230 COLLEGE BOTANY 



Temperature of the plant is approximately the same as the 

 temperature of the air, but in the presence of sunshine may be 

 slightly higher (see page 197). There is very little or nothing 

 in the way of plant structures for direct protection against 

 changes of temperature which may be injurious to plant growth. 

 It is true that buds may have well-developed coverings of scales, 

 abundance of hair or resin deposits; but these protect against 

 loss of water rather than against external temperature. How- 

 ever, some of the external structures protect to some extent 

 against sudden changes of temperature. 



Freezing. The freezing of most living plants is very gen- 

 erally recognized as injurious, but it should be remembered that 

 many plants, such as wheat, rye, clover and vetch, remain green 

 and grow to some extent during the winter. Their ability to 

 thrive at low temperatures makes them valuable as cover crops. 

 In the first stages of freezing, the ice crystals are formed on the 

 surface of the cells, usually in the intercellular spaces. It is 

 very generally believed that in this process the water of the cell 

 gradually passes through the cell walls into these intercellular 

 spaces. The extent of the injury due to freezing appears to be 

 due to the degree to which the cell gives off water. However, 

 thisi question is subject to differences of opinion. 



Light. The necessity of light for phot osyn the tic work has 

 already received consideration (see page 195) and we have 

 learned that sunlight (or its equivalent, i.e., the proper kind of 

 artificial light) is absolutely necessary for plant growth. How- 

 ever, the amount of sunlight necessary for plant growth is not 

 the same for all species of plants and, therefore, plants do not 

 always grow under the most favorable sunlight conditions. The 

 sunlight may be too weak or too intense for the best growth. 



