Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 292]

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286 Passing now to another series of facts pertaining to the structure of the solid parts of organic bodies, it may be first stated that water forms an essential part of the texture. In plants with which we are now concerned, all the solid parts are composed of cells usually % only to be seen with the microscope. These cells have walls or sack-like membranes which often enclose various substances more or less mixed with water. Sometimes the cell cavities are fall of liquid water, forced up from the earth by the roots. But aside from this liquid water contained in the cells the molecules of water help to form the solid parts, as of the cell-walls. This last is perhaps difficult to comprehend, but it is exceedingly important that we should understand the fact in order to rationally acquaint ourselves with what takes place when a plant freezes. It has already been said that water is really made of minute solid particles called molecules. The substance of the cell-walls, known * as cellulose, is likewise composed of molecules, but of more complex structure and undoubtedly of considerably greater size than those of water. In the natural composition of the cell-wall the cellulose molecules may be represented by the bricks in masonry, and the water molecules by the grains of sand in mortar. Between these different kinds of molecules there is a strong attraction or adhesion which binds the whole into a solid substance. There is plenty of water present but no liquid. The water molecules are as truly a part of the structure as are the cellulose molecules, iiipe seeds have no liquid water in them, nothing but this water of structure and comparatively little of that; so of the other parts of many plants in certain normal conditions of their existence; while on the other hand more than nine-tenths of the weight of rapidly growing shoots is water in both liquid and molecular states. Let us remember that every solid is made up of invisibly small molecules, and that these are held together by the attraction that exists one for another; that the force or effect of this attraction varies inversely as the square of the distance. This distance, at most, is so minute that any variation makes considerable} perhaps very much difference in the result. It is this molecular attraction that binds the cellulose molecules together with the water molecules into a cell-wall. The molecules, however, do not actually touch each other. Each is wondrously endowed with motion and swings back and forth in a limited path of its own, not unlike, on an infinitesimal scale, the planetary bodies; kept asunder by motion, but held from farther separation by attractive force. In the living plant the swinging water molecules always separate from each other the cellulose molecules to a certain limited extent, but if by any abnormal cause the water molecules are once forced out and the cellulose molecules approach so near to each other that their own attractions are greater than that between the cellulose and the water, the latter can not get back, the organization is destroyed, the tissue fails in its physiological functions, the plant, or the injured portion of it, dies* There is still another series of facts which, though more familiar, must be included for use in the explanation to follow. It is known to all that bodies shrink in size as the temperature decreases, and