126 



BULLE^TIN 79 



It will be interesting, therefore, to describe the outside layers 

 of each of the six species of cacti with which the writer worked. 



STUDY OF RELATIVE THICKNESS OF CUTICLE, CRYSTAL-BEARING CELL 

 LAYER AND THICK-WALLED CELLS OF DIFFERENT SPINELESS OPUN- 

 TIAS 



1. Opuntia castillae has a cuticle 15 to 20 \k thick, the cells of 

 the crystal-bearing layer are from 30 to 42 [x thick, and the following 

 thick-celled layer islOO to 120 [x thick. Individuals from the Uni- 

 versity Farm, from the cactus garden on the campus of the University 

 of Arizona, and from a garden in the vicinity of Tucson were obtained. 

 These plants all showed the same general structure. In the writer's 

 experiments, plants from the University Farm were used, as the 

 other species which follow were also grown in the introduction gar- 

 den at the University Farm under similar conditions. This species 

 was damaged at a temperature of — 14° C. (6.8° F.). 



2. 0. ficus indica, from Malta. This species shows, in compari- 

 son with the former, a cuticle 5 to 10 [x thick; the crystal-bearing 

 layer is 30 to 38 [jl thick, and the thick-celled layer is 55 to 75 {i,. 

 in thickness. This species is not hardy in the field and was injured 

 at temperatures below — 6° C. (21.2° F.) in the laboratory. 



Fig. 2. — Cross-section of a piece of joint of Opuntia casliUa* 

 about two years old. The integument is thick. 

 (Highly magnified) 



