STABLE AND MIGRATORY PLANT FORMATIONS 1 35 



requiring separation from other rock habitats and from the 

 moorland and woodland flushes, etc. , on soils capable of 

 easy erosion. 



The moorland flushes show relations to spring associations 

 on the one hand, and to the grassland of the sandy river 

 alluvia on the other, and many so-called bog plants are 

 practically confined to these flushes and the alluvial stretches. 

 The flushes form a connecting link and a route of migration 

 between the springs and the grassland and swamps of the 

 higher stream alluvia. On the slopes of some of our 

 mountains the flushes are so numerous and so constantly 

 migrating from one point to another, that large areas of 

 grassland are formed through their influence, as mentioned 

 above. 



The flushes arising from snow lie have well - defined 

 characters of their own, as pointed out in the last number 

 of this Review (41). 



Flushes are also encountered in woods, where they show 

 a complete change in the plant association, including 

 the trees. In this country the wet flushes in woods are 

 commonly dominated by ash trees in oak woods and by 

 alders in birch woods. The most frequent trees in flushes 

 are the ash, alder, various forms of sallow and bird-cherry, 

 and sometimes hawthorn, blackthorn, or hazel, but many 

 flushes form open gaps in the woods occupied by what might 

 be termed a spring-marsh vegetation. These woodland 

 flushes are recruited from spring associations and from the 

 bush swamps of the river alluvia. 



Not only has each type of river its natural history, but 

 so has each segment of its course. The source in a snow 

 patch, spring, or tarn, or from water-logged peat, aff"ects in 

 different ways the upper drainage waters and the plant 

 associations. In their middle courses rivers form rock-chasms 

 and waterfalls ; but again there are differences in plant 

 association, from differences in the nature of the waters or 

 the rocky stream channel. The submerged and amphibious 

 associations of the waterfalls, rocky gorges and boulders, 

 always show a close zonation, depending upon the force and 

 erosive power of the stream, the aeration of the water, the 

 amount of light, the depth and permanence of the stream, 



