Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 296]

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290 southern crack, if all other parts were exactly equal. Besides this, the center of the heart is commonly nearer the south side, on account of less wood growth on that side. There is, however, a far more effective cause for tjie phenomenon. Every change in the temperature of the tissues of a tree affects the quantity of water in the cells and spaces. Indeed, it is largely by such alternating changes of heat and cold that liquid water gets into and accumulates in the trunk of a tree, mainiy through the contraction and expansion of contained air. The corky bark is almost impervious to water and air, and forms a kind of sealed tube, whose lower end only is open in winter. If air at first occupies all the cavities in the wood, as it does in summer, and a reduction of temperature occurs, this air very considerably contracts in volume, making a vacuum, or would do so were the spaces not •concurrently filled up by an upward flow of air and water from the xoots, and through them from the soil. When the air expands pressure is produced, and gases being more mobile than liquids, the air rather than the water is forced down or away in any other direction, leaving another condensation by cold to act as before. In this way the tissues of the south side of an exposed trunk of apple or other tree gains more fluid than those of the north side. The increase of water and the consequent dilution of the dissolved substances causes earlier and greater congealing and its effects. I submit the following figures, obtained by Mr. Hewes, a student of the University, in April, 1883, from experiments upon a soft maple (Acer dasy carpum), about forty feet high and nearly one foot in diameter of trunk.

No. of Observation. Specific Gravity. JN. 1 8. JN. 1 S. IN. 1 S. (N. ! 8. 1.013 1.011 1.012 1.008 1.011 1.0088 1.006 1.006 P e r cent, of sugar. IN. ( 8. JN. 1 S. j N. 1 S. JN. " S. 1 Amount s of Liquid.

10 11 12 13 . "

3.3341 2.6485 3.2279 F o r nine c o n t i n u o u s 2.0004 o b s e r v a t i o n s : 2.9569 2.2731 3971 c. c. 4.4967 N . . . . 3.51508 7869 c. c.

If the south side of the trunk is well shielded from the sun splitting may occur, and then on any side; but the injury would not be so liable to happen. The condition of the heart of the tree must be an important item in the effect, but the last season's growth, whether vigorous or not, makes little difference. Passing now to the much more serious injury—the separation of bark and wood—we may say that neither the swelling of the interior nor the shrinking of the exterior layers of tissues can be accredited with the disastrous results, for these crowd the parts together instead of forcing them asunder in a radial direction. Neither is it in any way the undue shrinking of the interior arid extension of the outer portions, because no such thing occurs. The heart of a tree freezes first. No change of temperature is sudden enough in the 'open air in winter to cause the bark and younger wood to freeze before the pure water of the heart wood congeals, popular opinion