62 Postelsia 
Channel bed—Among the boulders of the 
river channel and in the hollows and crevices of 
the large rocks a few hardy types have gained a 
foothold . A few of these are perennials and as 
such must bear the yearly winter inundation. 
The forms most commonly noted in this habi- 
tab ane. 
Heuchera glabra Cladothamnus pyroliflorus 
Viola sp. Erigeron philadel phicus 
Chamenerion latijolium Luina hypoleuca 
Arnica amplexicaulis 
In no instance do these plants become closely 
associated into groups or societies but occur 
thinly and scattered where enough soil is present 
to sustain them. Cladothamnus pyroliflorus 
grows as a small shrub less than a meter high, 
upon the large rocks in the channel and also 
upon the rocky banks of the river. Chamae- 
nerion latijolium, Erigeron philadelphicus and 
Arnica amplexicaulis grow in sand upon the 
boulder strewn channel bottom:. 
San Juan river bed—-Owing to the fact that 
the descent in this valley is more gradual, typical 
mud banks are found along the river sides. 
