Plants as Affected by Cold. 123 



196. Root-Killing of trees. When a very dry autumn 

 passes to winter without rain or snow, tJie surface layers 

 of the soil sometimes freeze so severely as to destroy the 

 roots of young trees. Eoot-killing is usually most serious 

 on light soils, and on one-year-old, root-grafted (391) 

 nursery trees, especially when grafted with short cions 

 (386). "With very severe freezing on bare ground, root- 

 killing sometimes occurs on soil well supplied with 

 water. The destruction of the roots may be complete or 

 only partial. In the latter case, the tree, if of a vigor- 

 ous variety, may largely outgrow the trouble, though 

 complete recovery is rare. 



Treatment that prevents late growth (200, 201), or 

 mulching the ground about trees tends to avert root- 

 killing. 



197. Flower-Buds are often Destroyed by Cold while 

 other parts of the plant are uninjured. This frequently 

 occurs in the peach, apricot, nectarine and certain species 

 of the plum in climates of rather severe winters, espe- 

 cially after the buds have been somewhat excited by 

 unseasonable warm weather. Flower-buds thus destroyed 

 are dark -colored at the center. 



198. Flowers are Especially Sensitive to Cold. Fruit 

 crops are , usually wholly or in part cut off if a slight 

 frost occurs during bloom, and in certain fruits, as the 

 apricot and some species of the plum, the blossoms some- 

 times appear to be destroyed by a degree of cold that 

 does not descend to the freezing point, possibly through' 

 interference with pollination or pollen germination (151). 

 When the freezing is accompanied with snow, however, 

 open flowers may escape without harm, probably owing 

 to the slow extraction of the frost (190 b). 



