26 



PLANT PHYSIOLOGY AND ECOLOGY 



moist winds exert an opposite influence. The most important 

 action of wind is the removal of the more humid air ordinarily 

 occurring above the plants of a habitat, and its replacement by a 

 drier air. This has the effect of keeping the immediate humidity 

 low. This may be readily verified by taking readings of humidity 

 in a sheltered area and in one exposed to the full effect of a strong 

 wind. The moist winds that blow across a large body of water or 

 those that precede a rain either do not have this effect or possess it 

 in but a slight degree. The exact influence of wind upon humidity 

 is best shown in a hilly habitat, such as an undulating prairie. If 

 the velocity of the wind is determined by means of a hand ane- 

 mometer for ravine, slope, and ridge, and simultaneous readings of 

 humidity are taken, the relative humidity will be found to de- 

 crease with the increase in wind velocity. 



34. Influence of pressure and physiographic factors. Pressure 

 modifies humidity by varying the density of the air, and hence its 



Fig. 7. Station on the summit of Mount Garfield (3800 m.) for deter- 

 mining the effects of altitude by means of plants and instrmncnts., 



power to hold moisture. The daily fluctuations which constitute 

 weather are slight and are of little importance, except in their rela- 

 tion to rainfall. The effect upon humidity is much more pronounced 

 when differences in altitude bring about permanent differences in 



