River-bed.. 5 163 
 Terraces’) in most places bound the lowland and montane river-beds, while 
these latter may be ı.6 km in width. It is obvious that as the stony plains 
themselves have been subject to inundation in all parts during their formation, 
the vegetation-dynamics of river-bed at the present time must be very similar 
to that of the plains during their construction. 
Typical river-bed consists of a more or: less flat expanse of stondk which 
vary considerably in size and are mixed with a large but varying proportion 
‚of sand and silt. The river-proper wanders from side to side of the bed in 
anastomosing streams, its path restricted‘ only by the terraces or adjacent 
. mountains slopes. During the frequent floods, the streams may change their 
course, so turning stable ground into flood-plain, but rendering the abandoned 
5  surfaces are ancient flood-plains, be in process of formation. Thus it can 
the development of the land-surface, commencing with the peopling of the 
bed as the water recedes, and ending with the vegetation of the oldest flood- 
_W. forest. 
“The ecological conditions are comparatively simple. The stony substratum 
: favours rapid drainage, the water-content close to the surface being extremely 
small. At a depth of about 30 cm, there is always a certain amount of moisture; 
stony bed fit for plant-colonization. Moreover, the stream may cut into its 
bed, and terraces, such as now exist far from the river’s influence, but whose 
be seen that various associations will exist, each marking a certain phase in 
x kin, the climax-association of the E. being steppe or heath, and er iu 
Re 
