RAINFALL 101 



some of the desert shrubs, such as Foiiquieria splendens, the ocotilla, 

 when they are leafless. It has been shown in the case of several 

 kinds of emersed water plants that water is lost more rapidly from a 

 given area occupied by plants than from an equal area of free water 

 surface. In the case of water lilies, however, the reverse was true. 



The process of transpiration is thought to be of very great impor- 

 tance to the plant because of its cooling effect. Yet the greatest 

 danger to which many plants are subjected is that of too great a 

 loss of water. The various ways in which plants are protected from 

 this danger are discussed in Chapter IV. 



104. The Mechanical Effects of Water.— Water sometimes has 

 destructive effects upon plants. Heavy storms of rain or hail may 

 beat down and destroy seedlings or they may tear or knock off 

 leaves, flowers, or fruits. Snow, which is a form of water, may break 

 down plants or plant parts by its weight. In some places in moun- 

 tainous regions snow collects near the mountain top and when thaw- 

 ing starts in spring the snow comes down the mountain in a gigantic 

 snow-slide, breaking down the woody plants in its path. When this 

 is repeated in the same path year after year the development of a 

 forest is completely prevented. Such snow-slide paths are often 

 several hundred feet wide. Snow, on the other hand, may be of very 

 great value to plants by serving as a protective covering during the 

 winter. A sleet storm is very destructive in breaking down woody 

 plants. There is also danger of trees being smothered when sleet 

 remains on them for a long time, though probably this does not 

 happen often. 



Water is of considerable importance as an agent of pollination and 

 in the dissemination of seeds and fruits, but these matters are dis- 

 cussed elsewhere, 



105. Rainfall.— Whenever air is cooled to such an extent that it 

 cannot hold in a vaporous state as much water as it contains, the 

 water falls as rain, hail, or snow, or is deposited as dew. These forms 

 of precipitation are the chief source of water for most plants. Some 

 plants, such as numerous epiphytes and many lichens and mosses, 

 especially those growing upon rocks, absorb all of their water directly 

 from atmospheric precipitations. Ordinary land plants, on the other 

 hand, get most of their water indirectly from rainfall after it has 

 penetrated into the soil. 



The annual amount of rainfall varies greatly in different places 

 and from season to season. The locations of large bodies of water 

 and of mountain ranges and the direction of the prevailing winds 

 11 



