Jan. 1911.] | BOTANICAL SOCIETY OF EDINBURGH 125 
(d) AFFINITIES OF THE RIVER-BED ASSOCIATIONS. 
The river-bed, prior to its peopling, is more closely re- 
lated to shingle-slip than to any other land-form. It differs 
ecologically in its flat surface, stability of the substratum, 
and greater amount of surface-moisture, whereas the stones 
of the shingle-slip are always liable to move while the 
rain-water will pass off more rapidly. The consequence is 
that, contrary to expectations, true shingle-slip and river- 
bed have virtually no species in common, those of the 
former land-form being highly specialised and confined, or 
nearly so, to moving débris as their station. Where, how- 
ever, large stones fill the bottom of a shallow gully and are 
fairly stable, certain species of Raouwlia may be present (R. 
tenuicaulis, R. australis, R. lutescens), but R. Haastii is 
invariably absent, so it is possible that the first three- 
named species may tolerate a certain amount of burial, 
whereas it may be the intolerance to this which confines 
the plant in question to the river-bed, since its “seeds” 
must constantly be blown on to the bare mountain slopes. 
The subalpine river-bed Raoulia association is closely 
related to that of the lowlands and of the mountain steppe- 
climate, both ecologically and floristically, but in the 
lowlands Raowlia Haastii! is wanting; and as the other 
species of the genus do not nearly so much favour the 
presence of epiphytes, owing to the absence of peat within 
the cushions, or supply so much humus by their decay, the 
advent of tussock-steppe 1s a slower process. 
The steppe is floristically and ecologically related to 
that of the drier mountain slopes, but is made up of fewer 
species. Some of its plants, however (e.g. Pow anceps var., 
Carmichaelia grandiflora, Coprosma rugosa), do not 
generally occur except where there is an abundant rainfall. 
The scrub association, as already noted, is almost identical 
with the subalpine scrub of the immediate neighbourhood, 
and the Discaria thickets have their counterpart on low- 
land river-beds and sand-dunes, and may be considered a 
form of steppe rather than subalpine scrub, which latter is 
a mark of a forest-climate. 
1 In Westland R. Haastvi, thanks to the wet climate and proximity of 
lofty mountains, descends to at least 300 m. altitude. 
