342 Introduction to Botany. 



herbs, and the trees following the streams, are represented 

 for the most part by forms specially adapted to withstand 

 dry weather. 



Beyond the Rocky Mountains, to the Sierra Nevadas, 

 there is a high plateau where the rainfall is about 30 centi- 

 meters and less, and here we find xerophytes of a more 

 pronounced type, such as cacti. Agaves, and sage brush. 

 Across the mountains, on the Pacific slope, the rainfall in- 

 creases, amounting to about 93 centimeters in the north 

 Pacific states, and to 45 centimeters in the northern and 

 central parts of California, where, however, the rainfall 

 occurs in winter and so occasions a peculiar form of vege- 

 tation especially adapted to these conditions. The shrubs 

 and trees have leaves which are commonly leathery and 

 often entire, and so adapted to reduce transpiration and 

 perform their food-building function through the dry sum- 

 mer season. 



215. Light in Temperate Regions. — The intensity of 

 light diminishes from the equator to the poles; but since 

 the length of the day in summer is greater in the temperate 

 than in the equatorial regions, the total amount of illumina- 

 tion in the twenty-four hours of a summer's day may be 

 greater in the temperate regions. In latitude 30° on the 

 longest summer day the sun is above the horizon 13 hours 

 and 56 minutes; in latitude 50°, 16 hours and 9 minutes; 

 in latitude 60°, 18 hours and 30 minutes; and in latitude 

 66^°, 24 hours. Accordingly, in passing from the equator 

 to the poles, the diminishing energy of the sun which comes 

 to plants in a given period is, in part, compensated by the 

 increasing length of time in which the energy is available 

 each day. 



216. Vegetation in Arctic Regions. — The arctic region 

 is characterized by its low average temperature, its long 



