ROOT VARIATION IN DESERT PLANTS 



325 



were found to have a tap root 1.8 m. in length, and laterals 

 arising about 15 cm. beneath the surface of the soil. The lat- 

 erals extended outward 1.4 m., or less, and reached downward 

 78 cm., or less. There were other roots, secondary also, which 

 arose from the crown of the main root and which either ran 

 directly outward and horizontally, or downward to a depth of 

 1.3 m. The vertical extension of roots from the two localities 

 is shown in figure 1. 



The degree of variation in species having the third type of 

 root is naturally less than that in plants with generalized roots. 

 The range is variation can be illustrated by citing what has 

 been noted in a flesh}^ species, Echinocactus, and a non-fleshy 

 species, Opuntia versicolor, when growing under soil conditions 

 unlike as to depth. In shallow soils the root-system of Opuntia 

 has been found to consist of a fairly short tap root, 20 to 30 

 cm. in length, and laterals which may attain a length of 1 to 2 

 m. and which usually do not lie more deeply than 5 cm. Where 

 the soils are deeper a variation of root development consists 

 in there being a brush of roots in addition to the tap root, or 

 taking its place. In any case the roots are fairly well differen- 

 tiated into anchoring roots and absorbing roots, although, as 

 will appear directly, this differentiation is not so well marked 

 as in Echinocactus. The root-system of Echinocactus consists of 

 a main root, or brush of roots, which go directly downward, 

 and of laterals which arise close to the surface of the soil and 

 reach out widely. Sometimes the laterals are 3.5 m. in length; 

 they usually lie within 1.5 to 3 cm. of the surface. The greatest 

 var ation in the root-system of Echinocactus, which has been 

 observed in nature, consists in a large development of the anchor- 

 ing brush of roots and a relatively small development of the 

 absorbing and superficially placed roots. In this variation the 

 anchoring roots reached outward as well as downward and served 

 as absorbing roots in addition to the usual function. In fact 

 such a root-system closely resembles the generalized type of 

 other species. It is to be noted, however, that the roots of the 

 cactus do not attain a great depth. 



