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The mechanisms that control gas exchange (within flowering plants and mammals) vary considerably, as will be seen after this discussion. One observation that can be made initially, which represent a distinct difference between plants and mammals, is that plants take in carbon dioxide, and oxygen is the outcome, whereas this is the opposite process in mammals. The shape and structure of plants facilitate gaseous exchange by diffusion. A flowering plant has many air spaces between the cells of stem, leaf and root. These air spaces are continuous; oxygen diffuses into the air space through stomata and carbon dioxide and water vapour diffuse out. Also, oxygen enters the root tissue by the method of diffusion from the soil, after being dissolved in the surface film of moisture. Woody flowering plants (trees and shrubs) have an external, impervious bark. Here, gaseous exchange occurs through small, powdery patches in the stem, called lenticels. The mechanisms,...

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