204. BOTANICAL GAZETTE [SEPTEMBER 
A second region of root penetration is occupied by the relatively 
superficial laterals so common among the plants of the region. 
These in general are more deeply placed than the roots of the 
grasses and annuals. Dysodia acerosum, Aplopappus, Euphorbia, 
and Hymenopappus are the principal plants. 
' A third layer of roots is made up of the lateral roots of Ephedra 
and the deeper parts of the root systems of Dysodia, Allocarya 
Jamesii, Pachylophus hirsutus, Melampodium, and others. 
The fourth layer probably does not always occur, but near an 
arroyo it may contain more roots than any except the superficial 
layer. Here occur the ultimate branches of the tap root of Ephedra, 
Gaura coccinea, Berlandiera lyrata, Stephanomeria runcinata, and 
others. 
The zonation of the roots reduces competition and permits the 
growth of a larger number of species. The root systems of the two 
dominant plants compete but little, since the principal absorptive 
roots of Dysodia occur in the third layer and those of Ephedra in 
the third and fourth layers. This no doubt accounts for the joint 
dominance of the two plants. 
Th t Albuqu liffers from that of Thaceoetin havind 
aboek two-thirds as much rainfall and much lower winter tempera- 
tures. ‘The soil of the mesa is fluviatile in origin and very 
_ diverse in composition. The hardpan layer prominent at Tucson — 
is not well developed. The winter annuals and the larger 
shrubs and cacti are absent. Most of the plants are sponses oa 
_ herbs. . 
"The root systems poms penetrate rather deeply, but often 
ae Mine prominent laterals near the surface of the soil. The cacti and oe 
afew ot fici: J root system. Thelarger— - 
u oS cacti show : a differentiation into anchorage and absorptive | eee Le 
