A leaf has a leaf base, a petiole and a lamina, with veins in a network or side by side. Leaves are simple or compound, and are arranged on the stem as alternate, opposite or whorled. Some leaves change for climbing, defence, storage or trapping insects.
about 25 min1 simulation1 practice question1 Python program5 questions to check yourself
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What a leaf is
A leaf is a flat, green structure that grows from a node of the stem. It has a bud in its axil. Leaves make the food of the plant by photosynthesis. A typical leaf has three parts.
The leaf base joins the leaf to the stem. In many dicots it carries two small leaf-like parts, the stipules. In monocots the leaf base grows into a sheath that covers the stem partly or wholly. In some legumes, such as gram, the leaf base is swollen. This swelling is the pulvinus.
The petiole is the stalk. It holds the blade out to the light. A long, thin and flexible petiole lets the blade flutter in the wind, which cools the leaf and brings fresh air to it.
The lamina, or leaf blade, is the flat green part. The middle vein is the midrib. The veins give the blade strength, and they carry water, minerals and food.
A leaf attached to a stem. The flat green lamina has a central midrib and side veins. A stalk, the petiole, joins the lamina to the leaf base, which is attached to the stem and has two small leaf-like stipules. An axillary bud lies in the angle between the leaf base and the stem.
Venation
The pattern of veins in the blade is its venation.
In reticulate venation the veinlets form a network. Dicots such as china rose and peepal have it.
In parallel venation the veins run side by side. Monocots such as wheat and maize have it.
Two leaves side by side. Left: reticulate venation, in which a midrib gives side veins joined by a fine network of veinlets, as in a dicot such as china rose or peepal. Right: parallel venation, in which many veins run side by side from the base to the tip of a long narrow blade, as in a monocot such as wheat or maize.
Simple and compound leaves
A simple leaf has one blade. The blade may be cut, but the cuts do not reach the midrib.
A compound leaf has cuts that reach the midrib. They break the blade into separate parts, the leaflets.
A compound leaf is pinnately compound when the leaflets sit on a common axis, the rachis. The rachis is the midrib of the leaf. Neem has such leaves. It is palmately compound when the leaflets join at one point, the tip of the petiole. Silk cotton has such leaves.
Three leaves. A simple leaf has one blade, and any cuts in its edge do not reach the midrib. A pinnately compound leaf has several leaflets in pairs along a common axis, the rachis, with one leaflet at the tip, as in neem. A palmately compound leaf has leaflets that spread from one point at the tip of the petiole, as in silk cotton.
A bud lies in the axil of the petiole of a leaf, simple or compound. There is no bud in the axil of a leaflet. This is how you tell a leaflet from a leaf.
Phyllotaxy
Phyllotaxy is the way leaves are arranged on the stem.
Alternate: one leaf grows at each node, on opposite sides in turn. China rose, mustard and sunflower are like this.
Opposite: a pair of leaves grows at each node, facing each other. Calotropis and guava are like this.
Whorled: more than two leaves grow at each node and form a ring round the stem. Alstonia is like this.
Three stems with leaves. Alternate: one leaf at each node, with the leaves on opposite sides in turn, as in mustard. Opposite: a pair of leaves at each node, facing each other, as in guava. Whorled: more than two leaves at each node, spread round the stem, as in Alstonia.
Choose one leaf, then two, then more. The node gets the name of the arrangement.
Try it in Python
The program is a small key. Three questions about a leaf give its kind.
Practise with new numbers
Leaves with special jobs
Climbing. In pea, the leaves change into tendrils that help the plant climb.
Defence. In cacti, the leaves change into spines.
Storage. The leaves of onion and garlic are fleshy, because they store food.
Making food with the petiole. In Australian acacia the leaves are small and live a short time. The petioles expand, turn green and make the food. They are called phyllodes.
Trapping insects. In the pitcher plant and the Venus flytrap, the leaves change into a pitcher or a trap that catches insects.
Common mistakes
Counting each leaflet as a leaf. A leaflet has no bud in its axil. The leaf has one.
Thinking a cut blade must be compound. The cuts of a simple leaf do not reach the midrib.
Mixing up pinnate and palmate. Pinnate leaflets sit along a rachis. Palmate leaflets join at one point.
Giving monocots reticulate venation. Monocot leaves have parallel venation.
Remember these
Leaf parts: leaf base (with stipules, a sheath or a pulvinus), petiole, lamina with midrib and veins.
Venation: reticulate in dicots, parallel in monocots.
Simple leaf: cuts do not reach the midrib. Compound leaf: cuts reach the midrib. Pinnate: leaflets on a rachis (neem). Palmate: leaflets at one point (silk cotton).