Adaption of Leaves.

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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 in 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.

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. Behind each stoma is an air space 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.

Water Water is obtained from the soil. It is absorbed from the root hairs, and carried up to the leaf in the xylem vessels to the mesophyll cells by osmosis.

Sunlight The position of a leaf and its broad, flat surface help it to obtain as much sunlight as possible.

The cells that need the sunlight are the mesophyll cells. The thinness of the leaf allows the sunlight to penetrate right through it, and reach all the cells. To help this the epidermal cells are transparent, with no chloroplasts.

In the mesophyll cells, the chloroplasts are arranged to get as much sunlight as possible, particularly those in the palisade cells. The chloroplasts can lie broadside on to do this, but in strong sunlight, they often arrange themselves end on. This reduces the amount of light absorbed. Inside them, the chlorophyll is arranged on flat membranes to expose as much as possible to the sunlight.

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