l6] VEGETATIONAL TYPES OF SEAS 521 



It should be noted that whereas a considerable number of marine 

 Bacteria may be found in the pelagic zone associated with the 

 plankton, they are apparently not truly planktonic but attached to 

 other organisms. Some investigators, however, have claimed that 

 Bacteria are present in relatively small numbers in the body of free 

 water, for example at depths of around 5,000 metres, and to that 

 extent must be considered planktonic. But in any case in the 

 ocean, the main concentration of Bacteria occurs in the uppermost 

 few millimetres of bottom-deposit [see below, p. S39)- 



Of macroscopic marine phytoplanktonic phenomena the most 

 remarkable is the drift of certain higher Brown Algae belonging to 

 the genus Sargassum. This familiar genus is predominantly 

 tropical, fairly large, and well differentiated (Fig. 8, H). Although its 

 members occur normally as attached littoral plants, they sometimes 

 become detached and, with the aid of gas-filled bladders, may float 

 far out to sea with the currents. Most benthic Algae even with 

 floats will die in time and disintegrate under this kind of treatment, 

 but two of the species of Sargassum grow exclusively (so far as is 

 known) in this free-floating manner, though apparently they are 

 unable to reproduce except by vegetative fragmentation. The 

 species which thrive in this freely-floating state, Sargassum natans 

 and S. fliiitans, exhibit marked elongation of the branches as com- 

 pared with the normally attached species. With some other seaweeds 

 and animals living attached to or among their branches, they tend 

 to aggregate in one relatively quiet part of the western Atlantic 

 which is accordingly known as the Sargasso Sea, whose surface is 

 largely covered by ' floating meadows ' of these so-called ' Sargasso- 

 weeds ' or ' Gulf- weeds '. In this manner they constitute the only 

 such extensive ' drift ' that is known or likely to occur in the world, 

 fragments of other drifting Algae, such as detached Fucus, Laminaria, 

 or Macrocystis, even though sometimes forming extensive aggregates, 

 being rarely if ever found in a healthy state very far from their 

 shore or ' shallow ' of origin. 



Benthic Environments and Life- Forms 



The general features of the marine environment have already been 

 described in the first part of this chapter, but we must here add 

 some which are of particular importance to the attached plants 

 (benthos). These factors are best treated in four groups, namely, 

 physical, chemical, biological, and dynamic. 



