MESOPHYTES. 235 



effects due to these three factors. It is difficult, if not im- 

 possible at present, to say which are the effects of light and 

 which of .evaporation. 



Precipitaiion affects plants chiefly as it influences water 

 supply. A few plants only of the higher forms are able to 

 absorb moisture directly from the air, except as a last resort. 

 (See *\ 165.) Many of the lower plants, such as the algae, 

 lichens, and mosses, absorb rain instantly by their aerial 

 parts. Some plants have adapted themselves to frequent and 

 prolonged rainfall, bearing it often for months at a time ; 

 other plants under such conditions lose their leaves very 

 quickly. Rain-loving plants have their leaves furnished with 

 elongated tips or with grooves and hairs to carry off the rain 

 quickly. Their surfaces, also, are not readily wetted by 

 water. Others protect themselves against the rain by adjust- 

 ing the direction of their leaves to it so that a heavy, splash- 

 ing rain strikes them at an acute angle. Others, by a move- 

 ment of their leaves as soon as the sky is clouded, avoid 

 injury from heavy rains. The branching of leaves in certain 

 cases may be looked upon as a protection against heavy rain- 

 fall. 



The snow cover through cold periods is for many plants 

 essential as a protection against low temperatures during the 

 dormant period. Others have adapted themselves to growing 

 even in the midst of snow, putting forth their leaves and 

 blossoms while still surrounded by melting snow. 



332. 5. Soil. — Both the chemical composition and the 

 physical properties of the soil affect plants. The latter are, 

 however, by far the most important. Here, again, the rea- 

 son is to be found in the relation of the physical qualities of 

 soil to the water supply. 



The water which permeates the soil takes up from it certain 

 substances, and becomes thus a dilute solution of various 

 salts. That the salts thus present in the soil water may affect 



