498 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



and hydrogen sulphide, are subsequently oxidized by chemosyn- 

 thetic Bacteria which utilize, for carbon-assimilation in the dark 

 (using carbon dioxide as the source), the energy liberated in this 

 oxidation. However, under acidic conditions, as in bogs, cellulose 

 is not broken down to methane, and under anaerobic conditions (of 

 lack of free oxygen, such as occur in the profundal of permanently 

 stratified eutrophic lakes) decomposition in general stops short at 

 intermediate organic stages. 



In running water, the slower the current the more closely the 

 benthos approaches in type one or another of the communities of 

 standing water. This is seen in streams in which rapids alternate 

 with ' lentic ' stretches having the same substratum ; for in the rapids 

 the stones are commonly overgrown by bright-green Algae and 

 Mosses, or in warm waters by thalloid Podostemaceae, but in the 

 stretches of slow current the growth at least of benthic lower plants 

 is liable to be markedly less luxuriant. The difference appears to 

 be due to the fact that whereas in quiet water the organisms are 

 surrounded by a film of liquid that soon becomes depleted of the 

 substances they need, in rapid currents the absorbing surfaces of 

 plants are continually brought into contact with new bodies of water 

 and hence with new sources of materials. 



The benthic plants of the so-called torrential communities have 

 to be attached sufficiently strongly to resist the mechanical forces 

 of the current, which may be considerable when it is rapid. Par- 

 ticularly effective and common as a type is the flat thallus applied 

 closely to the substratum. This is well exemplified by many 

 Cyanophyceae and Green Algae as well as by the Red Alga Hilden- 

 hrandia rivularis and many members of the peculiar dicotyledonous 

 family Podostemaceae. Also prevalent are gelatinous layers and 

 hemispherical colonies such as those found in the surf-belt of lakes ; 

 when lime is plentiful they may be held together by it. Attached 

 floating growths must be particularly strong, with powerful hold- 

 fasts, as in the cases of tufts or ' streamers ' of Mosses or the larger 

 Algae, while in the more delicate types such as benthic Diatoms 

 the stalks of inhabitants of swift currents tend to be much shorter 

 and thicker than those of their lentic relatives. This keeps them 

 out of the rigours of the main stream. However, since the velocity 

 of a water-current rapidly decreases as the bottom is approached, 

 and at a minute distance from the bottom theoretically becomes 

 zero, the tiniest organisms can easily remain attached to this bottom 

 or even lie loose and undisturbed. Others can, and often in great 



