ROOT VARIATION IN DESERT PLANTS 335 



ation of the roots of the cactus type in mind, to plan experi- 

 ments that would test the extremes of the variation as it was 

 not supposed this was reached either in nature, or in the 

 experimental plot. The method employed was that of causing 

 cuttings, or seedlings, to grow in glass, or metal tubes of a vary- 

 ing diameter and varying length. By this means deep penetra- 

 tion, following lateral restraint, would be induced, if such pene- 

 tration were possible. Further, the effect of a variety of factors, 

 such as variation in aeration, temperature, and moisture, could 

 be accurately observed. In the present paper, however, only the 

 actual penetration of the roots is recorded. 



Jatropha cardiophylla 



Aside from the cacti there is in the vicinity of the Desert 

 Laboratory, only one species with the superficial type of root- 

 system, namely, Jatropha cardiophylla. The roots of Fouquieria 

 splendens, which are reported on below, are in a measure inter- 

 mediate between the cactus-type and the generalized roots. The 

 behavior of the roots of both species are of interest in the pres- 

 ent connection. 



Jatropha is a low growing shrub with willow-like shoots which 

 bear leaves in summer oxAy. It is a plant with a water balance 

 by means of the roots which, as will appear inunediately, are 

 fleshy. The root-system of Jatropha, in its habitat on the do- 

 main of the Desert Laboratorj^, is peculiar in that anchoring 

 roots are wanting and the main root is placed horizontally and 

 is fleshy (compare Plate 13, The Root Habits of Desert Plants). 

 The laterals arising from the main root are also fleshy. All of 

 the roots lie within about 5 cm. of the surface of the ground. 



Early in the spring of 1912 a young specimen of Jatropha, 

 with a shoot 10 cm. long, was brought to the greenhouse and 

 placed in a glass tube 2.5 cm. in diameter and 43 cm. in length. 

 The soil in the tube was equal parts of sifted sand and adobe. 

 The culture was moistened thoroughly by sub-irrigation and 

 emersed for a portion of its length in one of the beds of the green- 

 house. During the summer the plant was watered frequently. 

 The culture was taken up September 11. 



