ROUT-SVSTEMS OF PKKIiWIAI.S. 55 



THE BAJADA. 



The- plants of the bajada which were studied cither were peculiar to or 

 characteristic of the bajada although not confined exclusively to that habi- 

 tat, or, being characteristic of other formations, occurred nevertheless on 

 the bajada where for different reasons they were found most convenient for 

 studv. CoviUca tiidcntata is found in greatest numbers on this habitat, 

 where it may in places produce almost a pure growth, but the species is to 

 be found on the other areas also and in fact attains its largest development 

 on the flood-plain or close by streamways. Riddcllia coopcri occurs mainly 

 on the bajada, and Opuiiiia arbitscula and O. jiilgida grow only there. Of 

 other species whose root-svstems were examined, Carnegiea gigantca, Fou- 

 quieria splcndcns. Franscria dcUoidea, Kvamcria canesccns, and A.', glandii- 

 losa are found on Tumamoc Hill also, and occasionally, on the flood-plain 

 save Fouquien'a which was not seen on the last named formation. 



Two species of Yucca, an Agave, and a Dasyliymn from the bajada to the 

 west of the Rincon mountains, about 20 miles east of Tucson, were also 

 studied. 



Carnegie.v Gig.\xtea. 



The root-svstems of several specimens of the sahuaro have been observed, 

 but the studv has been carried on mainh" on plants from the bajada imme- 

 diateh- west of West Wash, and from the flood -plain of the wash itself. The 

 soil conditions comprise A'ery shallow as well as fairly deep surface soils, as 

 a result of which the extreme variabilit\- of the roots might be expected. 

 The water relations, also, were very diverse, and probably the temperature 

 conditions as well. However, it should be stated that this cactus not only 

 avoids the habitats where the soils are shallowest, but also the opposite 

 extreme, where they are the deepest : or, briefly, it does not occur as a 

 general rule on the bajada or the flood-plain, but rather on rocky areas, 

 chieflv on the southern, the western, and the eastern exposures. 



The bajada habitat is at the eastern end of the long slope which leads 

 awav from the main range of the Tucson mountains. The soil at the place 

 where the cactus was studied is of a dark color, closely resembling the 

 malpais of Tumamoc Hill, and is rather thicklv strewn with large and small 

 stones. A section of the soil shows the following leading characters: The 

 uppermost soil, about 30 cm. in thickness, is of the adobe-malpais referred 

 to above ; beneath the superficial stratum is a thinner one of caliche which 

 rests on bed rock. (>See figure to the left, plate 14.) Numerous stones and 

 boulders are embedded both in the adobe and in the caliche layers. The 

 contour of the ground at the place is such that little if any water comes to 

 it by seepage from higher ground, or bv superficial run-oft", the plants 

 obtaining all of the water supply directlv from the rains. 



The root-system of the plant is differentiated into an anchoring portion, 

 which is the more deeply placed, and a superficial portion, which is pre- 

 eminently the part engaged in absorption, although it has other functions. 



