CHAPTER III 

 FRESH-WATER VEGETATION 



i. Systematic and Oecological Survey. Families of plants in fresh water. Sub- 

 division of forms into five oecological types. 2. Fresh-water Lakes, i. General. Pre- 

 dominance of the photic region, ii. The Differentiation of the Vegetation. I. The Limnetic 

 Benthos of Lakes. Belt-like arrangement in the photic region. Lime-excreting Cyano- 

 phyceae. Dysphotic region. 2. TJie Limnetic Plankton of Lakes. Flora and oecology. 



3. Flowing Water. Floating vegetation. Lithophytes of torrents. Podostemaceae. 



4. Periodic Phenomena of the Vegetation of Fresh Water. Benthos and plankton 

 at different seasons. 5. The Vegetation of Snow and Ice. Causes and distribution of 

 red snow. Sphaerella nivalis. Other microphytes on snow and ice. 



1. SYSTEMATIC AND OECOLOGICAL SURVEY. 



Whilst the macrophytic flora of the sea is composed chiefly of Phaeo- 

 phyceae and Rhodophyceae, to a less extent of Chlorophyceae, and to 

 a very slight degree of Phanerogamae, that of fresh water, at least as regards 

 mass, exhibits the reverse relation. In fresh water Phanerogamae pre- 

 dominate, and are accompanied by Pteridophyta and Bryophyta, which are 

 absent from the sea. The importance of Chlorophyceae may be about the 

 same in salt and fresh water ; on the other hand, in fresh water Phaeophy- 

 ceae and Rhodophyceae are represented only by rare and isolated forms. 



The microphytic flora likewise exhibits great differences in the two kinds 

 of water. True it is that the Bacillariaceae maintain a dominant position also 

 in fresh water, but the Peridinieae, so common in the sea, are represented 

 in fresh water by only a few forms, whilst the Desmidiaceae, which are 

 completely absent from the sea, are very abundant in the benthos and hemi- 

 plankton of fresh water. Cyanophyceae and Bacteria are more generally 

 distributed in fresh water than in the sea. 



The Phanerogamae of fresh water, in contrast with those of the sea, 

 belong to the most diverse orders of Angiospermae. Of special importance 

 among Dicotyledones are the Nymphaeaceae, ail of which are inhabitants 

 of fresh water ; in addition there are the aquatic Ranunculaceae (species of 

 Batrachium), Ceratophyllaceae, Elatinaceae, Callitrichaceae, Podostemaceae, 

 Haloragidaceae (Myriophyllum, Fig. 497, 5), Utriculariaceae. Among 

 Monocotyledones the following families are mainly or exclusively composed 

 of aquatic plants: Alismaceae, Juncaginaceae, Potamogetonaceae, Apono- 

 getonaceae, Naiadaceae, Hydrocharitaceae, Lemnaceae, Mayaceae, Ponte- 



