CHAP. XLII BENTHOS OF LOOSE SOIL 177 



is assumed that not an animal lives in this, and that the organic consti- 

 tuents of the mud are not consumed by animals ; but it is highly probable 

 that a rich anaerobic vegetation of bacteria occurs here. 



In like manner in north European seas and fiords there are muddy 

 spots where saprophytic vegetation may lurk. The black mud so extremely 

 common in lakes, on sea-coasts, as well as in the depths of the sea, is 

 usually very rich in iron sulphide. According to Beijerinck, 1 and van 

 Delden, 2 the reduction of sulphates in water on ferruginous soil is accom- 

 plished by definite anaerobic bacteria, Microspira desulfuricans and M. 

 Aestuarii. 3 



Also in north European fresh-water lakes in deep situations one 

 encounters but scanty vegetation of an elaborate type ; here many lowly 

 organized animals, including worms and larvae, thrive, and the eel waxes 

 fat by preying on them. Here, we anticipate (but do not know) there will 

 be found vegetation consisting of saprophytic Schizomycetes. Accord- 

 ing to Forel 4 , in Lake Geneva at a depth of a hundred metres there is 

 a brownish layer of lowly organized algae, mainly Schizophyceae and 

 Diatomaceae, which thus form an organized carpet. 



ii. Enhalid-formations. 



To this formation belong all communities of Spermophyta, and larger 

 algae growing on loose soil in salt water. 



FLORA 



Of Algae there are very few : in tropical seas species of Caulerpa 5 

 and Penicillus, and in European waters (especially if these be brackish) 

 Characeae, all of which send capillary root-like organs into the soil. (The 

 algae which casually occur here are attached to stones, and belong to 

 the lithophilous formation.) 



Spermophyta, all of which are herbaceous, preponderate in number 

 and dimensions, though the number of species present is small (twenty- 

 seven). They belong to two families : Potamogetonaceae, with Zostera, 

 Phyllospadix, Posidonia, Cymodocea, Halodule, Althenia, also in brackish 

 water Ruppia and Zannichellia ; and Hydrocharidaceae, with Halophila, 

 Enhalus, and Thalassia. Divers epiphytic Algae occur. 



ADAPTATIONS 



The grass-wracks, though belonging to two different families, are 

 externally so alike that mistakes have often been made in the identifica- 

 tion of flowerless specimens. 



The typical form is well illustrated by Zostera ; like this, all the represen- 

 tatives are submerged ; true floating-leaves are wanting, probably because 

 of the violence of the waves ; the leaves, except in Halophila, are ribbon- 

 like, rounded or blunt at the tip, and entire. The ribbon-like or zosteroid 

 form of leaf is well fitted for existence, not only in currents, but also in water 

 that is deep and therefore ill-lighted ; accordingly, it reappears in similar 



1 Beijerinck, 1895. * van Delden, 1903. 



3 See also Warming, 1904. * Forel, 1891. 



5 Svedelius, 1906; Borgesen, 1907. 



WARMING JJ 



