316 PLANT LIFE, 



the sun's rays. The influence of the temperature of the soil 

 is mainly indirect, acting through its effect on the water 

 supply of the plant. 



431. (e) Moisture and precipitation. — The amount of 

 moisture in the atmosphere largely determines the amount of 

 evaporation from the surface of the plant. The relative 

 amount of moisture in the atmosphere is exceedingly variable, 

 and bears a direct relation to its temperature. Indeed, so 

 closely related are the conditions of temperature, light, and 

 moisture in the air, that the adaptations of shade plants, 

 mentioned above, may be considered as the sum of the 

 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. 



Precipitation 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 ^f 196.) Many of the lower plants, such as the algas, 

 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 adjusting the 

 direction of their leaves to it so that a heavy, splashing rain 

 strikes them at an acute angle. Others, by a movement 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 rainfall. 



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 



