Whether it be desirous to retain water or to lose it by gradual evaporation, or expel an excess of it, each species of plant has developed the apparatus to best preserve its individual life. While only the barest outline of these adjustments to the water requirements of plants has been given here, the details form an almost dramatic picture of struggle of the different kinds of plants for survival. The extremes are the desert plants on the one hand and those of the rain forests in the tropics on the other. The chapter on Plant Distribution will show how important these water requirements of plants have been in determining what grows on the earth to-day.
With carbon dioxide going in, oxygen, water vapor and, as we have seen, even liquid water coming out of the stoma of leaves, it might be surmised that these busy little pores and their guard cells had done work enough for the plant. And yet there is still one more act to play and the stoma have much to do with it. For this process of photosynthesis and the closely related one of supplying food and water to the leaf cannot go on without respiration, which is quite another thing. In plants respiration or breathing has no more to do with digestion than it does in man. Digestion in man is not unlike photosynthesis in plants, except that plants make food in the process while men destroy it. But plants must breathe just as we do, and, as we need oxygen to renew our vital processes, so do they. While respiration is a necessary part of plant activity it is not such an important part as photosynthesis, for which it is often mistaken. The thing to fix in our minds is that photosynthesis makes food, uses the sun’s energy and releases oxygen in the process, while respiration uses oxygen and might almost be likened to the oil of a machine—necessary but producing nothing.