Plants as Affected by Cold. 119 



temperature, determined by the degree of concentration 

 of the solution, or the intimacy with which it is com- 

 bined with the tissues of the plant. The more thoroughly 

 dormant the condition of a plant, or part of a plant, the 

 less water does it contain, and the better is it able to endure 

 cold (175). 



190. The Power of Plants to Endure Cold depends upon 

 various conditions, aside from the amount of water con- 

 tained, as 



a — Heredity. Plants by nature possess widely differ- 

 ing powers to endure cold. The Anwctochilus (a-noec'- 

 to-chi'-lus) perishes when exposed for a considerable 

 time to a temperature of 42° P., while other plants, as 

 the common chickweed,* are uninjured by prolonged, 

 cold, far below the freezing point (176). 



b — The rate of thawing of the frozen tissues. The 

 more slowly the thawing takes place, the less likely is 

 the frozen part to suffer injury. Many bulbs, tubers and 

 roots which survive the severest winters within the soil, 

 where they thaw slowly, are destroyed by moderate 

 freezing if quickly thawed. Prost-bitten plants are sel- 

 dom injured wh^n sheltered from the morning sun by a 

 dense fog, which causes them to thaw slowly. Apples, 

 covered in the orchard in autumn by leaves, sometimes 

 pass a severe winter with little harm. 



When the water that is withdrawn from the tissues in 

 the freezing process is gradually set free by slow thaw- 

 ing, it may be absorbed by them again and little or no 

 harm results. 



c — The length of time the tissues remain frozen. A com- 

 paratively slight degree of frost, if prolonged, may act 



* Stellaria media. 



