120 TRANSACTIONS OF THE [Szss. LXxv. 
actual river. In a few places there are the silvery mats, 
40 em. in diameter, more or less of Raoulia tenuicaulis, 
the most rapid - growing of the species, thanks to its 
heterophylly and the more mesophytic reversion-shoots. 
The low mat-form (patch-form) of the plant offers a 
minimum of resistance to the wind, its tomentose leaves 
check transpiration, its numerous short adventitious roots 
can secure the warmer water, while its descending main 
root serves as ananchor. Other species of Raoulia present, 
but much fewer in numbers and with similar “adapta- 
tions,” are R. australis and R. glabra. 
If this pioneer colonisation has taken place on a part of 
the bed liable to flood, it can proceed no further, but if the 
ground remain dry enough, or a change towards dryness 
comes about, then other plants arrive and the second phase 
of colonisation begins. 
On many river-beds, especially in the drier localities 
or by the sides of mountain torrents, the Epilobiwm 
association is more important than that just described, £. 
glabellum, of a similar growth-form to EF. melanocaulon, 
being especially abundant, while mats of E. pedunculare 
will be everywhere, and the beautiful EL. macropus where 
shallow still-water lies. 
(¢) STABLE RIVER-BED. 
a. Raoulia Association. 
When for some reason or other a portion of the river- 
bed has got quite beyond the influence of floods, as when a 
stream has receded or terrace-building has provided a dry 
station, then there is a gradual procession of events. First 
of all, a dark-coloured crustaceous lichen coats the stones 
large and small, Epilobium melanocaulon and Raoulia 
tenuicaulis appear, and are rapidly reinforced by R. 
lutescens, R. australis, R. glabra, and, above all, by R. 
Haastii, which soon becomes dominant and gives a striking 
character to the association through the presence every- 
where of its great green cushions, which grow either singly 
or merge into one another. Nor are they confined to the 
flat surface of the river-bed and the small shingle, but they 
spread over the larger stones and even rocks, conforming to 
