Laml Plants 233 



conceivable that the supply could have resulted from such tenuous 

 food chains as are described above, even if the fish were able to retain 

 the vitamins in their bodies as mammals are not. Nor has any method 

 been shown by which an animal could manufacture vitamin D with- 

 out the presence of ultraviolet light. The actual source of the large 

 quantities of vitamin D in the fish liver oils thus remains a mystery. 

 It emphasizes again how little we know about ecological influences 

 of this region of the spectrum, both in the water and on land. 



Ecological Aspects of Photosynthesis 



Light is fundamentally important as the essential direct source of 

 energy for the growth of all green plants and the purple bacteria. 

 The photosynthetic plants form the first step in the ecological cycle 

 in every natural situation. They are the first link in the food chain 

 and hence are the base of the production pyramid that will be dis- 

 cussed in greater detail later on. 



Land Plants. On the surface of the land enough light is received 

 everywhere for the growth of plants of some sort. Within the soil 

 and in the interior of caves light is insufficient to allow photosynthesis. 

 A striking illustration of the limiting action of the light factor is found 

 in caves to the walls of which electric lamps have been secured for 

 the benefit of tourists. Within the cone of light immediately around 

 each lamp mosses have grown from spores brought in by air currents. 

 These "islands" of plant growth present a sharp contrast to the com- 

 plete Idck of vegetation in the remainder of the cave. 



Although illumination is generally sufficient for photosynthesis over 

 the entire surface of the globe, the local distribution of individual 

 species of plants is definitely influenced by differences in the avail- 

 ability of light. Species that can grow in shady places are termed 

 tolerant, and, although the degree of tolerance is affected by soil 

 moisture, temperature, and other factors, the illumination is the prin- 

 cipal controlling influence. Plants that require strong illumination 

 and will not survive or develop in reduced light are referred to as 

 intolerant species. Trees such as the spruce, hemlock, beech, and 

 sugar maple, shrubs, such as spicebush, and herbs, such as blood- 

 root, can grow in deep shade and are examples of tolerant species. 

 The sugar maple can photosynthesize adequately in situations where 

 the illumination is reduced to less than 2 per cent of full sunlight. 

 Birch trees, poplars, willows, and several species of pine, as well as 

 many shrubs and herbs such as sumac, bluestem grasses, and sun- 



