7? THE HABITAT 



ings of the same leaf were .5-i higher when made upon the upper smooth 

 surface. This immediately suggests that the lower surface may be modified 

 to protect the leaf from the great heat of the gravel, which often reaches 

 50 C. (122 F.). 



PRECIPITATION 



105. General relations. As the factor which exerts the most important 

 control upon water-content and humidity, rainfall must be carefully con- 

 sidered by the ecologist. It is such an obvious factor, and is usually spoken 

 of in such general terms that the need of following it accurately is not evi- 

 dent at once. When it is recognized that the fluctuations of water-content 

 are directly traceable to it, it becomes clear that its determination is as 

 important as that of any indirect factor. This does not mean, however, 

 that the amount of yearly rainfall is to be taken from the records of the 

 nearest weather station, and the factor dismissed. Like other instruments, 

 the rain gauge must be kept at the base station of the area under study, 

 and when this is extensive or diverse, additional instruments should be 

 put into commission. While the different parts of the same general cli- 

 matic region may receive practically the same amount of precipitation dur- 

 ing the year, it is not necessarily true that the rainfall of any particular 

 storm is equally distributed, especially in the mountains. Nothing less 

 than an exact knowledge of the amount of rain that falls in the different 

 areas will make it possible to tell how much of the water-content found at 

 any particular time in these represents merely the chance differences of 

 precipitation. 



The forms of precipitation are rain, dew, hail, snow, and frost. Of 

 these, hail is too infrequent to be taken into account, while frost usually 

 occurs only at the extremes of the growing season, and in its effect is 

 rather to be reckoned with temperature. Snow rarely falls except during 

 the period of rest, and, while it plays an important part as cover, it is 

 merely one of several factors that determine the water-content of the soil at 

 the beginning of spring. The influence of dew is not clearly understood. It 

 is almost always too slight in amount and too fleeting to affect the water- 

 content of the soil. It seems probable that it may serve by its own evapora- 

 tion to decrease in some degree the water loss from the soil, and from be- 

 dewed plants. If, however, the dew is largely formed by the water of the 

 soil and of the plant, as is thought by some, then it is negligible as a re- 

 inforcement of water-content. From the above, it is evident that rainfall 

 alone exerts a profound effect upon the habitat, and it is with its measure- 

 ment that the ecologist is chieflv concerned. 



i^xV 



