Trunks of Apple Trees. 181 



cules 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 inlinitesimal scale, the planet- 

 ary 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 attrac- 

 tions 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 phys- 

 iological 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 explanations to follow. It is known to all that bodies 

 shrink m size as the temperature decreases and expand or swell with heat. 

 The rails of the railway are perceptibly shorter in cold than in warm weather ; 

 the mercury in the thermometer sensitively obeys the heat changes upon the 

 same principle. The tissues of trees form no exception to this rule. The varia- 

 tion in the circumference of an apple tree over four inches in diameter can 

 be readily demonstrated by a common tape line. A box elder stick just cut, 

 three and a half inches in diameter was found by myself to be a fourth of an 

 inch greater in circumference between zero and plus 70° Fahr. In a large 

 trunk the difference is much greater. It is also well known that water from 

 plus 39° Fahr. to plus 32° Fahr. is an exception to the general rule, and ex- 

 pands as the temperature decreases from the first named degree. In the act 

 of freezing, further expansion takes jjlace and in both cases with, what may 

 be designated, resistless force. Iron pipes are split like fragile reeds; the 

 thickness of their walls constitutes no safeguard ; the iron itself shrinks, the 

 water within expands, and bursting follows. If we should bore a hole in the 

 heart of a tree and fill it with water, exactly the same result would follow 

 upon a decrease of temperature below 32° Fahr. A tape line would indicate 

 a constant decrease in the size of the trunk with the decrease of temperature 

 until relief from the enormous pressure finally came by a longitudinal split. 

 But such splitting does not take place in living trees, no matter how much 

 water they naturally contain, until the temperature is far below 32° Fahr. 

 The reasons for this have already been given and reference may now be 

 made to our salt solution ; but further discussion must first give place to still 

 another peculiarity of the freezing process of aqueous combinations. 



The plant tissues, in common with other things, containing water in a state 

 of molecular association with other molecules, are subjected to certain low 

 degrees of temperature, the outside or exterior water molecules first freeze 

 or arrange themselves to form a crystal. The temperature at which this 

 takes place depends upon the molecular attractions. When the amount of 

 water is sufficient to fill the molecular interspaces to saturation, the crystal 

 begins to form at very little below 32° Fahr. ; but when the proportion is 

 much smaller than suffices to equalize the attraction for the water molecules, 

 the crystalline arrangement of the latter begins at the surface of the organic 



