rigid and carry water, minerals and food.

Leaves differ in the shape, margin, apex, surface and depth of incision of the lamina.

Venation is how veins and veinlets are laid out in the lamina. Reticulate: veinlets form a network, usual in dicots. Parallel: veins run side by side, typical of most monocots.