110 TRANSACTIONS OF THE [Sess. Lxxv. 
for more than a week or two at a time; in other words, 
the river-bed lies at some distance below the winter 
snow-line. 
As for the heat factor, the average temperature is doubt- 
less less than‘in the lowlands, nor will there be nearly as 
much sunshine as even at the same altitude, or higher, 
further to the east. But on clear days insolation is very 
powerful, as evidenced by the heat of the stones of the 
river-bed. Judging from other parts of alpine New 
Zealand, the minimum temperature probably does not 
exceed —18° C. But it must be pointed out that frost 
may occur during any month of the year; that in the 
shade the ground may remain hard-frozen for several days 
at a time, and that even in winter the plants will fre- 
quently lack the protection, not only from cold, but from 
the excessive transpiration that a snow-covering affords. 
The wind blows with extreme violence from the north. 
west, the prevailing quarter. Its effect on increasing 
transpiration is frequently much lessened through its 
being charged with moisture and its not being a dry 
wind, as further to the east. On the river-bed the violence 
of the wind is intensified, hemmed in, as the valley is, by 
the contiguous high mountains. 
From the above it may be seen that the climate is partly 
hygrophytic and partly xerophytic, for the effect of the 
excessive rain is neutralised by the insolation, the frost, 
and, above all, the high winds, so that although there are 
periods when typical hygrophytes could thrive, there are 
also frequent periods which demand xerophytic structure. 
(b) EDAPHIC. 
The soil conditions of a river-bed are in a perpetual 
state of change. This arises from three causes: the first 
of which is the humus-making power of special growth- 
forms, especially of certain cushion-plants which rapidly 
convert their old leaves and shoots into soil; the second, 
the catching power of the plants with regard to wind- 
borne dry glacial silt (rock-flour); and the third, an ever- 
varying distance of roots from ice-cold water. 
Leaving the humus out of the question for the present, 
the soil consists chiefly of stones, large and small, mixed 
