124 BOTANICAL GAZETTE [AUGUST 
of progress the following notes on the creosote bush and its relations 
to water supply. 
The creosote bush, Covillea tridentata, is, as is well known, one of 
the most characteristic species of the Lower Sonoran zone, and through 
its wide range, from California eastward to Colorado and Texas, 
and southward into Mexico, it is perhaps, of all the species of this 
zone, the one most constantly present and most firmly established 
(fig. r). It occupies extended areas where its removal would leave 
a bare waste, but at the same time shares, on mesa and foot-hills, a 
great variety of soils and exposure with other species that exhibit far 
less capacity of accommodation than itself. 
This power of accommodation is particularly noticeable as regards 
water supply. One has only to pass from the mesa east of Tucson, 
for example, to the low ground of the Rillito near Fort Lowell, obsery- 
ing the specimens of creosote bush as he goes along, to be convinced 
that the differences presented by them are due to the meager supply 
of water in the one case and its abundance in the other. More strik- 
ing still are the changes that take place when individual plants are 
well watered. In contrast with the specimens around them to which 
no water is given, their leaves become deep green and undergo a 
marked increase in size, while the whole plant presents the appearance 
of robust health and remarkable vigor, very different from the pinched 
specimens with narrow, pale leaves, branches more or less defoliated, 
and other marks of a struggle that, however successful, is manifestly 
one of great severity. Plants that have been well watered for @ 
period of years are far more fruitful than their companions standing 
in dry ground near by, and from their vigor, fruitfulness, and habit 
of retaining a greater number of healthy leaves and branches, there 
can be no question as to which is the normal condition; the creosote 
bush reaches its normal development where there is a full supply of 
water; arid conditions are indeed tolerated to a remarkable degree, 
but the plant is dwarfed and suffers in other ways while it endures 
them (jig. 2). 
These facts, though matters of every day observation, are highly 
significant. Provisionally they may be interpreted as indicating that 
the creosote bush, living over much of the territory where it is now 
found from the period of maximum precipitation to the present time, 
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