CHAP. XLii BENTHOS OF LOOSE SOIL 179 



In lagoons on the coasts of the Danish West Indies, according to 

 Borgesen,^ there are species of Thalassia, Cymodocea, Halophila, and 

 Halodule, creeping algae, such as Caulerpa, and non-rhizomatous Algae, 

 such as Penicillus, Udotea, and Halimeda. 



The vegetation of brackish water on many coasts is closely allied to that 

 just described, but it also includes other more slender species and other 

 genera, such as Chara, Zannichellia, Batrachium, Naias, Potamogeton 

 pectinatus, and Myriophyllum, which reappear more bountifully in 

 fresh water. Several of the species mentioned grow only in shallow 

 water, down to a depth of two metres at most. 



Grass-wracks play an important part in the biology of the sea as homes 

 for marine animals, in connexion with oviposition by fish, and as food- 

 material in the case of Thalassia testudinum, which is eaten by turtles. 



Many sahne waters near salt-works have a peculiar algal vegetation 

 often intermingled with Ruppia and Zannichellia. 



iii. Limnaea-formations. 



To these formations belong aU those fresh-water communities of auto- 

 phytic spermophyta, and other plants of considerable size, the individuals 

 of which live on loose soil, whether sand, clay, or mud, and are completely 

 submerged, or at most possess floating leaves. (Their flowers, however, 

 in nearly all cases rise out of the water). They are therein distinguished 

 from marsh-plants, whose assimilatory organs for the most part project 

 above water. But there is no sharp hmit between these formations. 



I 



' FLORA 



The flora is composed of 

 ' Green Algae, namely Characeae, which occur particularly on marl 

 ! soil, and clothe this with a dense, peculiar-smelling carpet (characetum). 

 Musci, including Fontinahs, Hypnum, and Sphagnum. 

 Pteridophyta, including Marsiha and Pilularia among Hydropterideae, 

 IS well as Isoetes. 

 j Spermophyta, including more numerous species of Potamogetonaceae 

 ' than occur in the sea, of Potamogeton for instance, also Elodea, Vallis- 

 neria and Hydrilla representing the Hydrocharidaceae, Sparganium 

 minimum, S. affine, and other species, in addition to many Dicotyledones 

 such as Nymphaeceae, Cabombaceae, most species of Batrachium, 

 Myriophyllum, Helosciadum, Callitriche, Subularia, Elatine, Montia, 

 Limosella, and others. 



Epiphytes, among which are many Diatomaceae and Cyanophyceae 

 which are often enveloped in mucilage. 



ADAPTATIONS 



The diversity of form among Spermophyta, in contrast to that in 



I the enhalid-formation, is extreme. This is to be attributed to the great 

 variety of environment, which includes not only powerfully streaming 

 water but very often very calm water, sucli as is never met with in the 



' Borgesen, 1900. 

 N 2 



