STABLE AND MIGRATORY PLANT FORMATIONS I4I 



reed-swamp, the while the gradual eastward extension of a 

 spit of shingle from the "Crumbles," further and further, 

 embayed the silts brought down by the streams, and turned 

 the exit of the drainage further and further to the east. The 

 greater elevation of the most recently formed fulls may point 

 to a slight rise in sea-level since the marshes were reclaimed, 

 but the levels have in all probability recently subsided over 

 wide areas owing to artificial drainage and interference with 

 the rivers at their head waters and points of exit seaward. 

 Salt water still percolates through the shingle barrier at high 

 water and affects the plant association in the adjoining 

 marsh ditches. 



Apart from areas of wind erosion, sand only accumulates 

 at temporary base levels in the swifter courses of streams, in 

 lakes entered by such streams, and in bays or near river 

 exits on exposed coast-lines. The plant associations which 

 take possession of the sand depend partly on its mineral 

 composition and humus content, but chiefly on its physio- 

 graphic relations to flooding or water-level, the fairly constant 

 or rapid rise and fall of the latter, and the nature of the 

 water, stagnant or otherwise. Plant associations on sand 

 alluvia may vary, according to these conditions, from open 

 formations to grassland or heath, from marsh to bush swamp. 



Accumulating sand is subject to wind drift and dune 

 formation, but this only occurs to any extent in this country 

 in connection with lakes and the sea coast. The plant 

 associations following such blown sand are of an entirely 

 different nature to the stable vegetation on sands in this 

 climate. The former are specialised for obtaining their 

 water supply at a depth and for resisting sand erosion and 

 overcoming sand burial (43), whereas the latter are plants 

 that can live on superficially leached soils subject to periods 

 of drought and with a limited tendency to accumulate acid 

 humus. 



Flood gravels and shingle beaches are peculiar to the 

 swifter reaches of rivers and to sea coasts. Those of recent 

 accumulation rarely shelter any vegetation, but where less 

 liable to frequent movement they are generally covered with 

 species of encrusting lichens capable of growing on very 

 smooth, hard surfaces subject to periods of drought and 



