The slender petiole or leaf stalk is present in the leaves of most flowering plants.
It is a continuation of the stem to the rest of the leaf.
Leaves without a petiole are called sessile.
The blade is the flattened, expanded portion of the leaf and is usually green.
Some leaf blades are needle like as in pines or scale like as in cypresses.
Some leaves have small, leaf like stipules as outgrowths at the base of the petiole.
Running through the center of the blade is the midrib which is the continuation of the petiole.
The midrib is the main structure which acts as the backbone of the leaf and as a passage tube for liquids between the petiole and veins.
The veins act as the framework of the leaf and as passage tubes for liquids between the midrib and the blade.
The veins also distinguish a monocot leaf from a dicot leaf.
Leaves may be simple, compound, or bicompound.
Simple leaves have a single expanded portion.
In compound leaves, the leaf blade may be subdivided into several separate expanded parts, or leaflets.
Each leaflet may consist of an extended portion and a short stalk attached to the rachis, which is a continuation of the petiole.
Leaves may be pinnately compound or bipinnately compound.
Leaf venation, the arrangement of veins in a leaf, may be parallel or netted.
Parallel venation is a characteristic of most monocotyledonous plants, such as corn, onion and common grasses.
In such plants, numerous veins of approximately equal size extend side by side from the base to the tip of the blade and are interconnected by small and inconspicuous veins.
Net venation is found in dicotyledonous plants such as mangoes.
In these plants one or more veins are prominent and the smaller veins form a conspicuous network.
If a leaf has one main vein from which the others branch off, it is termed pinnately net veined.