34 THE BIOLOGY OF FLOWERING PLANTS 



necessitates a better absorbing system. Both classes must 

 be regarded as short-lived plants, which draw their water 

 from the upper layer of soil during the brief periods when this 

 is relatively moist. 



Of the desert perennials the most distinctive root systems 

 are shown by the Cactuses. Thus Optmtia versicolor 

 (Fig. 3) sends down a stout anchoring tap root with few 



^ 



Fig. 3. — Root system of Opiintia venicolor ; above, plan of about half 

 the root system, the large black dot representing the position of the 

 shoot; below, section showing the short anchoiing root and the depth 

 of the shallow system on one side. (After Cannon, modified.) 



branches to a maximum depth of i ft. A small number of 

 laterals arise from the tap just below the surface of the soil. 

 These do not branch much, but they give rise to bunches of 

 fine rootlets which are renewed yearly. The laterals and 

 their branches spread out in every direction, keeping within 

 a few centimetres of the surface, for a very great distance ; 

 in one instance they reached a distance of 14 ft. from the 

 tap root. The absorbing system of this cactus, which is 

 typical of the family, is thus very superficial ; in Optmtia 



