70 Postelsia 
This is especially to be observed upon stream 
deltas for the uneven stream deposits and: the 
changing contour of these bars do not permit 
the various inhabitants to establish themselves 
with permanency. Where on the other hand, 
the beach does not undergo comparatively rapid 
changes, vegetation is not of such transient char- 
acter and here too less hardy forms are able to 
compete for the ground and maintain them- 
selves with varying success. ‘The group as a 
whole is of more or less open growth. The 
various species of grasses enumerated grow in 
scattered tufts and do not build up a character- 
istic turf. The struggle here is not so much of 
species against species as that of the entire 
society against adverse edaphic conditions. 
That the different representatives share much 
alike in this fate is shown by the fact that no 
one species preponderates in any marked degree 
over the others. 
CG. Jback beach: 
This zone at Lake San Juan can be separat- 
ed, according to present topographic conditions, 
into a rocky and a non-rocky region. ‘The char- 
