328 W. A. CANNON 



Opuntia arhuscula and Opuntia neoarbuscula 



The center of interest in this culture lay in -the character of 

 the roots as regards being fibrous or fleshy. 0. arhuscula, a 

 cylindropuntia, has, in nature, fibrous roots, while those of 0. 

 neoarbuscula, also a cylindropuntia, are fleshy. In addition, the 

 roots of the latter species lie very near the surface of the ground^ 

 It was desired to learn whether variation in soils, or an espe- 

 cially good supply of water, would influence root development 

 away from that characteristic of each species. 



In its habitat on the bajada near Sacaton, Arizona, 0. arhus- 

 cula has a root-system similar in character to that of 0. versicolor 

 as sketched in a preceding paragraph. That is, there is a brush 

 of anchoring roots which penetrate the ground 25 cm., more or 

 less, and laterals which radiate from the central root axis and 

 assume a position close to the surface of the ground. As stated 

 above, the roots are fibrous tliroughout. 



The Tucson species, 0. neoarbuscula, has roots of a widely 

 different kind from the species at Sacaton. Not only are the 

 roots fleshy, as above given, but, also, they lie unusually close 

 to the surface of the ground. For example, the laterals are 

 mostly 1.5 cm. deep and can easily be torn from the ground, 

 or exposed by heavy rains. The anchoring roots, however, are 

 not unlike those of the Sacaton species. 



The character of fleshiness in the cacti is of especial interest. 

 It has been observed that the roots of seedling cacti, particularly 

 of the cylindropuntia type, as well as the roots that arise from 

 the ''joints" of such cacti, are fleshy, although, as is well known, 

 this character is soon lost. Fleshiness in young plants was at 

 first thought to be an embryonic condition simply, without ap- 

 parent and immediate external cause. At the same time it was 

 noted that the most vigorous growth occurred when the tem- 

 perature was high and the soil moist. Among those cacti whose 

 cuttings have fleshy roots was found to be 0. arhuscula, from 

 Sacaton, and, of course, the Tucson species. With these facts 



- W. A. Cannon. The root habits of desert plants. Publication No. 131, 1911, 

 Carnegie Institution of Washington. 



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