10 BULLETIN 367, U. S. DEPARTMENT OF AGRICULTURE. 
inches. The certainty of producing a crop of seeds and the ability 
of these seeds to endure desiccation and to plant themselves are 
factors which probably account for the distribution of these grasses. 
They are easily crowded out by even the least aggressive of the 
perennials, yet they occur as scattered individuals among nearly all 
the other grasses almost any season, though this habit is not so well 
shown by the Bouteloua as by the Aristida. Wherever, for any 
reason, the perennial grasses are killed out these grasses occupy the 
ground for the short summer growing season. 
The six-weeks grasses now occupy at least six or seven sections of 
the reserve as an almost pure stand, while they form a very impor- 
tant part of the assemblage here referred to as the black-grama 
association (No. 2 in fig. 3) and the crowfoot-grama association 
(No. 3 in fig. 3), especially along the boundaries of these areas. 
It is difficult to say definitely in many places just where the six- 
weeks grasses cease to dominate the association, and there is cer- 
tainly no such well-marked boundary line between this association 
and those adjacent to it as is suggested by the more or less arbitrary 
divisions made in this bulletin. When the range reserve was first 
inclosed, this association was of greater extent than any other grass 
association in the reserve. It has been replaced primarily by the 
crowfoot-grama association from above, but the black-grama asso- 
ciation has also crowded in from below, and the latter, while much 
slower in its encroachments and much more easily checked by graz- 
ing, may in the end dominate both the others if the area be protected 
for a sufficient time, especially if fire be prevented. 
Considered as a forage crop and from the standpoint of their eco- 
nomic importance, the grasses of this association are not of great 
value. They produce a light crop of forage; the crop lasts but a 
short time and loses feeding value rapidly, being almost valueless 
by the middle of the winter; and for some reason (probably because 
the plants pull up easily and thus get dirt in the animals’ mouths) 
stock do not eat them while at their best, unless there is nothing else 
to be had. However, the growth of spring annuals is usually heaviest 
on this area, and they add considerable good feed to the total annual 
crop. 
THE BLACK-GRAMA ASSOCIATION. 
Across the northwest corner of the reserve and extending along 
much of the west side is an area in which the most conspicuous 
grass is what is called “ black grama” in this region (No. 2 in fig. 3). 
The grass in question is not a member of the genus Bouteloua, which 
contains what are usually called gramas, its scientific name being 
Muhlenbergia porteri (Pl. II, fig. 2). Its importance in the early 
days of the stock business in this region has been discussed by 
