UNFAVORABLE TEMPEBATUBE 113 



THE PLANT AS AFFECTED BY EXCESSIVE COLD 



187. Effect of cold. — The immediate effect of cooling 

 the plant is to check the activity of its vital processes. 

 When a certain degree of cold is reached, the protoplasm 

 loses its power to imbibe water (62) ; hence the plant 

 tissues become less turgid, and the foliage droops some- 

 what (102). With a sufficient reduction of temperature, 

 ice crystals form within the tissues and the succulent parts 

 of the plant assume a glassy appearance. The foliage of 

 many plants, as celery, parsnip and the like, assumes an 

 abnormal position when frozen. 



188. Relation of water to freezing. — The more water 

 plant tissues contain, the sooner they freeze. Since the 

 water of plants is not pure, but is a solution of various 

 substances, it does not freeze at the freezing point of pure 

 water (32° F.), but at a lower temperature, determined 

 by the degree of concentration of the solution, or the in- 

 timacy with which it is combined 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 (174). 



189. The power of plants to endure cold depends upon 

 various conditions, aside from the amount of water con- 

 tained, as 



Heredity. — Plants by nature possess widely differing 

 powers to endure cold. The Anoectochilus perishes 

 when exposed for a considerable time to a temperature of 

 42° F., while other plants, as the common chickweed 

 (Stellaria media), are uninjured by prolonged cold, far be- 

 low the freezing point (175). 



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

 slowly the thawing takes place, the less likely is the frozen 



