144 THE SCOTTISH BOTANICAL REVIEW 



mountain top debris, apart from its porosity, is chiefly the 

 result of the intense evaporation and erosion caused by wind. 

 Scree plants are those that can find their water at a depth 

 by means of rhizomes and can send up shoots to the light of 

 day despite the slipping of the surface. The plants of the 

 plateaux debris, on the other hand, depend chiefly on their 

 powers of resisting desiccation, erosion, and bodily removal 

 by the wind. Block screes are highly favourable for the 

 growth of special types of lichens and bryophytes. The 

 blocks long resist removal from the position where they 

 come to rest, but the surfaces are exposed to the maximum 

 of light and atmospheric precipitation and undergo slow 

 decay, while the general surface is for long quite uninhabit- 

 able by higher plants. 



Rock formations are mainly the results of stream erosion, 

 coastal erosion, or position in relation to gravit}', frost, and 

 poverty of vegetation. The\' may therefore be roughly 

 classified into : (i) Stream gorges and waterfalls, (2) Coastal 

 cliffs and platforms, (3) Alpine crags. 



All these are naturally migratory under the influence of 

 the geological agents, their surfaces being liable to destruction, 

 whereupon a fresh initiation of plant succession is invited (44) 



(3) (26). 



In the case of stream gorges the type of rock vegetation 

 furthest removed from the influence of the stream depends 

 on the physiographic relations of the rock to the sun, wind, 

 and frost, and its chemical and physical relations to leaching, 

 water capacit}', and drainage. The lower parts of the gorge 

 under the influence of spray, flooding, or erosion, each have 

 their own type of vegetation, which may further depend on 

 the amount of light, the aeration, and the acid, neutral or 

 alkaline nature of the water afforded by the up-stream part 

 of the drainage basin. 



The vegetation of sea-cliffs is perhaps even more complex, 

 since the late relative displacements of sea-level and the 

 structure of the rocks in their relation to sea erosion in 

 forming vertical cliffs plunging directly into deep water, or 

 more irregular cliffs with the accumulation of beach at their 

 base, has a marked influence on the range of the vegetation 

 and its composition (26). 



