PLANT ASSOCIATIONS AND HABITATS. 7 
The various elements of the flora that have been mentioned are grouped 
in societies, or associations, evidently related to different combinations 
of physical factors, such as water-supply and temperature, and also to 
historical factors which have been concerned in their establishment in 
their present habitats. In the following pages, which of necessity are 
largely descriptive, the part played by these several factors is the ultimate 
object of inquiry. 
(1) THE RIVER AND IRRIGATING DITCHES; ASSOCIATION OF HYGROPHYTES. 
The Santa Cruz River has the general character of streams in the south- 
western United States. For months at a time its bed is empty, but at 
the period of summer and winter rains it is not infrequently filled with a 
raging torrent, which has measured no less than 4,012 cubic feet of water 
per second at the Tucson bridge, flowing at the rate of 9 feet per second.! 
When thus swollen by rain it is a turbid, impetuous stream, bearing along 
branches of trees and débris of various kinds to be deposited at different 
points as the water lowers. At these times of flood its power of erosion 
is very great, and in a few hours the banks in places are deeply cut away, 
acres of fertile land being swept into the devouring current. 
In the course of a few days a small and harmless stream, a few inches 
in depth, is quietly flowing in its channel, and later, after the rains are 
over, even this disappears, leaving the bed of the river, as far as appears 
on the surface, only a collection of variously assorted and mingled masses 
of sand and gravel. Beneath the surface, however, is still at some depth 
a body of water, and from this, even in the driest times, the cottonwoods 
and willows along its banks draw an abundant supply. (Plate 3; com- 
pare plate 2, the Salt River after high water.) 
Irom what has just been stated, it is evident that the conditions are 
in general unfavorable for the growth of aquatic vegetation. The rapid- 
ity o1 the current when the river is “up, the turbidity of the stream, 
and the long period when no water appears above ground, are not condu- 
cive to the normal development of water-plants. Yet there are places 
where these do secure a foothold and grow with remarkable vigor. Espe- 
cially is this true of the irrigating ditches, in which green alge frequently 
accumulate in such quantities that they have to be cleaned out to prevent 
the channels from becoming choked. Great masses of Cladophora are 
often thrown out on the banks for long distances. Hydrodictyon is also 
abundant. Potamogeton pustilus is frequently conspicuous, and in pools 
of the river channel, where the force of the current is lessened, the water- 
cress (Radicula nasturtium-aquaticum) grows in luxuriant masses.? 

1Measurements by Prof. G. E. P. Smith, of the Arizona Experiment Station. The 
date of this flow was January 17, 1907. ‘The next highest flood measured was 3,200 
cubic feet per second on November 28, 1905. 
Synonyms and authorities for plant names used in this paper are given in the sec- 
tion on vegetation groups by Prof. J. J. Thornber. In his list plant names recently 
changed by systematists are given, as far as possible, in their latest accepted form. 
