ROOT SYSTEMS 35 



arhusciila the roots lie within 2 cm. of the surface. Obvi- 

 ously such a system can serve to absorb water only when the 

 surface layers of the soil are saturated after rains. The 

 immense water storage capacity of these plants makes this 

 method possible. The cactus root system is very strongly 

 specialised in Cannon's sense. 



A specialised root system with a strongly developed 

 tap is exhibited by Koeberlinia spinosa, a leafless spiny 

 member of the Capparidaceas. Most of the desert shrubs 

 have, however, generalised root systems, or systems more or 

 less like that of the cactus. The mesquite or locust, Prosopis 

 velutina, grows as a small tree on the flood plain of the 

 rivers, Vv'here its tap root may reach to ground water at a 

 depth of 25 to 40 ft. On the shallow soils of the desert it 

 remains a small bush ; the tap may penetrate deeply if the 

 soil permits, but it is often kept short by the underlying 

 hard pan. A mass of laterals is then developed close to 

 the surface and may stretch to the enormous distance of 

 50 ft. from the parent root. The creosote bush, Larrea 

 tridejitatay the most abundant shrub of the region, grows 

 both on the flood plain and on the true desert. In the 

 shallow soils of the latter it sends a strong tap to the depth 

 of 2 ft. ; where this meets the pan it forks and runs horizon- 

 tally for a considerable distance. A number of sparingly 

 branched laterals arise within 6 in. of the surface and 

 spread to about 12 ft. from the plant. Most of the shrubs 

 resemble the cactus in producing bunches of fine absorbing 

 rootlets which last through the periods when water is 

 available and then die oif. 



It will be seen that the v/ater supply of all these plants is 

 typically drawn from the surface 2 to 3 ft. of the soil. 

 Deep-going roots are usually impossible. The same con- 

 clusion is reached by Fitting (191 1) for the extreme desert 

 near Biskra in Algeria. He did not examine the root systems 

 in detail, but he points out that rock underlies the soil at 

 depths of 3 to 9 ft., and that there is no water reserve at 

 these depths. He found many of the root systems to be 

 poorly developed. Cannon (1913), in a more detailed 



