The foregoing paragraphs have shown that the process of tissue respiration is comparable with the internal combustion in a car ; that breathing is a process comparable with those which are carried out by the carburettor and exhaust; that the refining of food closely resembles the refining of crude petroleum, and that control is necessary for the sucoessful working in both cases. The living organism shows certain features which are not to be found in man-made machines, or, indeed, in anything which is not alive. The most important difference is the fact that organisms are controlled and reproduced. Although many machines are designed to produce other machines, these are never like the machines that produced them. Further, not all the useful food carried in the blood is used up by the muscles. Some goes to form reserves stored in the liver and around the muscles and under the skin, and around certain organs such as the kidneys, helping to embed them in their natural position so that they do not hang loosely inside the animal. The heart, however, is prevented from working efficiently when excess of fat is stored on the outside of it. Digested food is also used to build up new living and dead parts, e.g. skin and hair. In the young animal this not only enables repair to be effected but results in growth. Crystals, e.g. of copper sulphate, will grow from a solution of copper sulphate in water. There is one big difference between these two types of growth. The crystals will only grow in a solution of copper sulphate when the solution is saturated to start with, and when the solution either cools or evaporates. The material which forms the increase in the bulk of a young animal is nothing like that which the animal takes in as food and that which is carried in its blood stream. Finally, all machines ultimately cease to work, for the man-made ones wear out and the living ones die. A man-made machine is yet to be invented which will repair itself, although the efficiency of our present-day machinery is largely due to the fact that one can now use and work with much harder and therefore lasting alloys such as stainless steel and high-speed chromium steel.

The comparison of the animal with a machine such as a motor car may be extended to a plant, although in this case the activities are not nearly so obvious. Plants do their work unobtrusively. Seeds on germinating have to burst through the seed case. Such seeds are known to farmers and horticulturalists as ’hard ’seeds. The roots of trees can push their way through pavements. Their trunks by growth will push over a wall. To do this work much energy is required, which is obtained by the tissue respiration of the plant.

With few exceptions, oxygen is essential in the tissue respiration of plants, the mechanisms involved being the stomata on the surfaces of leaves and of young stems, the lenticels on older stems and the root hairs of young roots. Those parts above ground take in oxygen by gaseous diffusion, and it dissolves in the fluid of cells bounded by intercellular spaces, whereas the roots obtain theirs largely by the diffusion of the oxygen dissolved in soil water. The transpiration and food-conducting streams convey the necessary oxygen to the living tissues of the plant, acting in a similar capacity to the blood stream of an animal. The same mechanisms also serve for the getting rid of carbon dioxide produced by tissue respiration. The breathing organs of most animals are active ones, e.g. the air-breathing animal carries out movements which force air into and out of its lungs, whereas those in plants are passive, depending upon the rate at which oxygen will diffuse. The quantity of gas which diffuses in or out, in the case of oxygen and carbon dioxide, can be controlled by the changing of the size of the stomata. Tissue respiration of such parts of plants as seeds and bulbs is readily shown by comparing their dry weights before and after storage. Such structures seem to be quite inactive, yet their loss of dry weight on storage shows that some material must be used for tissue respiration, which must go on in order that life may be maintained. This emphasizes the fundamental feature of an active machine that liberates and utilizes energy so that work may be done. In this respect tissue respiration becomes the most important biological process, and must not be confused with external respiration, by which is meant breathing, e.g. the passing of air in and out of the lungs or skin of an air-breathing animal.

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