106 



PLANT SOCIOLOGY 



Wiesner concluded that the Ught requirement of a plant is greater 

 the nearer it approaches its polar boundary, but it must not be for- 

 gotten that Wiesner's method measured heat rays along with light. 



The light climate of the far north is characterized by a great uni- 

 formity in the intensity of the light and by its increased duration. The 

 intensity of direct sunlight is low, because of the low altitude of the sun. 

 The importance of diffuse light, however, increases greatly from the 

 tropics to the poles. The ultra-violet radiations unfavorable to plants 

 are greater toward the poles than in Central Europe or in the United 

 States. In consequence, the boreal and arctic plants possess adapta- 



FiG. 58.— Mean annual cloudiness. Degrees of (.loudiness 1-10. (After Szymkirwicz.) 



tions against too intense and too prolonged insolation. These consist 

 of reduced chlorophyll content, red and yellow pigments in cell wall and 

 in cell sap, and waxy coverings which reflect the hght (Lippmaa, 1929). 

 The greatest intensity of direct sunlight as compared with diffuse 

 light is found in the equatorial zone. Hence in this region the effect 

 of light upon vegetation is most pronounced. In the deserts and 

 steppes of the subtropics the total light is not so strong as would be 

 supposed (Riibel, 1910). This is doubtless due to the dust content of 

 the air, which is seldom cleansed by rain. But in these regions the 

 summation of light reaches significant values because of the very slight 

 cloudiness. The mean cloudiness at 60° N. lat. is 61 per cent; at 

 30° N., 42 per cent, at the equator 58 per cent; at 30° S., 46 per cent; 

 at 60° S., 75 per cent. The extreme cloudlessness of arid regions is still 

 better shown in Fig. 58. 



