Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 294]

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•288 When this water does freeze—in some of our trees at about 12° t o 15° Fahr.—-a minute, thin plate is first formed on the surface of the structure, or rather multitudes of such thin plates of regular shapes are thus formed near together. With a little further decrease o i temperature other water molecules are wrested from their attractions in the woody structure, and arrange themselves beneath those first formed, and in so doing push the latter outward. This escape of some of the water from among the cellulose molecules causes the latter to approach nearer each other, and at the same time to hold with stronger power the remaining water molecules, which only join their fellows in the crystal at a still lower temperature. As, however, the cold increases, the crystal pushes out, not .gaining in diameter, but increasing in length by constant additions to its base, just as we may conceive of the erection of a chimney by successively placing bricks under those already laid and pushing •upward the whole structure. The final length of the crystal depends upon the amount of water and the degree of cold. Sometimes the stems of frozen succulent plants may be seen thickly coated with a crust of such crystals a fourth of an inch or more long, but so slender that a magnifier is needed to identify individual ones, the whole presenting to the naked eye a somewhat velvety appearance. Similar crusts are formed in the interior of the tissues on the surfaces of certain kinds of cells, and pushing into cavities caused by the shrinking of the material. We may now consider that we have the chief facts upon which the explanation of the two forms of injury to tree trunks by freezing rests. We have compared the splitting of the trunks to the familiar bursting—too familiar—of iron water pipes and water pitchers. It is only necessary that a sufficient amount of water, in the liquid state exists in the central parts of the tree, and that a sufficient degree of cold be reached to shrink the woody fibre and congeal the fluid. If the water, though as a liquid in the ducts, cell-cavities, and intercellular spaces, contains substances in solution like- sugar, -earthy salts, etc., freezing will be more or less below 32° Fahr. ; and this is normally the case. If the water exists only in the imbibed state in the cell-walls, a much lower degree than this will be required to produce crystalization, and this is the normal winter state of a hardy and sound tree. It is only in spring-time, or in a spring-like condition of things, that any liquid water exists in such trees. On microscopic examination in winter, no water, as liquid, can be found in the cell-cavities, or other openings of the living tissues; yet by heat under 212° Fahr,, forty per cent, by weight of water can be driven off. It is molecularly distributed among the elementary bodies of cellulose, protoplasm, starch, etc., all of which are much too minute to be seen with our best microscopes, but which as certainly exists as do worlds beyond the reach of telescopes, and both are as certain as human knowledge is at its best. I append a table of the proportions of water determined by evaporation in an oven kept below 212° Fahr., in the trunks of several trees cut in December, 1883.