Page:Popular Science Monthly Volume 63.djvu/243

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HE aquatic origin of all living things is now a generally accepted conception. The arguments in its favor are: (1) morphological, based on comparative studies of the vegetative and reproductive parts; (2) biological, based on observation of the habits of plants and animals, especially at the breeding season; (3) paleontological, based on the now known fossil remains of formerly living organisms; (4) physiological, based on the absolute dependence of all living things on water. These last arguments appeal to me more strongly than any others. When we realize that all food, all the materials of which the body is constructed, and all the substances which its cells use, can enter the cells only in solution in water, we see at once how indispensable water is. When we realize besides that the form and size of the cells, and therefore of the body, depend upon the pressures within the cells which are due to the presence of aqueous solutions therein, we see how necessary water is in another way. Upon the tension of the cells depends the mechanical force which they, the tissues, and the organism, can exert. The absolute dependence of all living things upon water is one of the two most important characters which they possess. The amount of water which different cells, organs and organisms use varies greatly, but they all require some water. The ease with which different organisms, organs and cells obtain water also varies, though not necessarily in a degree corresponding with the amounts used. If we compare the conditions under which water and land plants live, we shall see some reasons for the differences in the structure and habits of these two classes.

The constantly submersed aquatic, whether fresh or salt water, is buoyed up with a very considerable force. A solid mass of plant tissue from which all air and water had been pressed would be buoyed up in water by a force from seven to eight hundred times as great as would be exerted if it were in the air. This is in accordance with Archimedes' well-known law in physics—a body in a fluid is buoyed up with a force equal to the weight of the volume of fluid which it displaces. Any part of a land plant, therefore, which rises into the air is supported with say only one seven hundredth of the force which supports the submersed aquatic. This difference is met by the land plant in two ways. It develops tissues which mechanically support it, which carry that part