PLANT SUCCESSION AND RANGE MANAGEMENT. 
37 
species to announce the waning or retrogression of the foxglove-sweet- 
sage-yarrow cover are certain inconspicuous, rather short-lived per- 
ennials which usually occur very sparsely in association with the 
dominants of the second-weed stage. By far the most aggressive 
and reliable indicators of degeneration are low pea vine {Lathy rus 
leweanthus) , evening primrose {Lavauxia flava), false cymopterus 
{Pseudocymopterus tidestromii), Mexican dock {Rumex mexicanus), 
false Solomon's seal {Vagnera stellata), and tongue-leaved violet 
{Viola lingucefolia). These rather temporary second-weed-stage 
species obtain the greater part of their moisture supply below the 
first foot of soil (figs. 12, 13, and 14.) Except in the case of low 
pea vine and false Solomon's seal, their root systems are on the tap- 
root order, most of the species having well-developed laterals, many 
being several inches long. With the exception of low pea vine and 
false Solomon's seal, which reproduce profusely by rhizomes, new 
individuals arise only from seed. Considering the elevation and the 
conditions of growth, the four species whose regeneration depends 
entirely upon seed have strong seed habits. Low pea vine and false 
Solomon's seal, on the other hand, produce only a small amount of 
seed per plant, but the viability of the seed is nevertheless relatively 
high, as Table 3 shows. 
Table 3. — Viability of seed crop of short-lived perennial weeds produced in 
1914-1916, inclusive. 1 
Plant. 
Viability of seed. ' 
Average. 
Maximum. 
Minimum. 
Per cent. 
52.7 
33.1 
29.4 
19.9 
16.3 
Per cent. 
78.4 
35.4 
36.8 
28.7 
28.1 
Per cent. 
20.5 
Evening primrose 
26.4 
17.3 
False cymopterus 
11.2 
6.3 
1 Seed was collected from a number of specimens of each species grown in different soils and in different 
exposures so that, presumably, the figures given are representative for the seasons in question. 
In view of the fact that tongue-leaved violet is the most aggressive 
invader immediately subsequent to the destruction of the foxglove- 
sweet-sage-yarrow cover, it is interesting to note that the viability 
of the seed of this species is superior to that of its associates. While 
it may be true that enough more viable seeds are produced by the 
competing plants to equal, or indeed exceed, the total number of 
viable seeds of tongue-leaved violet, it is not improbable that under 
field conditions, the seeds of the violet, with their higher germina- 
tion strength, may outnumber those of high germination power in 
the species having an appreciably lower percentage of viability. 
