CLIMATIC FACTORS : RADIANT ENERGY 



135 



ply plant needs anywhere on the earth. In fact, light intensities 

 may be too high for some plants to grow in full sunlight, their 

 seedlings being especially subject to injury. Such plants might well 

 be restricted to habitats with partial shade; if their photosynthetic 



FlG. 68. Trenched plot in a loblolly pine stand (40 T 50 yr.) four years after 

 initiation (see Fig. 12). Contrast vegetation on trenched plot with floor of 

 surrounding forest and control plot in foreground.— Photo by C. F. Korstian 



146 



efficiency is insufficient to maintain them in forest shade, they 

 might thrive in regions where light intensity is reduced by cloudi- 

 ness or fog. Probably the range of a species is rarely determined 

 by light intensity alone, however, for it must be remembered that 

 light effects are apparent in several processes and activities, which 

 can rarely be considered independently. The production of chlor- 

 ophyll, the opening and closing of stomata, and the formation of 

 auxins are examples of light-conditioned phenomena with widely 

 differing effects, but these activities must be considered in rela- 

 tion to each other when interpreting plant responses. 



The production of chlorophyll, although (with a very few ex- 

 ceptions) accomplished only in the presence of light, is perhaps 

 more apt to become limiting or significant in high than in low 

 light intensities. Available evidence indicates a greater production 



