70 ROOT HABITS OF DESKRT PLANTS. 



responding roots of all other opuntias seen, branched repeatedly through- 

 out their course, in places forming a network closely covering the ground. 

 The laterals extended for about 3 meters and lay so near the surface of 

 the ground that the tips could be lifted out with a walking-stick, and they 

 could be torn out of the ground their entire length. More exactly stated, 

 the roots lie from 2 cm., and even less, to 8 cm. beneath the surface. 



Fig. 11. — Horizontal extension of root-system of Opuntia arhuscula. Black dots indicate position 

 of Covillea tridentata which had invaded the root-area of the cactus. 



As compared with the roots of other opuntias, those of arbiiscula are 

 large (plate 18). This will be appreciated from the following measure- 

 ments. A meter from its base one of the larger roots had a diameter of 

 2.5 cm., 2 meters distant it was 2 cm., 3 meters distant it was 1.5 cm. The 

 tips of these roots, excluding the most recent growth, were not less than 

 I cm. in diameter. From the size and tuber-like appearance of the roots it 

 was concluded that they function as water-storing organs. This conclusion 

 was strengthened by the observation that inside of 48 hours after the roots 

 of this plant were removed from the soil they were rapidly losing their 

 plump aspect and had begun to shrivel strikingly. A noteworthy peculi- 

 arity of the larger roots was their frequent abrupt ending through the death 

 of the more distal portion. 



The fleshy roots of arhuscula, in addition to functioning as water-storing 

 organs, a rare root-character among mature opuntias, further serve as prop- 

 agating organs, and to this function the occurrence of the species in masses 

 is due. The procedure, in brief, is this. One or more shoots may arise 

 from a single lateral while the latter retains its connection with the parent. 



