112 TRANSACTIONS OF THE [ Sess. Lxxv. 
of the river's flow is subterranean. The presence of the 
underground stream makes itself plainly visible in flood- 
time through the rapid formation of new streams and the 
filling of dry channels, while, on the other hand, a diminu- 
tion of the supply, as in winter, may cause the suppression 
of the streams altogether. A river-fan is frequently quite 
dry, yet at its termination streams gush forth. Similarly, 
some river-beds are normally without surface-water in one 
part, while in another, higher up, there will be a rapid 
stream which, all on a sudden, disappears beneath the 
surface. 
The river-water, issuing as it does from the ice, and 
in large part the result of its melting, is, at most, but a 
few degrees in temperature above the freezing-point, and 
is therefore of little moment for plant-life. Nor can the 
underground stream be much warmer. A glacial river-bed, 
then, near its source, appears to be both a physical and 
physiological dry station. The physical dryness seems 
in itself sufficient to govern the plant-life, smce far from 
the river’s source in the lowlands, where the river-water 
is much warmer, the plant-life is very similar in character, 
many species being identical, as well as growth-forms; 
indeed, on the whole, the subalpine river-bed, owing partly 
to its more abundant rainfall, is the less xerophytic station 
of the two. In winter, when the streams dry up more or 
less, the subterranean water must be considerably lowered, 
and lie beyond the reach of most of the plants. 
How far the heat from the sun can penetrate into the 
soil I cannot say, but probably to no noticeable depth, 
except between the chinks of the stones. These latter 
become so strongly heated during a period of insolation 
that one can hardly bear to touch them with the hand, 
and the reflected heat must be very considerable so far 
as the ground plants are concerned. On the other hand, 
the stones rapidly lose their heat when the sun’s rays 
are obscured. 
As for aeration of the soil, that will be abundant. 
From the above it is evident that a river-bed is a 
strongly xerophytic station, and that, notwithstanding an 
abundant rainfall, the conditions primarily resemble those 
of a desert. 
