82 ROOT HAI5ITS OF DESERT PLANTS. 



down to its end, 1.36 meters deep; 18 laterals, i cm. or less in diameter, 

 were given off from the tap root within 15 cm. of the surface of the ground ; 

 at a depth of i meter a larger lateral was given off. None of the laterals 

 bore filamentous roots along their course, as did those of Encelia, Franscyia, 

 and other plants (plate 21/)). 



From this study of the root-systems of Coiulalia and Zizypliiix it will be 

 seen that, more than in any other plants examined, unless perhaps it is 

 Ka'berlima, a well-developed tap root appears to be an essential character, 

 and it is due to this fact that these species are confined to places where the 

 soil has considerable depth. 



Ephedra Trikurca. 



Ephedra, on the domain of the Desert I^aboratory, occurs exclusively on 

 the flood-plain, especially that by West Wash, where it attains large size. 

 It is there associated with plants characteristic of the wash, Acacia greggii, 

 Prosopis vclnti)ia, Parkinsonia torrcyana, and a few specimens of Peniocereus 

 greggii, shown in ])late 21, which is somewhat rare in the vicinitvof Tucson. 



Fig. 5. — Root-s\steni of liplicdra trijiirca showing position of tap root and typical laterals. 



The Ephedra whose roots were examined was about i meter high and the 

 shoot was composed of numerous slender branches. The root-system was 

 characterized by a stout tap root and few relatively slender laterals. The 

 main root was 13 cm. in diameter at the crown, and was traced 75 cm. to 

 the place where it forked; one fork was followed 25 cm. deeper, a total 

 depth exceeding i meter, and the end was not found. The laterals were of 

 two sorts, a larger and a smaller kind. The latter were confined to the 

 crown and resembled the slender roots in an analogous situation in the plants 

 of Franseria, growing by the Wash. The larger laterals originated from 

 15 to 50 cm. beneath the surface of the ground, and, as is usual with the 

 roots of plants which grow where the soil is deep, either went straight 

 down or fairly horizontal!}-. The depths and the lengths of two of the lat- 

 erals may be taken as being representative of all of the rest ; one arose 1 5 

 cm. from the surface and kept this distance very closel}- to end, 1.8 meters 

 from the main root ; the other lateral, which arose at about the same depth, 

 after running straight out 10 cm., turned downward at a sharp angle to a 

 depth of 57 cm., when it took a horizontal course for a short distance. 



