Trunks of Apple Trees. 179 



or less considerable wound, which becomes much more evident as the new 

 layer of growth pushes out the torn edges. These cracks are much more 

 frequent on the south or southerly side, but sometimes are otherwise located. 

 Numerous kinds of trees are known to suffer in this way, and among others 

 the following have, in my observation, been quite commonly cracked: Ap- 

 ple, sweet cherry, plum, box-elder, hard maple, butternut, iron-wood (carpi- 

 nus), chestnut, occasionally the black walnut, white willow, tulip poplar, sev- 

 eral oaks and linden have been seen similarly burst. No doubt the list can 

 be greatly extended. But this form of injury is not nearly so destructive as 

 the next to be mentioned. Trees are rarely severely checked in growth, and 

 probably never through this cause alone killed, though the wound may be 

 deep and long and slow to heal. 



The second form of injury by frost is from the separation of the bark 

 from the wood, occurring for the most part near the ground, and also usu- 

 al!}' most common on the side next the sun; but very often, much more so 

 than in the former case, seen on any side or entirely around the trunk. This 

 is the injury 'which has been most disastrously destructive in the apple or- 

 chards throuuhout a wide area of our country within recent years. It no 

 doubt occurs in other trees, but I have seen it, since speciallj' looking for it, 

 only in the apple and the white willow. When only the bark cracks, as in 

 the first form of injury, a bare strip of wood is often exposed by the shrink- 

 ing of the bark and by subsequent enlargement of the trunk by growth; but 

 this need not lead any to confound the kind of injury directly done by frost 

 with that now considered. In the latter, the bark may or may not be split, 

 usually not in any conspicuous degree; but separation from the wood is 

 more or less complete over the affected area. Very often there is for months 

 no external evidence of the injury, and the sickly appearance of the leaves, 

 perhaps after mid-summer, first attracts attention. The bark itself is not al- 

 ways killed, and there occurs an irregular growth of wood on the inner side 

 but separate from the older wood-layers. 



The same sort of separation sometimes occurs between the annual layers 

 of wood, and what are called " wind shakes,'' are often no doubt really due to 

 frost acting in the manner now described. Sometimes these injuries take 

 place in very old and very large trunks, but whether the splitting occurs 

 near the surface and is afterward thickly covered by other layers, or occurs 

 in the heart-wood as such, is not known to me. 



Having thus endeavored to describe the injuries, I now attempt an expla- 

 nation, based to a considerable extent upon studies bearing directly upon 

 the problem, but admittedly inweaving more or less of theory, and thus lia- 

 ble to be partially incorrect. In the first place some facts need to be stated. 



Water freezes at the temperature marked by 32° in Fahrenheit's thermom- 

 eter ; that is, the exceedingly minute, ultra-microscopical, but solid and firm 

 particles (molecules) composing liquid water, at this temperature arrange 

 themselves in certain regular positions with respect to each other, and cohere 

 so as no longer to be as before, freely movable upon each other. What 

 was a liquid is now a solid, though the component molecules are not in them 



