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In order to provide the cells of the leaf with sufficient CO2 and to allow for the escape of O2, the bottom side of the leaves contain countless openings, the stomata. Each stoma can be opened and closed by the expanding and contracting action of two guard cells. The stomata open during the day to allow for the free movement of gases into and out of the leaf. At night, when photosynthesis does not take place, the guard cells close the stomata to minimize the loss of water. Exceptions do exist, such as the CAM (Crassulacean acid metabolism) plants, which grow in hot and dry environments. These plants close the stomata during the day to further reduce the water loss. These plants then open the stomata at night to allow CO2 to enter the leaves. The CO2 is fixed and the product is stored in vacuoles to be used for photosynthesis during day.

Hydropassive Control - simply put, as the plant looses water, the turgidity of the leaf cells, including guard cells, decreases and this results in stomatal closure. The plant is not "intentionally" closing the stoma, it is simply the consequence of drying out.

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The stomata and associated guard cells can be made visible in several ways. Thin leaves can be placed directly on the slide for observations. The light intensity must be sufficiently high to pass through the leaf. The distinctive shape of the stomata and guard cells can then be seen, but the quality of the image is naturally not high. Carefully microtoming the leaf would be another possibility, but this can become a problem, especially when leaf observation is to be done with students or children, when safety issues start to play a role. Making leaf impressions is probably one of the least complicated and easiest methods to make the surface texture of the leaves visible.

A typical deciduous plant leaf is composed of several layers. The top most layer of a leaf is a cell-free layer of wax, the cuticle. Its function is to reduce the loss of water from the cells. The waxy cuticle is produced by the upper epidermis. This layer of cells does not contain chloroplasts in order to allow sunlight to pass through unhindered. The palisade mesophyll is a layer of packed cells beneath the upper epidermis. The cells are arranged vertically and contain many chloroplasts. Their vertical arrangement and high density makes them an efficient layer for photosynthesis. Spaces between the individual palisade cells allow for the diffusion of CO2 gas. Beneath the palisade mesophyll, one can find the spongy mesophyll. It is a loosely packed layer of cells, containing (as the name “spongy” suggests) many air spaces. This is the place where the gases CO2 and O2 are stored. Many cells are in contact with the air spaces and therefore the total surface area to loose water is also quite high. The lower epidermis is a single layer of cells on the bottom part of the leaf. It is the plant tissue which contains the guard cells and the stomata. With the exception of the guard cells, the lower epidermis is free of chloroplasts.

SBI3U Grade 11 University Biology Exam— onstudynotes

The leaves are the photosynthetic organs of a plant. The cells are packed with chloroplasts, which convert CO2 gas from the atmosphere and water to glucose, using the energy of sunlight. The produced glucose is then either respired and used as an energy source, or it is converted to other organic substances needed for plant growth. The cells of a leaf also produce molecular oxygen (O2 gas) as a waste product. This gas is either given off into the atmosphere, or stored inside the leaf for the night, to be used for cell respiration.

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