538 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



from the rocks or otherwise prevents them from developing properly. 

 Consequently the types occurring are mostly small, though repre- 

 sentatives of Green and Red as well as Brown Algae commonly occur 

 — including, particularly, crustose Corallinaceae. Only these and 

 other calcareous Algae seem to be unaffected by the almost constant 

 surf and rubbing ice-floes of open coasts, though in sheltered pools, 

 lagoons, and coves the vegetation may be quite luxuriant even in the 

 eulittoral. The sublittoral is again characterized by gregarious 

 Brown Algae of substantial size, commonly including species of 

 Desmarestia and CystospJiaera, down to a depth of some 30-50 metres, 

 with usually an assortment of associated Red Algae. The larger 

 species among the Brown Algae include Lessonia simulans up to 

 5^ metres long, and the general richness is said to be not inferior to 

 that of the arctic sublittoral. Although doubtless some Red Algae, 

 particularly, grow at greater depths, the examples dredged from 

 hundreds of metres down were evidently not growing there, drifting 

 Algae being commonly encountered in the Antarctic as elsewhere. 

 As no large rivers exist in the Antarctic, the discharge being 

 chiefly in the form of icebergs which float far before melting appreci- 

 ably, there is little freshening of the water near the coasts in the 

 manner which apparently impoverishes the algal flora of many 

 boreal seas. On the other hand in both the Arctic and Antarctic, 

 wherever the inland-ice or glaciers extend down and calve into the 

 sea, no eulittoral vegetation can exist and even sublittoral Algae are 

 liable to be injured. Thus vegetation here, and on shores invested 

 with shelf-ice, is chiefly found about stretches of beach that are free 

 from ice in summer. Even on such beaches there is a freezing more 

 or less ' solid ' in winter — which does not, however, preclude the 

 existence of large Algae. 



Aphotic Bottoms 



The average depth of the oceans being computed at nearly 4,000 

 metres, most areas of sea-floor are deep and dark, being reached, 

 if by any daylight at all, only by an insuflicient amount of it to allow 

 the normal growth of benthic Algae. Phosphorescence is entirely 

 inadequate. Such Algae as have been dredged up on occasion from 

 the aphotic zone have drifted there and, being unable to photo- 

 synthesize, will not long persist. Nevertheless, these ocean depths 

 have their flora of saprophytes, parasites, and chemosynthetic 

 organisms that form vegetation of a sort. This deep-sea-floor 



