xxiii] COLONISATION 289 



Moss Loch, it has been recorded that the floating leaves of 

 Potamogeton natans cover the surface in the parts of the loch 

 which are protected from the prevaiHng winds; where the 

 water is much exposed, however, such broad-leaved plants are 

 absent, their place being taken by Myriophylium^ whose highly 

 divided foliage is uninjured by wave-motion^. Submerged 

 plants, as a rule, form a special zone farther from the shore than 

 the floating-leaf association, because the latter shades the lower 

 layers of the water so much that the subdued sunlight, that 

 penetrates it, is insufficient to supply a deeper flora. An ex- 

 ception to this rule is afforded by Aldrovandia vesiculosa^ a. 

 typical shade plant, which grows among reeds, or protected 

 by the leaves of Waterlilies, in order to secure the dim light 

 which suits its requirements^. 



In addition to the examination of well-established aquatic 

 floras, another branch of the ecology of aquatics consists in 

 the study of the process of colonisation of newly formed waters. 

 We shall return in the next chapter to the methods by which 

 this colonisation is achieved, but we may mention here an 

 account, recently published by a Cambridge botanist^, of an 

 ecological experiment on a large scale which was carried out in 

 the fen country, by Nature herself, not long ago. In January, 

 19 1 5 an area of about 24 square miles became inundated, and 

 remained under water for nine months, until re-drainage was 

 accomplished; it was thus temporarily restored to something 

 like its original aquatic conditions. Even in the brief period 

 in question, water plants invaded the area, but, somewhat un- 

 expectedly, the new flora was confined mainly, as far as flower- 

 ing plants were concerned, to two species, Alisma Plantago and 

 Polygonum amphibium. Those were present in abundance and 

 tended locally to form "closed associations." 



The effect of altitude above sea level upon the water vege- 

 tation, may be considered as coming within the purview of 



1 Matthews, J. R (19 14). 2 Hausleutner, (1850I). 



3 Compton, R. H. (19 16). 



A. w. p. 19 



