DESERT REGIONS OF NORTH AMERICA. 33 
It reaches its greatest abundance along the Grand Canyon of the Colorado 
River. Here the heated air, rising from the river-bed under the influ- 
ence of a subtropical sun, loses about 20° F. in its ascent of about a mile, 
and as a consequence it pours over the mesa much cooler and with its 
relative humidity increased to near the point of saturation. The Raz- 
oumojskya finds the strip of territory where the full effect of the moist 
current is greatest most advantageous in the germination of its seeds and 
the attachment of the seedlings to the host plant. It is therefore most 
abundant in a belt 1 or 2 miles in width running parallel to the rim of the 
canyon, while it is comparatively infrequent at greater distances. Within 
this belt it is estimated to have gained a foothold on 60 to 80 per cent of 
the pines. 
THE DELTA OF THE COLORADO RIVER. 
The action of the Colorado River in its delta, in its occasional over- 
flows of the Salton and Pattie basins, and the interwoven conditions affect- 
ing the origin and development of the floras of the delta and contiguous 
deserts, are such that the principal features of the deltamust be considered. 
The main portion of this delta consists of an alluvial plain a few feet 
above low-water mark, cut in all directions by sloughs and bayous, 
which are filled at irregular intervals, but principally by the annual floods 
of May, June, and July, resulting from melting snows in the headwater 
region of the river. Occasionally, however, midwinter floods occur, due 
to rain and melting snow in the region drained by the Little Colorado, the 
Bill Williams River, and the Gila (plate 30). 
In contrast with the desert the delta is characterized by almost pure 
cultures of various plants in different areas. Thus in places seedling 
willows and poplars occupy areas of great extent to the almost total 
exclusion of all other seed plants. The cat-tail tule (Typha angustifolia) 
lines the shore of the river for many miles and extends back from it so 
densely that, except an occasional mesquite or screw-bean, nothing else 
may compete with it. In other places the arrow-weed (Pluchea sericea), 
quelite (Amarantus palmer), wild hemp (Sesbania macrocar pa), salt-grass 
(Distichlis spicata), Cressa, and wild rice (Uniola palmert) occur 1n similar 
density. Filling in the interstices, as it were, between the larger blocks 
occupied by these colonies are the mesquite and screw-bean (Prosopis 
velutinea and P. pubescens), cane or carrizo (Phragmites phragmites), 
Scirpus fluviatilis, and S. californicus, the two sedges of the delta, cow- 
pumpkin (Cucurbita palmata), Lippia cunetfolia, Eclipta alba, Echinochloa 
crus-gallt, Diplachne tmbricata, and dock (Rumex). 
The tidal action of the waters of the Gulf of California is felt as far 
up the river as Colonia Lerdo, 75 miles from the mouth, and shortly 
below the limit of such action the trees begin to find the soil too highly 
charged with sea-salts, and great flats occur in which an occasional 
mesquite and salt bushes (Aérzplex) find a foothold. Within the memory 
