Jan. 1911.] BOTANICAL SOCIETY OF EDINBURGH 109 
It is evident that a river-bed such as is here described 
offers a quite unstable substratum for a plant-covering ; 
but, generally speaking, the changes wrought by the shift- 
ing streams take place slowly, and there is a gradual 
procession of events from the merely temporary occupation 
by plants of ground liable to frequent submersion to closed 
associations of considerable age. These latter, where a 
river flows through a narrow valley with mountains de- 
scending steeply on both sides to the flat bed, are always 
liable to destruction, but nevertheless extensive stony 
flats, situated at no great distance apart, and occupied by 
grass, shrub, or even forest formations, are quite common 
and bear the appearance of perfect stability and consider- 
able age. Notwithstanding the extreme instability of 
river-bed vegetation, a complete cycle of events is always 
in view, every progressive phase of the development of the 
vegetation existing at various spots from the first insecure 
occupation of the ground to its final conquest by steppe, 
shrubbery, or forest. 
3. ECOLOGICAL CONDITIONS. 
(a) CLIMATIC. 
Although no statistics are available as to the rainfall 
and number of rainy days at the source of the river Rakaia, 
there can be no doubt that both are excessive, the former, 
judging from statistics elsewhere, doubtless exceeding 
250 cm. a year. There is, in fact, a forest-climate, which 
is defined by the average distance reached by the heavy 
rainfall from the west, as distinguished from the dryer, or 
steppe-climate, of the area lying to the east of this line. 
Heavy downpours, accompanied by high wind, constantly 
occur on the mountains and in the valleys near the sources 
of the glacial rivers, while at a kilometre or two to the 
east there is no rain at all, but a fierce gale favouring 
excessive transpiration will rage. 
Snow must fall in great abundance at the higher levels 
during the months of June, July, and August, and, indeed, 
heavy falls occur both earlier and later than those months. 
But at the altitude of the river-bed here dealt with it is 
probable that snow does not usually lie upon the ground 
