ROOT-SYSTEMS OF PERENNIALS. 



Opuntia Vivipara. 



In the vicinity of Tucson the cacti usually occupy the more arid situations 

 as the bajada or Tumamoc Hill, and avoid the flood-plain, but among the 

 exceptions to this condition should be included Pcnioceyeus greggii and 

 Opuntia vivipara. The former, owing to the fleshy main root, requires con- 

 siderable earth-room for its full development, and usually occurs 

 where the soil is relatively deep. Opuntia vivipara is'k.nown only 

 from a locality not far from the 

 mouth of a wash which debouches on 

 the flood-plain of the vSanta Cruz near 

 the Nine-mile Water-hole. 



Opuntia vivipara is of the cylind- 

 rical type and is remarkable especially for the great 

 number of young plants which take their origin from 

 fallen "joints" and which cover the ground, beneath 

 every large individual of the species, with a fairly 

 dense growth. The manner of origin of most of the 

 plants is indicated by the figure in plate 21, which 

 also shows something of the fleshy nature of the 

 young roots. 



The root-systems of several plants were examined 

 and what follows is merely a resume of the observa- 

 tions made on them. In most regards the root- 

 system of the mature plant of vivipara is similar to 

 other arborescent opuntias, e.g., that of O. versicolor, 

 as described in a foregoing paragraph ; that is, there 

 is an anchoring system and an absorbing system, both well differ- 

 entiated. But in a certain particular the roots of this species are unlike 

 those of any other cactus examined : they are usually slender, but occasion- 

 ally one is found which is fleshy, although the more distal portion as well 

 as the proximal portion may be of the usual type. The departure from 

 the usual root-type Avill appear in the following measurements. A lateral 

 of the usual form, that is, 3 to 4 mm. in diameter up to a point 50 cm. from 

 its place of origin, may then become 1.5 cm. in diameter and retain this 

 larger size for 25 cm., while nearer as well as farther from the main root the 

 lateral will be of the slender type. The peculiarit>' of the root-system of 

 the species, in addition to the fact that fleshy roots are present, is that both 

 slender and fleshy roots occur on the same plant. 



It was shown in the case of Opuntia arhuscula that the fleshy roots of that 

 species gave rise to plants as a regular habit, and it was learned that the 

 fleshy roots of O. vivipara are capable of doing the same thing, although 

 the characteristic has not been observed in nature. 



Fig. 16. — Horizontal exten- 

 sion of root-system of 

 Opuntia vivipara. All 

 laterals in this specimen 

 were slender. 



