CHAPTER X 



RESISTANCE OF PLANTS TO UNFAVORABLE 

 ENVIRONMENTAL CONDITIONS 



80. Effect of Low Temperatures on the Plant. Freezing of 

 Plant Tissues and Processes Accompanying It. — It is only in 

 tropical climates that plants remain all of the time under tem- 

 perature conditions favorable for their life activity and can 

 develop unhindered the year round. In temperate and even in 

 subtropical climates, they have to endure the influence of tem- 

 peratures below freezing, when their sap freezes and the plants 

 that are not sufficiently resistant to the changes occurring in their 

 tissues with freezing are injured or killed. The existence of 

 plants in temperate climates is closely connected with their frost 

 resistance. Some of them possess so short a vegetative period 

 as to be able to accomplish their entire life cycle in the frostless 

 period of the year. Such are the annuals, which possess a short 

 vegetative period and begin their development after the termina- 

 tion of the spring frosts and complete it before the beginning of 

 the first autumn frosts. Most of the plants of temperate 

 climates that live through the winter in the vegetative condition 

 have the ability of hardening sufficiently to endure the low tem- 

 peratures of winter or have parts that vegetate underground and 

 thus escape the low air temperatures. Among crop plants, the 

 winter cereals and grasses, as well as several other groups that 

 are of great agricultural importance, overwinter under the snow 

 in the vegetative phase of their development. 



Ability to survive the winter may be dependent upon several 

 different growth habits of plants. Annual plants overwinter in 

 the form of dry ripe seeds that will not freeze at low temperature. 

 Most of the perennials lose their aerial organs and overwinter 

 in the form of bulbs, tubers, and rhizomes well protected from 

 the frost by a layer of soil and snow. But winter crops with 

 parts above ground and woody plants must inevitably endure 

 the influence of frosts; their tissues sometimes freeze to the 



350 



