GENERAL CONSIDERATION OF PLANT DISEASES 283 



freezing, is noteworthy. The death of cells rich in water, when exposed 

 to low temperatures, seems to depend upon the conversion of the water 

 extruded into the intercellular spaces into ice. The parenchymatous 

 tissues are ruptured and crystals of ice are formed. The water, there- 

 fore, which is in the cell reaches the surface and the cell sap diminishes 

 in amount and there may be chemic changes in the cell as a result of 

 freezing, for in some cases .the leaves assume a leathery brown color. 

 Long exposure to cold may lead to the actual disorganization of the 

 protoplasm. It, however, does not always follow that the formation 

 of ice in the intercellular spaces necessarily involves death. Slow 

 thawing may be followed by a return of the water to the cells until 

 the normal equilibrium is restored and the cells continue to live. A 

 rapid thawing, however, causes death of the cells, because the water is 

 not reabsorbed. Frost-killed twigs and branches are more susceptible 

 to the entrance of saprophytic fungi such as species of Nectria, Dasy- 

 scypha, and Valsa. The exposure of roots during a snowless winter 

 may lead to their disturbance by freezing. The anatomic changes 

 induced by freezing are frost blisters, such as appear on the leaves of 

 fruit trees and cereals, and frost cracks, which may ultimately heal 

 over, producing an external ridge or enlargement. The fruit-grower 

 can distinguish four kinds of winter injury to his trees. First, the 

 frosting of the blossoms after they begin to open; second, the freezing 

 of the buds in winter; third, the freezing of the twigs and branches; 

 fourth, root freezing. It may happen that early in the spring the 

 peach trees come into bloom. Then on a cold cloudless night with no 

 wind the temperature sinks below freezing and the partially open 

 flower buds are nipped by the frost. About twenty years ago the 

 upper Mississippi Valley was visited by an unusual cold wave. The 

 frost penetrated to great depths and the cold was so intense that the 

 tree roots were actually frozen in the soil.^ 



The formation of ice fringes upon plants has been investigated 

 exhaustively by Coblentz,- with the dittany, Cunila mariana. He 



* Consult Waugh, Frank A.: Jack Frost's Tricks. The Country Gentleman, 

 Feb. 6, 1915, p. 213. Wilson, Wilford M.: Frosts in New York. Bull. 316, 

 Cornell University Agri. Exper. Stat., June, 191 2. Chandler, W.H.: The Kill- 

 ing of Plant Tissue by Low Temperatures. Research Bull. 8, Coll. of Agric, 

 Univ. of Mo., Dec, 1913. 



2 Coblentz, Wm. W. : The Exudation of Ice from Stems of Plants. Monthly 

 Weather Review, 42: 490-499, August, 1914. 



