Chap. Ill] FRESH-WATER VEGETATION 817 



tracts of Pistia Stratiotes, in which no progressive movement was perceptible. 

 In such places the water was shallow and its flow weak. On rapid streams 

 such floating vegetation is excluded, or consists, as in the case of the 

 floating islands not uncommon in tropical rivers, of constituents of the flora 

 of shallow calm places that have accidentally been conveyed into the 

 current. A plankton consisting of microphytes cannot possibly maintain 

 itself in running water. The investigation of river water has accordingly 

 usually led to negative results ; only in a few cases have some floating 

 Bacillariaceae been discovered. The fact that in almost similar localities 

 at different times results have been obtained sometimes positive and some- 

 times negative, renders it probable that the plankton of rivers (potamo- 

 plankton) consists of the transported constituents of plankton of lakes and 

 ponds. At the most is plankton maintained and capable of further 

 development in very slowly flowing streams or in still bays. 



In calmer streams, as in still water, the macrophytic vegetation is chiefly 

 rooted in the ground ; but in torrents, especially in mountain cascades and 

 cataracts, naturally only lithophytes occur. The rooted plants of the 

 streams of Europe, and, with few exceptions, of the temperate zones in 

 general, are specifically identical with those occupying still water, but are 

 somewhat modified in their structure by the movements of the water. The 

 axes and shoots are always elongated in the direction of the stream, for 

 instance in Ranunculus fluitans and Scirpus fluitans. Potamogeton fluitans 

 develops peripheral bundles of fibres, which are always absent in still water 1 . 

 Moreover, the current arrests the formation of flowers. 



The lithophytes in streams, with one exception about to be mentioned, 

 are solely Musci, Algae, and Lichenes, some of which are characteristic of 

 flowing water, as, for instance, species of Fontinalis and Cinclidotus among 

 Musci. In the tropics, on the other hand, one phanerogamous family, the 

 Podostemaceae 2 , is limited to streams with stony beds, and indeed prefers 

 waterfalls. Only one species, Podostemon Ceratophyllum, is extra-tropical, 

 and occurs in North America. The Podostemaceae are all typical litho- 

 phytes, and, under ordinary circumstances, submerged. 



Fig. 501 represents two species of Podostemon, P. Schenckii, Warm., and 

 P. Mulleri, Warm., which Fritz Muller, H. Schenck, and I collected in water- 

 falls at Blumenau in South Brazil. Neither of these species exhibits the 

 most striking peculiarities of the family. Their shoots spring from riband- 

 like flat roots creeping on stones and attached by haptera ; the roots are 

 coloured deep green by chlorophyll, as is always the case in this family. 

 The part the roots play in assimilation is subordinate in Podostemon, 

 owing to their relatively feeble development, but in some other genera 

 they form the chief mass of tfie vegetative apparatus and consequently 



1 Schwendener, op. cit. 2 Warming, op. cit. 



SCHIMPER 3 G 



