SOCIETY OF NORTHERN ILLINOIS. 221 



suit is ill every way analogous to the bnrstiii<j^ of an iron pipe filled 

 with water and the latter permitted to freeze. The iron contracts or 

 shrinks with the inereasinj;' cold, while the water in passinj;" from 

 the liquid to the solid crvstalline condition, ex])aiids. and does so with 

 enormous force. Unless the enclosinj^' material is elastic enou<;li to 

 ^xe way to this ])ressure without ru))ture, the latter is sure to occur 

 no matter what the stren*>th may l)e. In the apple tree the same 

 shrinkin*ij of the plant tissues takes ])lace as the heat decreases, and 

 if the inner ])arts are full of water the same kind of expansion of the 

 latter in crvstallizin*; occurs, and the crack is inevitahle. The con- 

 ditions are: 1st. Sutticiency of water: and 2d. Low enough temi)era- 

 ture. In rej^ard to the first, it is not sufficient that the wood is sim- 

 )dy '■saj)py.'" or moist. It needs l)e coni])letely tilled with water, its 

 cell cavities and other openin*i,s *jjorged. so that as coiif^ealation takes 

 place the s])Hce is entirely occupied, with no room for ex])ansioii 

 within. It is, however, possible that the shrinking of the tissue ma- 

 terial may often he the cause of the rupture, just as we see a green 

 stick split when shrinking by drying. This contraction by cold 

 conies about in two ways: 1st. According to the natural law of all 

 bodies, and familiarly illustrated by the mercury in our thermome- 

 ters. 2d. By the withdrawal of water in the process of freezing. 

 We all know that plant tissues shrink and swell with the varying 

 amount of water, and all pr()l)al)ly know that a wet rag may " freeze 

 dry." In these two ways the shrinking of the outer {larts of a tree 

 may be sufficient to cause a crack without the internal j)ressure of 

 freezing water: but it is much more probable that Ijoth work toge- 

 ther when serious mischief of this kind occurs. 



In regard to the degree of temj)erature, or the intensity of cold 

 refjuired. nothing definite can be stated, save that for all our native 

 trees, and all others hardy enough to usually withstand our winters, 

 the thermometer must sink far below 32°. the freezing point for pure 

 water. The reason for this is not hard to find. The fact is. in all 

 such living trees the water normally in their tissues does not freeze 

 at zero of Fahrenheit's scale. Of this repeated examinaticuis this 

 winter have given abundant testimony. When the thermometer 

 stood at — 6°, I found, after careful looking, no ice in the mature 

 wood or bark of many trees examiiu'd. No crystals were to be seen 

 with the microscope, and though the fibers were more brittle than in 

 warm weather, they were still fiexible. At twelve below the pith in 

 several kiiuls. including l^artlett prar. peaches, and in the softer 

 shoots of ap])le, was completely hardened like a stem of ice. while 

 the wood and bark, in all except some apple tree " water shoots.'* 

 still remained fiexible. At twenty-eight below the wood of many 

 kinds of trees was rigid and full of miuute crystals, though in no 

 case was this found true with well-matured bark. This exemption 

 of the water in plant tissues from freezing with cold below 32° Fahr. 

 is of easy explanation, though at first sight a curious phenomenon. 



