332 



W. A. CANNON 



Opuntia discata 



Opuntia discata was the only platopuntia cultivated in the 

 experimental garden. The habitat of the specimen studied in 

 the garden was at Oracle, Arizona, but the same species occurs 

 on Tumamoc Hill, where the Desert Laboratory is situated, and 

 where the natural development of the root-system of the species 

 was seen. Briefly given, the root-system of 0. discata consists 

 of an anchoring portion, which attains a depth of 15 to 25 cm., 

 and an absorbing portion, which reaches outward for a distance 

 of 1.5 m., more or less, and which lies within 10 cm. of the sur- 

 face of the ground. The roots are fibrous. Thus the species 

 has a well marked absorbing and an equally well defined anchor- 

 ing system, as in the cylindropuntias. 



In the garden the specimens of 0. discata were planted in the 

 garden soil, in sand, and certain of them had roots both in the 

 sand and in the soil of the garden outside. A part of the plants, 

 also, projected a portion of theii^ roots away from the plot, nad, 

 as this portion of the main garden was not irrigated, there re- 

 sulted in the culture an unusually wide range in water relations 

 as well as in soil conditions. Upon taking up the culture it 

 was not surprising to find that the roots showed considerable 

 variation in direction as well as in extent of their development. 

 The main characteristics of the roots of the plants are as follows. 

 Such forms as were placed in the natural soil of the experimental 

 plot had roots which were fairly well differentiated into anchor- 

 ing and absorbing portions, as in nature. Certain of the absorb- 

 ing roots attained a length of about 1 m. Such specimens, on 

 the other hand, as were wholly in sand had anchoring roots 

 only, that is, a generalized root-system. Finally, such specimens 

 as were situated on the edge of the experimental plot, and which 

 were largely apart from the culture conditions of tiie plot, organ- 

 ized the usual brush of anchoring roots, and absorbing roots 

 which extended so far as 4 m. from the main axis. One such 

 plant is shown in figure 3, D. 



