184 HYDROPHYTES 



ground-algae, which may include vast numbers of Cladophoraceae, with 

 which Diatomaceae and Fontinalis antipyretica may be mingled. 1 Brand 2 

 found a zone of this kind at a depth of 20 metres. Generally, characeta 

 form the deepest zone of macrophytes ; they are ranged in dense sub- 

 lacustral ' meadows ', in which no plants, other than possibly mosses, 

 may find place. In Lake Geneva they descend to 20-25 metres, according 

 to Forel, 3 while in Lake Constance they go down to 30 metres according to 

 Schroter and Kirchner, 4 but the most frequent depth is 8-12 metres. 

 The association next in depth is that of Elodea, which descends to 6 metres ; 

 then succeed submerged species of Potamogeton, including P. lucens, 

 at depths as great as 4-6 metres, and with them may be Myriophyllum. 

 In the next higher zone appear plants with floating leaves, nymphaeeta 

 and nuphareta down to 3-5 metres, and batrachieta to 2-3 metres. As a 

 rule, Spermophyta seem to cease at a depth of 10 metres. 5 



In North America 6 there are zones completely corresponding to these 

 European ones. 



By the shores of shallow waters in Europe, and especially where the 

 soil is sandy, there occurs a peculiar association composed of the limnaea 

 rosette-forms, Lobelia, Littorella, Isoetes, and Subularia, which are charac- 

 terized by shortness of stems and by rosulate leaves that belong to 

 type 2 b described on p. 182. 



The distribution of the associations is determined by 



a. The conditions of depth, light, and clearness of water. Some species 

 can descend to much greater depths than others. The limnaea-com- 

 munities in larger lakes are confined to a zone of slight depth which 

 borders the shore and entertains an abundant fauna. 



b. Distinctions in soil. Some species prefer sand, and others mud. 

 The Characeae are calciphilous, yet they can be found in pools on heaths 

 and moors where the water contains but little lime. 



c. Movements of the water. Some species, and particularly those with 

 floating leaves, live only in placid water. 



Limnaea-vegetation is allied to hydrocharid- vegetation. The boundary 

 between the two is not sharp ; they are often intermingled, and in both 

 there occur genera that are the same, although represented by different 

 species. Certain plants, including Eichhornia crassipes, Stratiotes 

 aloides, Hydrocharis, and Pistia, that usually are free-floating, may 

 on occasion become attached by roots ; and conversely other species, 

 of Ceratopteris 7 for example, that are normally fixed and rooted, may 

 become free-floating. Plankton necessarily must occur in limnaea- 

 formation : the two give rise to a combined formation. There is, of 

 course, no sharp distinction between aquatic plants that are fixed by 

 their roots and marsh-plants ; among them are many ' amphibious ' 

 species, including Polygonum amphibium, that can assume an aquatic 

 or a terrestrial form. Plants such as Montia rivularis living in springs 

 are in a sense transitional between land-plants and water-plants ; they 

 seek water that flows rapidly and is rich in oxygen and carbonic acid. 8 



1 (Schroter und) Kirchner, 1896; G. Huber, 1905. * Brand, 1896. 



3 Forel, 1891. Schroterund Kirchner, 1896-1902. 5 Magnin, 1893, 1894. 



According to Coulter, Cowles, Transeau, and Pieters. ' Gobel, 1889-92, II. 

 8 In addition to the literature already cited, attention should be directed to 



s by Chatin, 1856; Frtih und Schroter, 1904; Pond, 1905; Gluck, 1905-6; 



, 1908. 



