8i4 ZONES AND REGIONS [Pt. Ill, Sect. V 



hitherto unexplained manner with the formation of tortuous furrows on the 

 surface of the stones 1 (Fig. 499). 



Although a considerable literature exists in reference to ' sculptured stones ' in 

 lakes, two very different theories confront one another regarding their origin, both 

 of which may perhaps be true, as in this case similar phenomena may arise from 

 dissimilar causes. Kirchner, who investigated chiefly the stones of Lake Constance, 

 considers that the coating of Algae, by means of its calcareous investment, protects 

 stones against the solvent action of water, and that furrows arise wherever insect 

 larvae destroy the coating. Chodat, on the other hand, who observed the stones in 

 Lake Geneva, includes these investing Cyanophyceae among ' calcivorous ' or lime- 

 dissolving Algae, the existence of which has been clearly demonstrated in other 

 cases. The Cyanophyceae on sculptured stones observed by Kirchner were recog- 

 nized by him as Schizothrix fasciculata, Gomont, Calothrix parietina, Thuret, and 

 Phormidium incrustatum, Gomont ; Chodat mentions species of Schizothrix as 

 lime-dissolving. 



Microphytes also form the essential vegetation of the narrow emerging 

 belt of lakes. Kirchner mentions as occurring in the spray-zone, Cyano- 

 phyceae, Bacillariaceae, and only one larger algal form, Spirogyra adnata, 

 Kiitz. 



The dyspJiotic region of the fresh- water benthos as a rule possesses only 

 a microphytic vegetation consisting of Bacillariaceae, Oscillarieae, and Beg- 

 giatoeae ; exceptionally Chlorophyceae appear (Scenedesmus, Pediastrum), 

 and Peridinieae (in Lake Zurich at a depth of 60-90 meters according 

 to Imhof-); only once, by Forel '■'', was a higher plant discovered, namely 

 a moss, Thamnium alopecurum, var. Lemani, in Lake Geneva at a depth of 

 about 60 meters. Some species appear to be true denizens of the depths, 

 or dyspJiotic plants ; but the physiological characters leading to such a mode 

 of life are unknown. In the greatest depths investigated in Lake Constance 

 (160 and 240 meters) the mud was free from any vegetation except a few 

 individuals of the diatomaceous Cymatopleura Solea, Brebisson. This dis- 

 covery shows at all events that organisms dependent on light are capable of 

 existing at these depths. The dysphotic flora of Lake Geneva seems to 

 be somewhat richer, as Forel speaks of an organic coating in which Diato- 

 meae predominate. 



2. The Limnetic Plankton of Lakes. 



The hemiplankton of fresh water includes some phanerogams, those of 

 the Hydrocharis type. On the contrary the true plankton, as in the sea, is 

 exclusively formed of microscopic species of Algae, which differ or at any 

 rate occur in different proportions in the various lakes or even in the 

 various parts of the same lake, so that the limnetic plankton exhibits 



1 See Schroter und Kirchner, op. cit. ; Chodat, II. 2 Imhof, op. cit. 



3 Forel, op. cit. 



