56/ 



found no room to grow into tlie clet't and therefore were forced outward. 

 On this account we find the Hp-Uke processes made recognizable in the 

 cross-section. Such a process of healing lias not yet been observed in any 

 other trunk injury, so that its appearance may be considered absolutely 

 certain evidence of frost action. 



Caspary' has experimentally examined this phenomenon more closely. 

 He proved by direct measurement that the coefficient of the distention of 

 the fresh wood consideral)lv exceeds that of all solid bodies, even that of 



Fig-. 122. Ffost rid.iie on the trunk of Acer campestre 



iron, tangentially as well as radially, and is exceeded only by air. This 

 explains the sudden production of deep clefts. 



The extent to which a cleft opens varies in the same tree species and 

 with the size of the trunk, but all cases show that if the frost clefts have 

 once been produced (eyen after they have closed in thawing weather) a very 

 light degree of cold is sufficient to re-open them. This is explained by the 

 fact that the amount of strength necessary for the production of the cleft 

 may be sufficient to overcome the cohesion of the cell elements in the whole 



1 Caspary, Neue Untersuchungen liber Frostspalten, Bot. Zeit. 1857, No. 20-22. 

 In an earlier treatment, Bot. Zeit. 1855, p. 449, he also cites the earlier literature. 



