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. 5. RESTLESS AND IRRITABLE PLANTS In walking through the quiet cathedrallike stillness of a deep forest or over the fields and moors, perhaps our chief thought is how restful the scene is, and what a contrast the quiet, patient plants make to the darting insects or flitting birds that our walk disturbs. We found at the beginning of this book that ability to get about is one of the main differences between animals and plants. Like so many first thoughts, this is, however, only a half truth, for while most plants, seemingly by a kind of fatality, are anchored forever to the place of their birth, many of them do move certain parts of themselves and that quite regularly. While some of these movements have already been hinted at as a possible response to transpiration or too intense light, there are others where the advantage to the plant, if any, has yet to be demonstrated. These other movements, perhaps because their cause has never been discovered, seem the more mysterious as they certainly are more weird and interesting than almost any other of the curious things that plants do. Perhaps the most difficult thing in the world is to keep an active growing child perfectly still for more than a few moments at a time. There seems to be some impelling force that makes young growing things in a constant state of restlessness, and it is perhaps not so extraordinary, after all, that practically all young plants are restless in the sense that they are never quite still. And, like many grown-up people who do not know what repose in their waking moments really means, there are a goodly number of plants that are restless until the day they die.