S- Kixif iiAniTs I'l" i>i>i:kt im.ants. 



down to its (.-ikI, i . ;6 inctcrs (k-tp; i.S laterals, i c-in. or k-ss in dianicler, 

 were given off from the tap root within i 5 cm. of the surface of the ground ; 

 at a depth of 1 meter a larger lateral was given olT. Xone of the laterals 

 bore fdamentous roots along their course, as dirl those of I-^ucrhd. I'niinciid. 

 and other plants (plate 21/)). 



From this study of the root-systems of Comlnln: and Zizv(>htis it will be 

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

 Kacbaiinia, a well-developed ta]> root appears to be an essential character, 

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

 soil has considerable de]itli. 



Iu'iii;uu.\ 'rKii-rKC.\. 



Ephedra, on the doinain of the Desert Laboratory, occurs exclusi\el\- 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 qicggii, 

 Piosopis vilittiii ;. I'tnkinxonia tonrya)i(i. and a few specimens of Poiiocercus 

 greggii. shown in jilate j 1 , which is sonu what rare in the \icinit\' of Tucson. 



Fig. 5. — Root syslcin of Kfhctlra Irijiirca showing position of tap root and typiial latiTals. 



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 \vas 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 inthcjjlants 

 of Franseria. growing b>- the Wash. The larger laterals originated from 

 15 to 50 cm. Ix'neatli 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 horizontally. The depths and the lengths of two of the lat- 

 erals may be taken as being representative of all of the rest; one arose 15 

 cm. from the surface and kept this distance very closely 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., tiu-ned downward at a sharp angle to a 

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



