54 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
taken complete possession of limited areas, and in spots where water 
has been used close to the building, Bermuda grass has made its appear- 
ance within the past year. 
Numerous other weeds, chiefly annuals, have come to occupy, more 
or less conspicuously, such ground as they have found available, especially 
on the flood-plain. While, as indicated, annuals and foreign weeds are 
most prominent as invaders, there are various perennials, particularly 
certain species of grasses, which present every appearance of invasion, 
either successfully accomplished or in progress. Among these are Hilaria 
mutica, and H. cenchroides, which, on the north slope of Tumamoc Hill, 
occupy extensive patches in the midst of other vegetation which they 
are apparently displacing. In order to determine this fact definitely, a 
number of permanently marked areas have been established for the pur- 
pose of continuous observation for a period of years, and maps and photo- 
graphs of these are kept in the records of the Desert Laboratory. 
In general, it appears that on the flood-plain the invasion of various 
species has resulted in their almost complete occupation of extended 
areas, so that the vegetation of these areas is essentially different from 
what it was in earlier days; but no such radical change is observable 
on Tumamoc Hill and the slopes adjacent. The invaders of the plain 
have taken advantage of changes effected by the hand of man. They 
found a fertile soil not fully occupied, and took possession with a thorough- 
ness that seems likely to be successful against competition for an indefinite 
period. On the hill and slopes, on the contrary, no such preparation 
for their invasion has ever been made, except to a limited extent in road- 
making, and their advance has been relatively slow and inconspicuous. 
Thus far the discussion has chiefly involved introduced weeds, but 
certain indigenous species have a history in some respects quite similar. 
One of the most interesting of these cases is that of Bigelowia hartwegit. 
On the flood-plain and adjacent slopes, this plant has been observed within 
recent years, almost covering the ground where before it was relatively 
inconspicuous. ‘This, according to Thornber, seems to have been closely 
connected with a period of abundant and well-distributed rains during 
the year 1905 and several months of the preceding and following years. 
During this period rain fell in sufficient quantities to insure the growth 
of large numbers of seedlings, which became well established and at the 
time of special observation in April, 1906, were apparently well beyond 
the critical period. At that time the parent plants could be observed 
quite scattering and several times larger than the seedlings of the new 
crop, while between them the young plants almost covered the ground 
and have ever since maintained the foothold then acquired. 
The various invasions that have taken place on Tumamoc Hill and 
in the adjacent valleys have necessarily involved competition. It is 
