Plants as Affected by Cold. 125 



flow of gum. If the coloring of the wood does not ex- 

 tend to the cambium, the tree or branch may survive, 

 but the first season's growth thereafter is generally 

 feeble and the fruit or the seed crop often fails. Dur- 

 ing the second season, healthy growth may be resumed, 

 but the heart-wood is rarely or never restored to its 

 normal color. Black-heart often results from other 

 causes than cold, as from bacteria that gain access to 

 the heart-wood through wounds (418). 



Other chemical changes result from cold, as the 

 sweetening of potato tubers when chilled, the removal 

 of astringency from the wild grape and persimmon, 

 and the heightening of the flavor of the parsnip. 



193. Tree Trunks are sometimes Split Open by 

 Severe Freezing, the split remaining open until the re- 

 turn of mild weather. This most often occurs in hard- 

 wooded, deep-rooted deciduous trees, as the oak, and 

 appears to result from the more rapid contraction of 

 the outer layers of the wood in a sudden fall of. tem- 

 perature. The rents are usually overgrown by the next 

 annual wood layer (70). 



The splitting down of the main branches of certain 

 varities of the apple tree appears to be favored by the 

 expansive force of ice in narrow crotches, which retain 

 snow and water. Varities the branches of which leave 

 the trunk at a wide angle are not subject to this trouble. 



194. Bark-Bursting on the trunks of young apple 

 trees often occurs when freezing weather overtakes late- 

 growing and hence poorly-matured wood. In severe 

 cases, the bark splits longitudinally entirely through 

 the cambium layer and from the ground to the lower 



