FACTORS OF ENVIRONMENT. 



471 



certain extent has the same effect on the protoplasm as the loss 

 of too much water by transpiration. The bud-scales thus pre- 

 vent the loss of too much water from the cells of the delicate 

 tissues of the growing point of shoots in winter, although in 

 many cases freezing in these tissues takes place. 3d. In other 

 cases plants are killed by the actual freezing of the protoplasm, 

 though there are some plants which are not killed by freezing of 

 the water in the protoplasm. The ability of some plants to 

 resist this is probably specific. 



914. Wind. The effects of wind on vegetation are manifest 

 in several ways for harm or for good. Severe, frequent, and 

 long-continued winds stunt 



and deform trees and shrubs, 

 and destroy or reduce the 

 size of herbs. Winds also 

 have a drying effect, and this 

 is probably one of the reasons 

 why forests are not developed 

 on much of the prairie re- 

 gions. Every now and then 

 in normally humid climates 

 we have an illustration of the 

 blighting effect of dry winds 

 on vegetation. " Sand blast," 

 i.e., sand driven by the wind 

 is very injurious to vegetation 



Mam trunk straight, branches all bent 



in sandy areas where there are a "d fixed to one side by wind from one 



direction (Rocky Mountains). 



high winds, tearing the leaves 



and even eroding the shoots. On the other hand wind is one of 

 the great agencies for pollination, especially in the case of ane- 

 mophilous plants, and it is one of the important agencies for 

 seed distribution. 



915. Ground covers. Plants are protected by the covering 

 of the ground in various ways. Among the lifeless covers may 

 be mentioned the following: 









Fig. 486. 



