144 Bulletin 79 



is fatal. Injury b)^ frost of a normal cactus plant — i. e., one healthy 

 and of mature growth, is due to the temperature faUing below the 

 point which the protoplasm can endure. 



5. This study indicates that Opuniia castillae and 0. EUisiana 

 are resistant to lower temperatures than the other species of spine- 

 less cacti studied, being injured at temperatures of — 14° and — IG' 

 C. (6.8° and 3.2° F.), respectively, while Opuntia ficus indica and 

 Opuniia sp. Burbank Special were injured with temperatures of —5° 



and 6° C. (23° and 21.2° F.), respectively. These results agree 



in general with observations made on these same species under 

 field conditions. 



6. The temperature which damages a plant to any extent will 

 kill the plant if continued long enough or if repeated several times. 

 This may be regarded as the "killing" temperature. 



7. The reason that one species of cactus endures more cold 

 than another is because of a difference in the character of the 

 protoplasm, due allowance being made for the thickness of the 

 integument when the cold extends over only a short period. 



