COLD-RESIStANCE; IN SPINELESS CaCTI 



141 



however, would not result in the death of the cells, as plant tissues 

 which have been frozen but not killed often show more or less en- 

 largement of the intercellular spaces. This was true with all spe- 

 cies of cacti studied. It is clear, therefore, that the protoplasm was 

 killed by certain low temperatures which it could not withstand. 

 With our present knowledge of chemical and physical phenomena 

 of life, it is not possible to study the internal behavior of the pro- 

 toplasm at the time when the temperature is fatal. 



It is of both scientific and economic importance to determine 

 at just what temperatures these different species are injured and 

 killed by the cold, since some are able to withstand lower tempera- 

 tures than others. When a cactus plant is injured somewhat — i. e., 

 only parts of the joints are killed, the plant may recover and make 

 good growth the following season. In the numerous experiments 

 performed in this part of the work, the results of which are sum- 

 marized in the table below, it was found that the same low tem- 

 perature which would injure a piece of cactus stem would kill it 

 if continued for some time, or if repeated five to eight times. Such a 

 temperature, therefore, must be regarded as the killing tempera- 

 ture. The less hardy varieties, like Opuntia ficus indica and Opuntia 

 sp. Burbank Special, were injured with temperatures between —5° 

 and — 6° C. (23° and 21.2° F.) and were killed when subjected to 

 these temperatures several times. On the other hand, Opuntia 

 castillae and 0. Ellisiana were injured at temperatures of — -14° and 

 —16° C. (6.8° and 3.2° F.), respectively. It is interesting to observe 

 that there is no great difference between the temperature which 

 will injure a plant and the temperature which is fatal to the plant 

 in a short time. 



TABLE SHOWING TEMPERATURES AT WHICH PIECES OF CACTUS STEMS 

 WERE DAMAGED AND ALSO KILLED OUTRIGHT, WITH DIFFERENCES 



IN TEMPERATURE 



