60 The Plant World. 



ditions prevailing at the lower levels. The single individual of 

 Yucca elata, a plant of higher levels, that has been found on the 

 Laboratory domain, is at the edge of the wash, protected by 

 the higher vegetation around it. 



It should again be noted, as already suggested, that many 

 of the characteristic species of the habitats thus far discussed, 

 though exhibiting more or less plainly certain structural features 

 distinctive of xerophytes, are semi-mesophytic or mesophytic, 

 in some cases even hydrophytic, as regards soil relations. Thd 

 willows and cottonwoods of the river-banks are, in their habits, 

 what they are the world over. The mesquite of the flood-plain 

 sends its roots down to the water-table, and elsewhere is restricted 

 in its range to habitats in which a satisfactory water-supply is at 

 hand, and the palo verde and catclaw of the washes evidently 

 have much the same dependence on a sufficient amount of 

 soil-water. 



The same thing appears to be true of various species of 

 salt-bushes, which follow watercourses, or — in the salt-spots — 

 occupy places of seepage. In view of their habits, it may be 

 fairly questioned whether such xerophytic structures as some of 

 the salt-bushes exhibit stand primarily in relation to "physio- 

 logically xerophytic" soil conditions or not; in fact, it seems not 

 at all improbable that they may be found to fall into the same 

 category with the mesquite and catclaw, exhibiting certain 

 xerophytic structures which are useful in the dry air of the desert, 

 and which also in cases of extraordinary drought successfully 

 regulate transpiration and make less draft on the resources of 

 the plant. 



(6) The Mesa-Like Slopes. 

 (a) Creosote-Bush Association. 



The long, gentle slope of the "mesa," as it is commonly but 

 incorrectly called, rises from the flood-plain, at flrst with an al- 

 most imperceptible grade, becoming steeper as the ascent con- 

 tinues to the hill above. Its soil, as already stated, is mainly 

 gravelly or sandy, with but little loam and with a considerable 

 amount of caliche. The slope and the nature of the soil insure 

 perfect drainage and aeration. But with the poor soil, with 

 relatively little capacity for the retention of water, it is plain 

 that we have passed, almost at a step, from soil conditions that 

 may be classed as mesophytic, or semi-mesophytic, to those that 



