Jan. 1911.] BOTANICAL SOCIETY OF EDINBURGH 119 
bare shoot-axes have put forth roots, and the fifth, the 
final zone, which consists of creeping and rooting stems. 
These latter only give off roots in relation to wet soil, as 
when growing on silt or peat, since they are quite rootless 
when extending on a bare rock-surface. 
The species of Raouwlia show an interesting epharmonic 
gradation of forms from the rapidly-growing creeping 
patches or mats of Raoulia tenuicaulis, with its open 
mesophytic leaves of seedlings and reversion-shoots, to the 
highly differentiated dense woolly masses of the vegetable- 
sheep (R. eximia, ete.), denizens of wind-swept and sun- 
scorched alpine rocks; and it is easy to see how the present 
species have been evolved from mesophytic creeping herbs 
by xerophytic conditions arising from migration to xero- 
phytic stations, or, what is more likely, in the first instance, 
a dry climatic period. 
5. THE PEOPLING OF THE RIVER-BED. 
(@) GENERAL. 
The peopling of the river-bed, which station, it must be 
remembered, is absolutely new ground, resolves itself into 
several distinct stages, each of which is really a valid plant- 
association, always present on the river-bed, and although, 
like any presumedly stable plant-formation, capable of 
change, is just as much a regular feature of the landscape 
as is a forest. 
(b) THE UNSTABLE BED (Epilobiwm Association). 
On that part of the river-bed subject to occasional 
flooding there are, at wide intervals apart, occasional plants, 
about 16 em. tall, of Epilobiwm melanocaulon (Onagrac.), 
conspicuous through its dark purple stems and usually 
reddish leaves. The wind-borne seeds, their rapid germina- 
tion and the quick growth of the seedlings, enable this 
species to speedily establish itself. Although the station 
is fully exposed to the wind, the stones offer abundant 
protection to the quite tender young plants, and this applies 
to every part of the river-bed. Their roots, although short, 
will quickly reach the water under the uppermost stones, 
and this will frequently be much warmer than that of the 
