180 BOTANICAL GAZETTE [SEPTEMBER 
several times the normal amount, the figures would have had even 
less value than usual. Similar variations in water holding capacity 
and wilting coefficient would result from the lack of uniformity in 
the soil. 
On account of climatic differences, the flora is very different 
from that of the Tucson region. The larger cacti are absent here, 
all being low forms, excepting Opuntia arborescens. No tree is 
found on the mesa or its arroyos, and the bushes, except Chilopsis 
saligna, are seldom more than 5 ft. in height. The period of 
greatest growth follows the rainy season, whenever it may occur. 
There are no winter annuals here, the corresponding forms being 
either biennials or summer annuals. The annual plants are greatly 
in the minority, the most conspicuous part of the flora being 
composed of perennial herbs. 
The principal plant associations occurring within the area 
ane are as follows. 
. The Penton: association, _chaeactenized by the grama and 
| sees , but now much 
: invaded by ruderals, such as xs Cakes Scvothrae and Salsola. 
2. The Dysodia-Ephedra association, dominated by Ephedra 
ietjores and Dysodia acerosum. The majority of the plants have 
reduced aerial parts and a large root system. The association 
occupies the top and the upper portion of the sides of the gravelly 
oe -tidges between the arroyos, where exposure and run-off are maxi- : 
ae mum. The soil is atte Pee gravel and ne 8 surface is generally a 
oo es The Ch : th Pee 4 oe Pies by Chrysothamnus si 
: _— Bigelovii and occupying the stare of the sides of the antes a 
The soil 3 is generally adobe, often d and gravel. 
tnd form eae ‘a. narrow fringe along the beds of arroy0s, which are = 
mle sired wi ke ane tos : 
: association, dominated by Fallugia poradoxa - 
