252 THE PLAXT WOELD. 



genta iiowers. These are two quite closely related species wliicli 

 grow in the same habitat, an example of a principle which is 

 now under discussion among biologists. As we approach the 

 foot hills along the margin of which flows the Rillito River- — ■ 

 though one might more properly speak of the Rillito wash, since 

 no water flows on the surface except during short periods of the 

 year — the mesa slopes downwards very gradually. In the lower 

 levels grow the mesquite and cat's claw, and two species of 

 acacia, which are readilv distin<>'uish(Ml bv their flowers, the 

 former having cream-colored flowers in a raceme about an inch 

 long and without fragrance, the latter, the huisache, having 

 bright, yellow flowers in a rather close head, and giving out a 

 sweet fragrance which loads the air. Along the wash, cotton- 

 woods grow. together with a species of ash and the desert willow, 

 a relative of the catalpa. (Crossing the drv and sandv bed of 

 the Rillito the road follows a tributary wash, or rather draw, 

 in which the large-leafed palo verde ( /)<ih> hrclla) is a conspic- 

 uous figure. As one approaches the wash tlic (puiil in great 

 numbers take flight and jack rabbits run for their burrows. On 

 a really warm desert day there is no \A-m-v like a draw of this 

 kind to liet the full value of the heat, and the walking becomes 

 rather hard, both on account of the intense heat and the sand 

 under foot. 



('limiting the low escarpment one coiues at once into an 

 entirely dift'erent kind of country, as iudicatt^l ])r)th by the stony 

 soil and by the vegetation. Here we lind the giant cactus and 

 the prickly pears in great abundance. A tree-like cactus (0. 

 versicolor and others) is found on every liand. Occasional level 

 stretches of loose, stony soil are covered with a growtli of Fran- 

 serla delfoidea, a low half woody shrub with canescent leaves 

 and flowers rather similar to, th<»ugli smaller than, those of the 

 cockle burr. These patches of Franseria are favorite places for 

 the cottontails, which readily hide behind the bushes. Occa- 

 sionally the air becomes laden with a delicate perfume, the 

 source of Avhich T found verv difficult to trace until some days 

 later I discovered it to be the large species of Kramcria. a shrub 



