ROOT VARIATION IN DESERT PLANTS 



339 



Upon the termination of the culture it was seen that the main 

 root was 1.12 m. long and had penetrated to within about 25 

 cm. of the lower end of the tube. At the surface of the soil 

 there were many laterals, which were short and stout, which 

 made a dense weft and plugged the tube. At a depth of 20 cm. 

 a lateral arose which reached nearly to the tip of the main root. 

 Between the uppermost tuft of laterals and the long one, and 

 below to a depth of 95 cm., but few laterals were formed. Near 

 the tip of the root, however, the branches were numerous. 



Opuntia versicolor 



The usual condition of the roots of Opuntia versicolor has been 

 described above. To learn how far the root development could 

 be changed from that observed in nature a tube culture of the 

 species was set up early in May, 1912. The glass tube was 1.5 

 cm. in diameter and 1.0 m. long. It was filled with sifted sand 

 and adobe, two parts of the former to one part of the latter. 

 During the spring and the summer the culture was given water 

 in the greenhouse, where it was runn'ng, until October 8 when 

 the culture was ended. An examination of the root-system 

 showed that the main root had penetrated to a depth of 78 cm. 

 and that it bore short laterals, 2 to 3 cm. in length, throughout 

 its entire course. At the crown of the main root the laterals 

 were very dense and formed a heavy tuft, which is imperfectly 

 shown in figure 4 C. 



SUMMARY 



The present paper reports observations on root variation as 

 observed in nature, in garden cultures, and in especially planned 

 tube cultures. The leading results can be given as follows. 



Variation Under Natural Conditions. Root-sj^stems of 

 desert perennials may be roughly classed as being either gen- 

 eralized or specialized. The former type, of which the roots of 

 Franseria dumosa are given as an example, is capable of greater 

 variation than those of the latter. When growing in shallow 

 soils the roots of Franseria are necessarily shallowly placed, and, 

 in this regard, resemble those of the cacti. When growing in 



THE PLANT WORLD, VOL. 16, NO. 12, DECEMBER, 1913 



