Aquatic Plants 237 



When algae are grown in the laboratory in light of different colors, 

 they often tend to change color, and this response has also been 

 referred to as chromatic adaptation. Experiments have shown that 

 individual plants photosynthesize most efficiently and grow best in 

 that color and in that intensity in which they have been living. In 

 the course of time fixed algae can adapt themselves to habitats with 

 illumination of widely different conditions of color and intensity. In 

 view of the great range in the transparency of natural waters, it is not 

 surprising that the maximum depth at which attached plants can live 

 varies widely in different situations (Table 15). In general benthic 



TABLE 15 

 Maximum Depth in Meters for Growth of Attached Plants 



plants will not grow at depths at which the light intensity is less than 

 0.3 per cent of the surface value. 



A glance at a chart of the oceans or of a typical lake reveals that 

 only in a very narrow fringe around the margin is the water suffi- 

 ciently shallow for enough light to reach the bottom for the growth of 

 plants. We are forced to the realization that in any deep body of 

 water far more organic matter is synthesized by the phytoplankton 

 present everywhere in the surface layers than by the benthic plants 



