Cold-Resistance in vSpineless Cacti 



137 



The above facts, however, are entirely independent of both 

 cooling point and actual freezing point, and, as we will see later, are 

 also independent of the degree of cold required to kill the plants. 



Without doubt the duration of the freezing point is related 

 directly to respiration, which the plant is able to perform at low 

 temperatures until the water is entirely frozen, at which time 

 the protoplasm probably becomes dormant. This freezing of the 

 water takes place at the end or about the end of the nearly straight 

 line in the diagrams. As already noted, this line is shorter in 

 some species of cacti than in others. The curve is quite constant 

 for the same species, but there appears to be no relation be- 

 tween the length of this line and the, cold resistance of a species. 



1£. 



Fig. 9. — Curves showing the freezing of dead tissues of spineless cacti, including Opuntia 

 sp.Burbank Special, O. fusicaulis and O. ficus indica. Compare with curves of living tissues. 



It is interesting to observe the character of the curve when 

 pieces of recently killed cactus stems are substituted for the living 

 ones. These show a very much shorter line, which soon becomes 

 strongly curved downward, as shown in Figure 9. This line 

 is practically the same for Opuntia ficus indica, Opuntia sp. Bur- 

 bank Special, and 0. fusicaulis after the freezing point is reached. 

 Similar results were obtained with other species. 



