Chap. Ill] FRESH-WATER VEGETATION 811 



is always that of aquatic plants, whilst the above-mentioned semi-aquatic 

 plants form a transition stage to terrestrial vegetation as regards habitat. 



The above classification depends in the first place on distribution as 

 regards space that is so important in the study of formations. From other 

 points of view, it is natural that other methods of grouping should be pre- 

 ferred. I have therefore nothing to urge against the classification made by 

 Schenck \ but it corresponds less to distribution in space than to other no 

 less important oecological characters. 



Macrophytic fresh-water Algae are more or less of the Naias type. This 

 is true in particular of the Characeae, which show a great likeness to Naias 

 in their mode of life. 



Microphytic Algae and the few saprophilous aquatic Fungi are some of 

 them fixed to the substratum or to other plants of the benthos, and others 

 float or swim freely in the water as plankton. 



2. FRESH-WATER LAKES. 

 i. GENERAL. 



In contrast with their behaviour in salt-water seas the majority of macro- 

 phytes are rooted to the ground in fresh water. Lithophytes are very rare 

 among the higher and larger plants (Podostemon type), whereas many 

 filamentous Algae grow fixed to stones. The latter also sometimes occur 

 as epiphytes. This mode of life is specially exhibited by small superficially 

 extended Algae (Coleochaete and others). 



The deep regions of the vegetation in fresh water resemble those in the 

 sea ; but, in accordance with the smaller extent of the former, the photic 

 region occupies a relatively greater area, and is alone represented in small 

 tracts of water, such as ponds, ditches, and brooks. 



The boundary between the photic and dysphotic regions lies at very 

 different depths, varying from 5 to 30 meters, according to the variations in 

 the turbidity occasioned by particles in suspension. The flora of the dys- 

 photic region is composed almost exclusively of microphytes, Diatomaceae 

 in particular ; but exceptionally a few macrophytes have been observed in it. 

 The flora of the aphotic region is not known ; it is at all events very poor. 



Still and flowing water afford very different conditions to vegetation, and 

 are therefore dealt with separately. 



ii. THE DIFFERENTIATION OF THE VEGETATION. 



Fresh-water lakes represent the type of motionless fresh water, while 

 shallow water of ponds and ditches, as regards vegetation, is for the most 

 part allied to that of the calm bays of salt seas. In any case many small 

 sheets of water possess a peculiar flora not yet observed in lakes ; for 



1 Schenck, I. 



