130 



^Bulletin 79 



SUMMARY SHOWING TIME REQUIRED FOR A TEMPERATURE OE 0° C, 

 TO PENETRATE DIFFERENT CACTI 



Name of plant 



Opuntia castillae 



0. ficus indica, from Malta. . 



O. Etlisiana 



Opuntia sp. Burbank Special. 

 0. fusicaulis 



O. ficus indica, from Sicily I 32 



Average 

 Mins. 



63.3 

 35.0 

 79.3 

 32.6 

 38.6 

 40.6 



PHYSIOLOGICAL STUDIES 



In order to ascertain why these plants were injured by cold 

 it was necessary to consider four main problems: 



1. Are the plants killed at a temperature a little above the 

 freezing point of water, as is often the case with tender greenhouse 

 plants? 



2. Is the injury caused by the freezing of water in tissues of 

 the plant body? 



3. May the plant tissues be killed by poisoning caused by 

 the cell sap becoming more and more concentrated, as its wa- 

 ter gradually changes into ice? 



4. May the protoplasm withstand only certain temperatures 

 below 0° C. ^32° F.) without injury, and after having exceeded that 

 degree of cold, be killed? 



EFFECTS OF TEMPERATURE AT THE MELTING POINT OF ICE ON TH^ 

 ^ . TISSUES 



Beginning with the first problem — stems and parts of stems of 

 Opuntia castillae, 0. fusicaulis, 0. Ellisiana, Opuntia sp. Burbank 

 Special, O. ficus indica from Malta and from Sicily were put in 

 crushed ice in an ice box, where they remained for two days at a 

 temperature of 0° C. (32° F.). After six hours the material was exam- 

 ined, but was not at all injurjed. After 48 hours the material was 

 again examined, but none of the stems were killed or apparently 

 injured. Parts of these cactus joints were then placed in a constant 

 temperature oven at a temperature of 60° C. (140° F.) for six hours, 

 at the end of which time they showed no apparent injury. It is 

 evident, therefore, that these plants can endure a considerable 

 range of temperature without injury. It is quite possible, how- 

 ever, that they might have been injured by this latter temperature 

 had it been continued for a longer time. 



