167 



CHAPTER XLI. LITHOPHILOUS BENTHOS 



This type of vegetation is confined to rocks, loose stones, mollusc shells, 

 and similar solid substrata near shores and banks. Many of the species 

 growing on these substrata also li\'e as epiphytes. 



FLORA 



The salt-water communities are solely composed of algae, which here 

 reach their highest and richest development in all four colours (blue-green, 

 pure green, brown, and red), and exhibit extraordinary variety of form. 



The fresh-water communities, though much poorer, consist of algae 

 (nearly entirely Chlorophyceae, Cyanophyceae and Diatomaceae), of mosses 

 (Fontinalis, Dichel3nTia, Cinchdotus, and others), and of Spermophyta, 

 especially Podostemaceae. 



The chemical nature of the substratum plays a part that, so far as is 

 known, is shght and solely concerns the presence of calcium. Some algae 

 flourish only on Ume, which is perforated and corroded in furrows by their 

 hypha-hke filaments.^ Most of the others grow equally well on stones, 

 piles, shells, or on other algae. In addition, the inclination, illumination, 

 and physical nature of the substratum (rock, stones, or calcareous shells) 

 has its influence on the distribution of species.- According to Wille a sub- 

 stratum of shells is distinguished by special algal associations, for instance, 

 by Tilopteridaceae. 



ADAPTATIONS 



The general pecuharities of submerged hydrophytes, such as reduction 

 or absence of stomata, of lignified constituents, and of wood-vessels, the 

 production of assimilating chromatophores in the outermost layer of cells, 

 and so forth, have already been dealt with.^ The assimilatory tissue extends 

 to the surface ; but beyond this many algae possess an internal assimilatory 

 tissue, which undoubtedly utilizes the carbon dioxide produced by 

 respiration.^ 



Specialized adaptation is revealed in the undermentioned directions : 



Solidity of the substratum necessitates the possession of haptera,^ which 

 m connexion with algae are sometimes described as roots though they 

 arc widely different from true roots. They occur in two forms ; as circular 

 disks, in Fucus vesiculosus and Laminaria soHdungula and others, or as 

 branched digitate or coralloid structures, in Laminaria saccharina and 

 Agarum Tumeri and others. Here, too, may be placed the tufted rhizoids 

 of Fontinalis and of other aquatic mosses. The adaptive means of fixation 

 ha\'e been discussed by Wille.^ 



Haptera in some cases have the structure of root-hairs, but in others 

 they are solid, multicellular bodies. The firmest attachment is exhibited 

 by such crustaceous algae as Lithothamnium, Lithophyllum, Hilden- 

 brandtia, and Lithoderma, which form an incrustation on rock. Diato- 

 maceae and Desmidiaceae that are fixed to other bodies by mucilage belong 

 to a special type. 



' Chodat, 1902; Lagerheim, 1892; Colin, 1893; Nadson, 1900 ; M. le Roux, 



1907 ; P. Boysen-Jenscn, 1909. ' Sec Borgesen, 1905. 



^ See Chap. XXVIII. * Wille, 1885. 



' Warming, 1881-1901. " Wille, 1885, and others : see Oltmanns. 1905. 



