PLANT SUCCESSION AND RANGE MANAGEMENT. 
47 
mean temperature being 42°, and when the nocturnal temperature 
has dropped from 2° to 4° below freezing. Few perennial plants, 
even though restricted in their distribution to the subalpine zone, 
are capable of germination at such low temperatures. Being entirely 
dependent upon seed for their perpetuation, obviously only those an- 
nuals persist or are conspicuous which are capable of germinating 
and becoming established at the earliest advent of spring. 
&? 
Tolmie's orthocarpus 
(Orthocarpus tolmiei). 
Douglas knotweed 
(Polygonum douglasii). 
Tansy mustard 
(Sophia incisa). 
Fig. 18. — Plants characteristic of the first or early weed stage. 
Since the ruderal-early-weed stage represents the lowest or most 
primitive herbaceous cover possible, it will be instructive to compare 
briefly the conditions of growth of this type with those of the highest 
herbaceous successional cover, the wheat-grass consociation. For the 
purpose of ready comparison the average, maximum, minimum, and 
optimum depths of the roots of the most characteristic ruderal-weed 
species and of the chief wheat grasses are tabulated in Table 5. 
While the depth of penetration of the roots of wheat grasses as a 
whole is appreciably greater than in the ruderal weed species, small 
wheat grass obtains its water supply from much the same soil stratum 
