40 
BULLETIN 791, U. S. DEPARTMENT OE AGRICULTURE. 
No systematic attempt has been made to determine the viability of 
the seed crop of these secondary weed species, but various laboratory 
tests and field observations indicate that the germination strength of 
the seed is high. This lower successional stage of vegetation, like 
the second-weed consociation generally, 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- 
SiMk?ili 
'Tongue- leaved Violet 
Fig. 14. — Plants characteristic of the early second-weed stage. 
Mexican Dock 
{(gumex^mexiCdnus) 
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. 
