ROOT-SYSTEMS OF PEREXXIAI.S. 71 



The shoots put down roots, the connection with the parent plant is prob- 

 ably early cut off (when was not learned), and the daughter plant is inde- 

 pendent. The result is that arbuscula frequently forms clumps several 

 meters in diameter, to the total exclusion of other species. 



From the previous description of the root-system of Covillca it is perhaps 

 hardly necessary to point out that the roots of this species growing within 

 the root-area of the cactus are much deeper than the roots of the latter 

 plant, and hence are not in active competition with the cactus roots for 

 their water supply. 



Opuntia Fulgida. 



On the domain of the Desert Laboratory, Opuntia fulgida occurs only 

 on the bajada near West Wash, where in places it forms fairly dense growths 

 constituting the most prevalent species. This is an unusual habit for any 

 cactus in this vicinity, since usually the cacti are either scattered in an 

 apparently chance fashion, unless controlled by such external factors as 

 temperature or other physical environmental conditions, or at best but 

 few indi^•iduals of a species are congregated. 



The soil in the habitat of fulgida is a sandy clay with an admixture of 

 rocks of various sizes which appeared to be in stages of active disintegra- 

 tion. No caliche was found to a depth of 60 cm. The habitat, therefore, 

 presents certain striking differences from that of Franseria, not far distant. 



In the immediate vicinity of the Opuntia studied were Acacia constricta, 

 Echinoccreus fcndlcri, Covillea tridcntata, Opuntia fulgida, Opuntia versicolor, 

 Parkinsonia micro phylla, and Riddel I ia coo peri, and of these plants only one 

 example of Acacia and two of Riddel Ha occurred within the root-area of the 

 cactus. vSince, as will appear directly, the roots of fulgida extended as 

 far as 3 meters from the main plant axis, the perennial population of the 

 habitat is relatively light. 



The specimen selected for study was growing somewhat apart from the 

 typical fulgida habitat, but the plant appeared to represent thoroughly the 

 typical and mature species. The root-system, like that of Opuntia versi- 

 color, has a somewhat deeply penetrating portion, the anchoring roots, and 

 a widely reaching horizontal portion not far from the surface of the ground. 

 The horizontal and vertical extensions of the root-svstem are shown in 

 fig- 12. 



The anchoring system consisted of about six stout roots which ran down 

 at an acute angle to a depth of 35 cm. Of these the largest was 2.5 cm. at 

 the crown ; it tapered rapidly and gave off several branches which ran out- 

 ward for a distance of 5 to 10 cm. The anchoring roots did not reach hard- 

 pan or bed-rock, or other material which might prevent deeper penetration. 

 The superficial root-system consisted of 6 main laterals, of which 4 ran 3 

 meters, more or less, in an easterly direction (down-hill), and the others 

 took a fairly well-defined up-hill direction. The roots branched but little; 

 for example, one 3 meters long gave off no important branches, and a some- 



