ROOT VARIATION IN DESERT PLANTS 



329 



in mind it was of especial interest to observe root behavior of 

 these species when grown under parallel conditions of soils, water 

 relations, and of temperature. 



Examination of several specimens of 0. arbuscula, from Saca- 

 ton, grown in the experimental plot, showed the following fea- 

 tm"es of the roots. From the base of the stem there arose a 

 brush of supporting roots which extended downward and out- 

 ward for a distance of about 30 cm. The long, superficially 

 placed laterals, prominent in the species in its proper habitat, 

 were lacking. The root-system was essentially a generalized one, 

 although of a very limited extent. The roots were fibrous 

 throughout with no trace of the persistence of the embryonic 

 fleshy roots. The general appearance of the roots of one of the 

 garden-grown specimens is shown in figure 2. 



The specimens of 0. neoarhuscula, from Tucson, were grown 

 in the experimental plot both in sand and in the garden soil. 

 The roots in both situations were similar in character, except 

 only that those in sand lay about 5 cm. deeper than those in the 

 finer soil. The roots were not numerous and did not form a 

 brush. They lay 8 to 10 cm. beneath the surface and thus 

 were much more deeply placed than in the proper habitat. The 

 leading roots were small where they took their origin but they 

 thickened rapidly until toward the distal end they become narrow- 

 1}' spindle-form. Figure 3, A, gives the leading characteristics 

 of the roots of the species when grown in the experimental plot. 



A comparison of the root-systems of the two species when 

 grown under similar conditions in the garden shows no change 

 from what is characteristic of each in nature as regards the 

 quality of fleshiness, but it reveals the interesting condition in 

 both species of less differentiation where the soil is moist than 

 where it is dry. The fibrous root habit of 0. neoarhuscula and 

 the fleshy root habit of 0. arbuscula are thus deep seated and 

 are to be regarded of fiixed morphological rank. 



Opuntia spinosior occurs in the \dcinity of Tucson on the bajada 

 and has a root system which is similar to that of 0. versicolor 

 as sketched in an earlier paragraph. It was for this reason that 

 this, a representative cylindropuntia, was chosen for observation. 



