PLANT SUCCESSIOl^ AND RANGE MANAGEMENT. 



37 



species to announce tlie waning or retrogression of tlie foxglove-sweet- 

 sage-yarrow cover are certain inconspicnoiis, ratlier sliort-livecl 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 {Lathyrus 

 leucanthus), evening primrose {Lavauccm fava), false cymopterus 

 (Pseudocymopterus tidestromii) , Mexican dock (Bum-ex m^xicanus) , 

 false Solomon's seal {Vagnera steUata)^ 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 

 hiffh, as Table 3 shows. 



Table 3.- 



-Tkihility of seed crop of short-lived perennial u'eeds produced in 

 1914-1916, inclusive} 



Plant. 



Viability of seed. 



Average. Maximum. [ Minimum. 



Tongue-leaved violet . 

 Evening primrose . . . . 



Low pea vine 



False cymopterus 



Mexican dock 



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 

 26.4 

 17.3 

 11.2 

 6.3 



1 Seed was collected from a number of specimens of each species grown in different soils and in diflerenx 

 exposures so tbat, 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 nmnber 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 j)ercentage of viability. 



