CHAP. XLi LITHOPHILOUS BENTHOS 169 



FORMATIONS 



As the lithophilous Spermophyta are biologically \'ery different from the 

 algae, which are always submerged and are provided with quite other 

 methods of reproduction, lithophilous \-cgetation must be divided into at 

 least these two formations : 



1. Lithophilous Spermophyta. 



2. Algae (nereid formation). 



It may seem subsequently correct to establish several additional forma- 

 tions, for instance, those of mosses and of diatoms. 



I Formation of lithophilous Spermophyta. 



Two remarkable families only, the Podostemaceae and Hydrostachydaceae 

 (which were formerly combined into one) are represented in this formation. 

 They are adapted for existence on submerged rock, in powerfully disturbed 

 water (cascades) and have consequently evolved a number of structural 

 features, affecting their external morphology as well as their internal struc- 

 ture, which are unique in the plant-world.^ 



They are almost confined to the tropics ; they occur in America from 

 Uruguay to the southern United States, in Africa (where many most interest- 

 mg forms are found), in Madagascar, and extend from India to Java ; towards 

 the east they become rarer, so that only a single species seems to hxe in 

 Austraha. Mosses and algae may be intermingled with these Spermophyta. 



2. Formation of Algae (nereid-formation). 



This must be divided into at least two sub-formations : (a) Fresh-water 

 (Limno-Jiereid) ; (b) Marine (Halo-nereid). 



(a) Limno-nereid communities are poorer in species, individuals, and 

 shapes than are the marine ones. In comparison with the latter they display 

 far less luxuriance and variety of form. Nearly all belong to the Chloro- 

 phyceae and Cyanophyceae ; but there also occur diatoms, a very few 

 Phaeophyceae (including Pleurocladia lacustris) and Florideae (Lemanea, 

 Batrachospermum, for example). In accordance with the variety displayed 

 in environment, many associations (and possibly sub-formations) must be 

 distinguished. For example : 



Icy mountain streams have a quite peculiar flora, including Hydrurus, 

 Prasiola fluviatilis, Tetraspora cylindrica, and others. ^ 



On stones in shallow water along lake-shores there is an entirely different 

 flora, with species of Cladophora, Rivularia, and Diatomaceae. 



Sometimes associated with the algae are various mosses, among others, 

 Fontinahs. 



Schroter and Kirchner^ recognize in Lake Constance an cncyoncmduni 

 with different \-arieties, namely, spirogj^'retum, tolypotrichetum, and 

 schizotrichetum. 



Stones near the shores of fresh-water lakes are often encrusted by lime- 

 producing algae."* According to Wesenberg-Lund, incrustations of lime are 

 produced mainly by Cyanophyceae (Schizothrix, Rivularia), but also 

 by diatoms, Chlorophyceae (Cladophora), and the phaeophyceous Pleuro- 

 cladia lacustris. These incrustations occur especially on stones where the 



' See Warming, 1881-1901. * Lagerhcim, 1892. 



' Schroter und Kirchner, 1896, 1902. 



* Chodat, 1902 : Forel, 1901 ; Schroter und Kirchner, 1896. 



