l6] VEGETATIONAL TYPES OF SEAS 513 



green rays of the depths. The deep-water Red Algae, on the other 

 hand, are capable of growing under conditions of much lower light- 

 intensity, being able to utilize precisely those deeply penetrating 

 rays of shorter wave-lengths. Only when the light is sufficiently 

 weakened, do many of these deep-water forms appear able to grow 

 well in the red-containing light of the surface. Diatoms tend to 

 flourish best under relatively low light-intensities where the red 

 component is much reduced, often having their maxima at around 

 10 metres' depth and forming rich growths at 15 to 20 metres where 

 chiefly green and some blue rays prevail. 





Supralittoral>>\^^^ 

 Eulittoral(oftidesr^Lo 



Sublittoral (of large_^ 

 vegetation ) 



Abyssal-benthic 

 (of practically uniform conditions) 



Fig. 170. — Diagrammatic representation of typical sea-marginal profile. 



The motion of the water is another factor important to marine 

 vegetation, leading as it does to the occurrence of largely different 

 forms on surf-pounded and sheltered shores. Thus in exposed 

 situations, quite apart from the need for strong holdfasts and 

 ' leathery ' thalli to prevent detachment and injury by breakers, the 

 moving water tends to be better aerated than on sheltered shores 

 and consequently to favour increased biological activity. Moreover 

 sea currents, whether regular or irregular, commonly carry Algae, 

 often for considerable distances, and can be one of their chief 

 agents of dispersal, though for most benthic forms death follows 

 any prolonged period of detachment. 



For plankton, although mixing of waters by upwelling or con- 

 vection or other currents is widely important, a goodly supply of 

 nutrients being necessary for rich development, some degree of 



