Leaf structure A leaf consists of a broad, flat part called the lamina (Figure 6.2), which is joined to the rest of the plant by a leaf stalk or petiole. Running through the petiole are vascular bundles, which then form the veins in the leaf. These contain tubes which carry substances to and from the leaf. Although a leaf looks thin, it is in fact made up of several layers of cells. You can see these if you look at a transverse section (TS) of a leaf under a microscope (Figures 6.3, 6.4 and 6.5). midrib margin lamina vein
transverse section of leaf
cross-section of vein
The top and bottom of the leaf are covered with a layer of closely fitting cells called the epidermis (Figures 6.6 and 6.7). These cells do not contain chloroplasts. Their function is to protect the inner layers of cells in the leaf. The cells of the upper epidermis often secrete a waxy substance, that lies on top of them. It is called the cuticle, and it helps to stop water evaporating from the leaf. There is sometimes a cuticle on the underside of the leaf as well. In the lower epidermis, there are small openings called stomata (singular: stoma). Each stoma is surrounded by a pair of sausage-shaped guard cells which can open or close the hole. Guard cells, unlike other cells in the epidermis, do contain chloroplasts. The middle layers of the leaf are called the mesophyll (‘meso’ means middle, and ‘phyll’ means leaf). These cells all contain chloroplasts. The cells nearer to the top of the leaf are arranged like a fence or palisade, and they form the palisade layer. The cells beneath them are rounder, and arranged quite loosely, with large air spaces between them. They form the spongy layer (Figure 6.3).
Figure 6.2 The structure of a leaf.
cuticle
cell wall
vacuole
cytoplasm
nucleus
chloroplast
upper epidermis
palisade layer mesophyll layer xylem vein
spongy layer phloem
lower epidermis
guard cell
stoma
air space
Figure 6.3 Transverse section through a small part of a leaf.
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Cambridge IGCSE Biology
Original material © Cambridge University Press 2014