B4: Plant nutrition
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. epidermal cell cell wall
nucleus
cytoplasm
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 mesophyll. The cells beneath them are rounder, and arranged quite loosely, with large air spaces between them. They form the spongy mesophyll (Figure B4.02). Running through the mesophyll are veins or vascular bundles. Each vein contains large, thick-walled xylem vessels (Figure B6.01) for carrying water. There are also smaller, thin-walled phloem tubes (Figure B6.03) for carrying away sucrose and other substances that the leaf has made.
questions
B4.04 Which kind of cell makes the cuticle on a leaf? B4.05 What is the function of the cuticle? B4.06 What are stomata? B4.07 What are guard cells? stoma
thick cytoplasm cell wall nucleus chloroplast guard cell
Figure B4.03 Surface view of the lower epidermis of a leaf.
B4.08 List three kinds of cell in a leaf which contain chloroplasts, and one kind which does not. 39
Leaf adaptations Leaves are adapted to obtain carbon dioxide, water and sunlight.
Carbon dioxide Carbon dioxide is obtained from the air. There is not very much available, because only about 0.04% of the air is carbon dioxide. Therefore, the leaf must be very efficient at absorbing it. The leaf is held out into the air by the stem and the leaf stalk, and its large surface area helps to expose it to as much air as possible (Figure B4.04). The cells which need the carbon dioxide are the mesophyll cells, inside the leaf. The carbon dioxide can get into the leaf through the stomata. It does this by diffusion, which is described in Chapter B2. Behind each stoma is an air space (Figure B4.02) which connects up with other air spaces between the spongy mesophyll cells. The carbon dioxide can therefore diffuse to all the cells in the leaf. It can then diffuse through the cell wall and cell membrane of each cell, and into the chloroplasts. Image B4.03 The lower surface of a leaf, showing the Water closely fitting cells of the epidermis. The oval openings are Water is obtained from the soil. It is absorbed by the root stomata, and the two curved cells around each stoma are hairs, and carried up to the leaf in the xylem vessels. guard cells (× 450). Original material © Cambridge University Press 2017