148 



Science of Plant Life 



:^- 4y > 









79. 



Fig. 85. Eliects of wind on trees along the Bay of Fundy, Nova Scotia. 



The light. The amount of light available to a plant de- 

 pends primarily upon the intensity of the sunshine. This is 

 greatest in the tropics and least at the poles. The total 

 amount of light is influenced also by the length of the day. 

 At the equator the daylight lasts 1 2 hours ; at the poles 

 the light continues all summer. So tropical plants have in- 

 tense Hght during half of each day, while arctic plants have 

 weak light continuously through the growing season. 



Locally the light is modified by clouds and fogs. These are 

 much more prevalent along the seacoast than inland, and are 

 particularly common along the Pacific coast from Alaska to 

 Cahfornia. The slope of the land, especially in mountain 

 regions, may increase or decrease the intensity and the length 

 of daylight. Finally, plants may have their light reduced or 

 cut off by trees or other objects. 



