184 



Mississippi Valley Horticultural Society. 



otherwise destroying the vitality or healthfulness of the tree. I shall, there- 

 fore, only add an explanation of the crack so commonly occurring on the 

 south side or that most exposed to the sun. If this splitting of the trunk can 

 be properly compared to the bursting of a water pipe, how can it be that the 

 points of the compass have anything to do with it? The rupture of an iron 

 tube always occurs in the weakest place, and no amount of thawing and 

 freezing on one side, with the other less subject to such changes, can make 

 any difference in the result. Action and reaction are equal, pressure south- 

 ward means equal pressure northward, and so of east and west. Now so 

 far as the outer layer of bark is concerned the south side is the weakest, be- 

 cause of the drying effects of the sun ; cracks always being more numerous 

 here than elsewhere, and this difference in strength, slight as it is, should be 

 sufficient to cause the southern crack, if all other parts were exactly equal. 

 There is, however, a far more effective cause for the 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, mainly 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 ffrst occu- 

 pies 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 they not concurrently filled by the upward flow 

 of air and water from the roots, and through them from the soil. When the 

 air again expands pressure is produced, and gases being more mobile than 

 liquids, the air rather than the water is forced down or in any other diret-- 

 tion, 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 trees gains 

 more fluid than those of the north side. The increase of water and the con- 

 sequent dilution of the disolved substances causes earlier and greater con- 

 gealing and its effects. I submit the following figures obtained by Mr. 

 Hewes, a student of the Illiiiois Industrial University, in Aju-il, 1883, from 

 experiments upon a soft maple (Ace dasycarpum) about forty feet high and 

 nearly one foot in diameter of trunk : 



