SOCIETY OF XORTIIEUN ILLINOIS. 223 



show liv their foliage signs of injury in the s})rin<i;. but do so phiinly 

 (luring sonu'tinic in the stiniiiifr. some have attrihuteil the cause to 

 something else than frost. Hut whatever other factors may enter 

 the problem, we may rest assnred that the actual force which sent 

 off this bark is the expansion of water in freezing, together with the 

 concurrent shrinking of the plant tissues ])y the cold. The latter 

 alone cannot jjroduce the result, however it may be in the case of 

 longitudinal cracks, because such shrinking tends to tighten, not to 

 loosen, the bark. The internal parts never shrink away froiu the 

 external. 



What has heretofore been said about the crystallization of water 

 in })lant tissues must be recalled in the ]iresent ex])lanation. and one 

 other fact added. No ice is formed in the substance of the cell 

 walls, though nearly half their weight be water. The nudecules of 

 water at the surface first become arranged to form the beginning of 

 a crystal: next, other molecules are withdrawn from those of the 

 cellulose and ])laced under the first ami crowd them out. This con- 

 tinues until the crystal stands outward to some length — the success- 

 ive additions being ;d ways at the base and under those preceding — 

 ami until the crystallizing force no longer is suthcient to extract the 

 molecules of water from the attractions of those of the other mate- 

 rial of the cell wall (cellulose). Such crystals are very slender, but 

 usually stand thick together, presenting to the naked eye something 

 of the appearance of a close coating of hoar frost with a velvety re- 

 flection. Under a magnifier the distinct and wtdl-formed crystals of 

 solid ice are plainly discerjied. It is this crystalline growth, com- 

 bined with the shrinking of the tissues in the two ways already 

 pointed out. whic h rujjtures the cambium cells and se])arates the 

 bark. The cambium is ])y no means the first ]iart of the stem to 

 freeze: but the cells of this layer have mucdi thinner walls, while 

 their cavities are more nearly full of |)rotoplasmic and other mate- 

 rial, all ca])able of yielding water for crystallization. Sometimes the 

 annual layers several years old are separated in the same way. and 

 from similar causes. 



It now remains to notice the conditions under which these dis- 

 astrous results are ])roduced besides the abs(dute cold of the winter. 

 Probably we shall never find either form of injury without the ther- 

 mometer showing at least zero temi)erature. bu.t the mercury nniy 

 sink much below this while the trees are in certain favorable condi- 

 tions without injurious effects, to — 20° or — 80° or lower. 



We all know that growing shoots have by far a greatei- propor- 

 tion of water in them than ri))e twigs, and that rotting wood will 

 soak up much more water than sound wood. In well-ri])ened wood- 

 bark and seeds there is. as has been said, no water in a licpiid state. 

 neither will they readily absorl) such water at a low temi)erature 

 when immersed in it, though in molecular combination with the or- 

 ganic substances, there is. in the dryest seeds possessing the power 



