136 THE SCOTTISH BOTANICAL REVIEW 



and the range of spray. This may be well studied in the 

 aquatic and semi-aquatic bryophytes and algae of our own 

 moorland streams. 



The plants which colonise the deserted banks and crags 

 are, further, always the same under the same physiographic 

 conditions of exposure and substratum, and differ from the 

 surrounding more completely stabilised vegetation. 



The segments undergoing erosion slowly migrate up- 

 stream, alternating with alluvial stretches laid down on 

 temporary base levels, where lateral erosion has prevailed 

 over vertical cutting. The alluvial deposits of such tem- 

 porary base levels are chiefly composed of sands and gravels^ 

 or torn and washed-down sods, or undermined and subsided 

 masses of peat or soil interwoven with roots. These various 

 forms of alluvial deposits soon become colonised by plant 

 associations differing completely from the surrounding 

 vegetation. 



Parts which temporarily escape renewed erosion, form 

 caps to a succession of terraces, as vertical cutting is renewed. 

 So long as they remain subject to flooding, or differ in their 

 physiographic relations to moisture, plant food, etc. , from 

 the neighbouring stable areas, so long will they bear character- 

 istic plant associations, and meadow, marsh, bush, swamp, or 

 other associations hold the ground, but with time the climatic 

 soil-relations are restored and the ground is invaded by the 

 associations of the dominant stable formations (26). 



The lakes which are interposed in the stream belt form 

 rather more permanent base levels to erosion, and the 

 vegetation of their waters and shores differs considerably 

 from that in, or flanking running water. The plankton is, 

 of course, more abundant, and probably varies much with 

 the nature of the drainage basin of the entering streams, and 

 with the physiography of the lake, as affecting the food 

 supply, the depth, temperature, and other conditions of the 

 water. The floating and submerged aquatic flora differs 

 much from that of running streams, and varies greatly in 

 the high-level tarns and moorland and lowland lakes (39), (40). 

 The marginal plant associations further differ, according to 

 the rise and fall of the waters, the amount of incoming silt, 

 sand, or gravel, and other factors which require investigation. 



