40 



BULLETIN" 7&1, U. S. DEPARTMENT OF AGRICULTURE. 



No systematic attempt lias been made to determine tlie viability of 

 the seed crop of these secondary weed species, but various laboratory 

 tests and field obser\^ations indicate that the germination strength of 

 the seed is high. This lower successional stage of vegetation, like 

 the second- weed consociation generallj^, is early maturing, earlier, in 

 fact, by 10 days or so than the wheat-grass cover, so that as a rule 

 plants mature their seeds and the herbage dries up well in advance 

 of the occurence of killing frosts. The herbage of most of the 

 species, however, cures poorly. On some lands these species are so 

 closely associated with low pea vine, evening primrose, false cymop- 



Mexfcan Dock, 



[(Rumex^niex I cd n us) 



(Tongue- leaved Violet 



(Tongue- lea 

 Fig. 14. — Plants characteristic of the early second-weed stage. 



terus, Mexican dock, and tongue-leaved violet that it is difficult to 

 recognize where one set supersedes the other. Generally, however, 

 the line of demarcation is fairly distinct. As a rule the latter group 

 is associated with a larger percentage of annual plants than the 

 former, though this varies somewhat with the density of the cover 

 of perennial plants, moisture conditions, and numerous other factors. 

 In the absence of a nurse cover, such as shrubby plants or other 

 robust and conspicuously branched perennials, it is evident that a site 

 supporting the lower successional cover of the second-weed stage is" 

 at best severe and less favorable to germination and establishment 

 than are sites supporting a higher type of vegetation. 



