ROOT-SYSTEMS OF PERENNIALS. 73 



roots of the cactus, so that active competition between the roots of the two 

 species could hardly have taken place. 



The specimen of Acacia, which was about a meter distant from the 

 main axis of the cactus, had a generalized type of root-system. It con- 

 sisted of 2 main roots and about 12 smaller ones. Of the larger roots, one 

 ran north of the main axis of the cactus and extended beyond the east 

 boundary of its root-area, as fig. 12 indicates, and lay from 9 cm. to 40 cm., 

 where it was left, beneath the surface. The other roots descended rapidly 

 and evidently penetrated deep, although they were not traced more than 

 60 cm. from the surface. The disparity in the position of roots of O. fulgida 

 and of Acacia was so great that there could be no active competition 

 between the two species for ground water. 



RlDDELLI.\ COOPERI. 



Riddellia coopcri is one of the widely distributed shrubs in the vicinity 

 of the Desert Laboratory, occurring not only on Tumamoc Hill and on the 

 bajada, but on the flood-plains both of the Santa Cruz river and of West 

 Wash. It was studied on a slope separating the bajada west of West Wash 

 from the flood-plain of the wash and also on the flood-plain east of thewash. 

 The soil west of the washisred claycontaining coarse particles of sand, with 

 the caliche hardpan at a depth of 55 cm.; the soil to the east of the wash 

 is a sandy loam, apparently homogeneous to a depth exceeding 1.4 meters. 



Riddellia is about 30 cm. high and consists of numerous shoots, the ter- 

 minal portion only, equal to about half, bearing flowers, and the proximal 

 portion onlv bearing leaves. This should be borne in mind when consider- 

 ing the stature of the plant as presented in the succeeding measurements, 

 since, for various reasons, the measurements were made to include both 

 the flowering and the non-flowering portions of the shoots. 



The specimens situated west of the wash had a generalized type of root- 

 system. There was a tap root, the lower portion of which was dead, which 

 gave off a branch that descended gradually until it attained a depth of 48 

 cm. where its tip was 50 cm. distant from a line dropped straight down 

 from the crown of the tap root. In addition to this root, 5 laterals arose 

 within 20 cm. of the surface of the ground and extended in a fairly hori- 

 zontal direction for a distance of i meter from the main plant axis. These 

 laterals bore few branches and scarcely any filamentous roots, such as are 

 a feature of the laterals in Franscria, Encclia, and other species, and the 

 filamentous roots were not borne in groups, but occurred singly. 



The roots of Riddellia extended so short a distance in a lateral direction 

 that the only root found within its root-area was a single one of Acacia 

 const) icla (fig. 13). 



The specimen of Riddellia growing east of the wash had a shoot 32 cm. 

 high, or the same length of the plant just described, but, as will appear 

 directlv, in several particulars its root-system was unlike that of the other 



