174 HYDROPHYTES sect, iv 



Movements of the water, on the contrary, are of great morphological 

 and floristic significance. 



Salinity of the water is of even greater import. Vegetation in the sea 

 is morphologically and oecologically very different from that in the majority 

 of fresh waters (rivers, lakes, and pools). 



Loose soil, in contrast to a stony substratum, entertains very few 

 algae, but mainly Spermophyta. 



ADAPTATIONS 



Of the modifications of vegetation in such environment we may note 



Roots, or root-hke organs branching in the soil, serve to attach the 

 plant and absorb nutriment ; apart from these, special organs of fixation 

 are lacking. Roots do not attain the dimensions or degree of branching 

 displayed by land-plants, and some are devoid of root-hairs,^ for instance, 

 Hippuris (excepting at the 'collar'), Elodea, Hottonia and some others. 

 Some tropical algae (species of Udotea, Halimeda, Penicillus ^) on sandy 

 soil and mud are fixed to the loose soil, and obtain nutriment therefrom 

 by means of hypha-like hairs attached to the lower parts of the thallus 

 that penetrate the mud ^'^ ; the same is true of Characeae. 



Horizontal rhizomes, or their analogues (in Caulerpa, for instance), 

 creeping on or, more usually, in the soil are very common, and bring into 

 existence a dense vegetation composed of numerous social individuals, 

 such as is exemplified by the submarine ' meadows ' of Zostera and 

 other ' grass-wracks '. This mode of growth clearly harmonizes with the 

 loose texture of the soil.^ 



Gaseous interchange on the part of submerged plants is aided by the 

 large air-containing intercellular spaces which are peculiar to aquatic plants.* 



FORMATIONS AND ASSOCIATIONS 



Three formations may be distinguished : i. microphyte-formation ; 

 ii. enhalid-formation; iii. limnaea-formation. 



i. Microphyte-formations. 



Pure associations of microphytes, particularly of Cyanophyceae appear 

 in extreme circumstances, chiefly in hot springs, and in the shallow 

 beds of seas and fresh waters where organic constituents abound ; also 

 not infrequently in richly humous shallow waters of heaths. These 

 microphytic communities differ so widely from the limnaea-formation 

 that they must be regarded as constituting a distinct formation, or 

 perhaps two the autophytic and the saprophytic in which several 

 sub-formations may be distinguished. 



A. Autophytic Microphyte-formation. 



I. Sub-formation living in hot springs. This occurs in various 

 parts of the Earth. The temperature varies widely in these springs ; 

 at relatively low temperatures Phanerogamia still occur in them, but 

 high temperatures exclude all plants save Cyanophyceae (Beggiatoa, 

 Lyngbya, Oscillaria, Hypheotrix, and others). Many of these species 

 are ubiquitous. They form green, yellow, white, red or brown mucila- 



* See p. 97. ' Borgesen, 1900. ^ See p. 42. * See p. 98. 



