10 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
resulting in more or less pronounced changes of vegetation. Neverthe- 
less, although the formerly abundant saccaton has disappeared, and 
along with it doubtless other species, the vegetation of the flood-plain 
shows clearly enough what were in earlier times, and are still, its essential 
features. 
The dominant species is the mesquite (Prosopis), here in the form 
velutina, a highly characteristic species of the Lower Sonoran zone as 
defined by Merriam, but extending in its various forms far beyond.’ It 
is by preference a plant of low flats, though it occurs far beyond these 
on the uplands, in situations where a sufficient water-supply is obtainable. 
In the neighborhood of Tucson the mesquite ranges in size from a mere 
shrub a few feet high to a tree 2 feet or more in diameter and upwards 
of 4o feet in height. Such trees grow thickly on the bottom-land near 
the old mission of San Xavier, forming the fine forest that stretches for 
miles up the river, in the shade of which grows a rank vegetation similar 
to that of eastern mesophytic forests in luxuriance. 
The habits of the mesquite are popularly well known, and its presence 
is taken to indicate a good water-supply. Its roots extend widely, to a 
distance of 50 or 60 feet according to credible observers, and possibly to 
as great depth; and when cut green its wood tissue, which is hard and 
heavy, carries a large percentage of water, precisely as do the hardwoods 
of the eastern United States, and strikingly different from the creosote- 
bush, its near neighbor on the slopes beyond the flood-plain. At the 
same time, the general structural peculiarities of the mesquite are xero- 
phytic. It is commonly armed with spines, and its coriaceous leaves 
are well protected against excessive transpiration. Itisa plant requiring 
a better supply of water than many of its associates, yet well adapted to 
the low relative humidity of the desert air, and its occurrence beyond its 
own special area, ranging as it does to the top of Tumamoc Hill, in spots 
where a soil retentive of moisture affords the conditions it needs, corre- 
sponds with this peculiarity. Thus it is, in a sense, a desert plant, yet 
one of high water requirement—characteristics which it shares with 
various other species. 
The capacity of this plant for taking possession of wide areas beyond 
its earlier limits of distribution is of special interest. According to 
Bray (1904), “its spread northward and eastward from the Rio Grande 
country during the past 50 years has been a marked phenomenon. By 
its invasion, mile after mile of treeless plain and prairie have been won 
and reduced to a characteristic orchard-like landscape. It has traveled 
northward over the Staked Plains, covering half their area, and has 

‘IT have made no attempt to separate or delimit by geographical boundaries the 
forms of the mesquite which, under the names of Prosopis glandulosa, with its center 
in Texas, P. velutena in Arizona, and P. dulcis in northern Mexico and beyond, repre- 
sent the views of systematists concerning the closely related species or varieties of 
this interesting plant as it occurs in the southwestern United States and adjacent 
parts of Mexico. 
