Vegetation of Northern Cape Breton. 369 



the association-complexes of wet or dripping rock outcrops, 

 since these are especially characteristic of, though by no means 

 confined to, ravines. 



b. THE ASSOCIATION-COMPLEXES OF RAVINES 



The stream bed association-types. — The predominant plants in 

 the rocky stream beds which prevail in ravines, and to a large 

 extent elsewhere, are bryophytes. Characteristic species are the 

 following : 



Marsupella aquatica Oxyrrhynchium rusciforme 



Jungermannia cordifolia Hygrohypnum dilatatum 



• Scapania undulata Hygrophynum eugyrium 



Porella pinnata Hygrophynum ochraceum 

 Fontinalis dalecarlica 



The degree of luxuriance exhibited by the submersed 

 bryophytic vegetation varies greatly in different streams. The 

 aquatic mosses and liverworts are best developed in small brooks ; 

 in large streams they may be conspicuous by their absence. 

 This latter fact may be explained somewhat as follows. The 

 instability of the substratum might account for their absence on 

 small boulders and cobbles, but even where there is a firm rock 

 substratum, bryophytes are scarce. There seems little question 

 that, in general, the scarcity is attributable to mechanical factors 

 — to the erosive action of the sediment-laden water in flood time, 

 or, more likely, of ice-laden water in spring. Were the 

 phenomenon restricted to northern Cape Breton, one might feel 

 tempted to correlate it with the acidity of the water, which in 

 most streams commonly contains so much organic matter that 

 it is colored yellow or brownish (see, in this connection, Ganong 

 '98) ; but the same conditions can be observed in other regions, 

 e. g., in Connecticut streams, where the water is clear and color- 

 less. Often the rocky bottom in swift streams is utterly devoid 

 of plants of any description, but sometimes, in the absence of 

 mosses, there may be a considerable growth of Nitella sp. 



Stream bank association-types. — Along small ravine brooks 

 the banks, as a rule, are well shaded by overhanging foliage, 

 the air is always cool and moist, and the substratum con- 

 tinuously damp or wet. It is doubtful whether the plant cover 



