WATER AS FACTOR IN DISTRIBUTION OF PLANTS 1()3 



part, in some cases more than 80 per cent, falls in winter. The sea- 

 sonal distribution of rain not only has a marked effect upon the dis- 

 tribution of the types of vegetation, as will be brought out in the fol- 

 lowing paragraph, but is also one of the chief causes of certain 

 periodic phenomena in plants such as rest periods. An adequate 

 supply of moisture is absolutely necessary for active growth and, 

 therefore, if at any time it becomes inadequate it may bring about a 

 resting period, or it may cause death. 



106. Water as a Factor in the Distribution of Plants.— If we dis- 

 regard small local areas and consider only large regions, a birds-eye 

 view of the land surface of the earth shows three general types of 



11 f I 



Fig. 70.— Grassland vegetation, Oregon. (Photograph by H. L. Shantz. 

 Courtesy of H. L. Shantz and the U. S. Department of Agriculture.) 



vegetation; namely, forest (Fig. 69), grassland (Fig. 70) and desert 

 (Fig. 71). These rather distinct types are determined largely by the 

 amount of rainfall, the seasonal distribution of the rainfall, and the 

 frequency of rainy days or of effective rains. By effective rains we 

 mean rains that actually supply water to the plants. Light showers 

 are often entirely ineffective l)ecause the water evaporates before it 

 has penetrated the soil far enough to be available to the roots. On 

 the other hand, during a torrential rain there may be such a great 

 run-off that only a small fraction of the water that falls becomes 

 available to the plants. If, therefore, a large percentage of the rain 

 falls in light showers or torrential storms it may be considerably 



