Subaerial associations 419 



dependent upon the geological formation. In both small and large bodies 

 of water the algal associations often change with the seasons, so that a 

 succession of associations may occur in the same habitat. 



In recognition of the primary importance of habitat the subject is dealt 

 with under the following headings : 



I. SUBAERIAL ASSOCIATIONS. 



II. ASSOCIATIONS OF IRRORATED (or dripping) ROCKS. 

 III. AQUATIC ASSOCIATIONS. 



I. SUBAERIAL ASSOCIATIONS. 



Many freshwater Algae live under subaerial conditions, having adapted 

 themselves to a life in a damp atmosphere. Most of them are able to 

 survive a considerable period of desiccation, and it is amongst these subaerial 

 types that one meets with what is, perhaps, the nearest approach to an actual 

 plant-formation among freshwater Algae. 



1. ProfococcMS-formation. This is the most general and at the same 

 time most distinctive algal ' formation ' in north temperate regions. It 

 consists of a bright green incrustation of Protococcus viridis Ag. (= Pleuro- 

 coccus vulgaris auct.) covering the windward side of tree-trunks, branches, 

 walls, palings, etc. It is most conspicuous in those areas in which the annual 

 rainfall exceeds 30 inches, but may be almost equally abundant in low-lying 

 damp areas with a less rainfall, as in the fen-districts of the east of England. 

 The Alga is perennial and in some cases has a slight admixture of Stichococcus 

 bacillaris Nag. In parts of Canada the latter may almost entirely replace 

 the Protococcus. 



2. Zygnema m'ceforwm-formation. Extensive felt-like mats of this Alga 

 occur on the surface of the ground on heaths and moors, more particularly 

 where the soil is peaty. On almost pure peat-soil the algal cells develop 

 phycoporphyrin so that the mats assume a purple colour. In the British 

 Islands Zygnema ericetorum occurs at all elevations, but is seen in greatest 

 abundance on peat-moors, such as those of the Pennine Chain. On damp 

 heaths the Alga is not so conspicuous. It is perennial and can be found at 

 all times of the year, although its maximum activity is in the late spring. 

 As a rule it remains throughout the year in a purely vegetative condition, 

 but in the winter months aplanospores may be formed (West & Starkey, '15). 



3. Prasiola-foTm&tiou. This is very limited in extent except on certain 

 sea-coasts. Prasiola crispa, which is the commonest species, is a perennial 

 Alga able to withstand much desiccation, especially in the ' Hormidium- 

 state.' It requires an abundance of nitrogen and is a common Alga in towns 

 and villages. It sometimes occurs in quantity in the haunts of sea-birds, 



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