3i6 



Geors[e E. Nichols, 



hold in the interstices between the cobbles and boulders, rooting in 

 the sand and gravel which have accumulated in the shelter afforded 

 by the larger rocks. The pioneers are preeminently herbaceous 

 perennials: species which are able to tide over the unfavorable 

 periods by means of underground organs. Except for shrubby 

 willows (such species as Salix cor data, S. lucida, and 6". 

 humilis), which are able to survive considerable battering and 



Figure 21. — View along Middle River, showing boulder plain with 

 scrubby willows, etc. (left foreground), young flood plain with pioneer 

 tree stage (center, mid-distance), and mature flood plain, now under 

 cultivation (right, mid-distance). 



locally may form dense, low thickets (Fig. 21), woody plants are 

 scarce. 



On the higher parts of a boulder plain, the vegetation is much 

 more abundant, but always open. In addition to the willows, 

 species of alder, particularly Alnus incana, are ordinarily 

 conspicuous here, together with such other woody plants as 

 Rubiis idaeus canadensis, R. puhescens, and Spiraea latifolia. A 

 list of some of the more characteristic herbaceous plants of 

 boulder plains is given below. This list does not include weeds, 

 many of which occupy a very prominent position here. 



