FLOWERING PLANTS AND FERNS OF ARIZONA 13 



number of trees along the streamways, and a notably greater number 

 of cacti, which is doubtless due to the increasing amount of summer 

 rainfall encountered on going eastward from the Mojave Desert. 

 Sahuaro (Cereus giganteus) is very nearly limited in its westward 

 distribution by the Colorado River, but there are no other important 

 plants for which the river serves as a boundary. Several species of 

 Opuntia (0. echinocarpa, 0. bigelovii, 0. basilaris) are frequent in the 

 microphyll desert of Arizona but rare in that of California until the 

 desert slopes of the Chocolate, Chuckawalla, and Cuyamaca Moun- 

 tains are reached. 



Over much of the microphyll desert the surface of the ground is far 

 less stony than in the other types of desert, resulting in greater erosion 

 by wind and water. The sparse vegetation of the microphyll desert 

 is to be attributed chiefly to the low rainfall of 4 to 6 inches per year, 

 of which 70 percent falls in winter, and partly to the very adverse 

 conditions for seedlings. 



In the part of Arizona under consideration a very high percentage 

 of the total land surface is occupied by outwash slopes and slightly 

 tilted plains or bajadas. Over many areas of this character Larrea 

 tridentata (creosotebush) and Franseria dumosa form 80 percent of 

 the plant population. Usually the two are found together, but in 

 general Franseria is the more abundant. The monotony of the vege- 

 tation is occasionally broken by plants of Acacia constricta, Fouquieria 

 splendens (ocotillo), Echinocereus engelmannii, Opuntia echinocarpa, or 

 0. ramosissima. Perennial grasses are infrequent, except for colonies 

 of Hilaria rigida (big galleta) on sandy soil, and several species of 

 Aristida (pi. 1). * 



Some of the volcanic mountains near the mouth of the Colorado 

 River are unusually bare of vegetation, supporting little more than 

 occasional clumps of Heteropogon contortus (tanglehead grass), Bebbia 

 juncea, and Encelia farinosa (incienso). On the granite mountains 

 are to be found Cereus giganteus, Cercidium microphyllum (paloverde), 

 Olneya tesota (ironwood), Bur sera microphyll a, Echinocactus acanthodes 

 (bisnaga), Opuntia basilaris (beavertail cactus), and other plants, 

 which are either confined to this part of Arizona or else are found 

 farther east on the outwash slopes. In the latter case is seen an 

 excellent example of the common phenomenon of the occurrence of 

 a plant in the more favorable habitats of an unfavorable region and 

 also in the unfavorable habitats of a more favorable climate. . 



Some very sharp contrasts of vegetation are to be seen in the 

 microphyll desert by reason of the great river, which carries past it 

 such a large volume of water derived from a distant region of dissimilar 

 character. In many places the alluvial flats of the Colorado support a 

 forest of Populus jremontii (cottonwood) within a few yards of the low 

 sparse stands of Franseria. Along the Colorado and Gila Rivers 

 there are broad swamps of Typha angustifolia (cattail) and Scirpus 

 olneyi, and dense thickets of Pluchea sericea (arrowweed), the latter 

 often 10 to 12 feet in height. Where the moisture of the soil is more 

 deep-seated there are low forests of Prosopis julijlora (mesquite) or 

 Prosopis pubescens (screwbean). Along the small streamways there 

 are often remarkably large trees of Cercidium floridum (paloverde), 

 30 to 40 feet in height, as well as Olneya, and the smaller but very 

 striking Dalea spinosa (smoke tree). 



The mesquite often grows in circular clumps from 30 to 50 feet 



