118 PRINCIPLES OF PLANT CULTURE 



been somewhat excited by unseasonable warm weather. 

 Flower-buds thus destroyed are dark-colored at the 

 center. Often only a part of the embryo flowers on a tree 

 are destoyed. 



197. Killing of flowers. Flowers are especially sensi- 

 tive 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 sometimes appear to be destroyed by a degree of 

 cold that does not descend to the freezing point, possibly 

 through interference with pollination or pollen germi- 

 nation (150). When the freezing is accompanied with 

 snow, however, open flowers may escape without harm, 

 probably owing to the slow extraction of the frost (189). 



198. Destruction by ice. Low plants are often de- 

 stroyed by ice, especially when the ice layer forms in direct 

 contact with the soil about them and remains for a time 

 after the return of warm weather. The same effect 

 results sometimes from a covering of snow, of which the 

 top has formed into a crust of ice. Winter grain, straw- 

 berry plants, lawn grass, alfalfa, and red clover are often 

 smothered in this way. Surface drainage of ground de- 

 voted to such crops is highly important. 



199. Heaving. Frost may heave plants out of the 

 ground so forcibly as to break or tear away the roots or 

 else so loosen the roots from the soil that the plant perishes. 

 This is especially likely to occur on heavy clay soils and 

 with young plants having a tap-root, as red clover, or with 

 bulbs when planted shallow and left without a mulch. 

 Such heaving comes with alternate freezing and thawing, 

 especially in the spring. It is due to the expansion of 

 the surface layer of soil which by buckling upward lifts 

 tne plants just as it heaves fence posts out of the ground. 



