Cold Resistance in vSpineless Cacti 143 



were injured. No damage was done to plants of Opuntia castillae 

 or O. Ellisiana, which remained in excellent condition throughout 

 the winter. Plants of Opuntia castillae growing on the University 

 grounds and also in several parts of Tucson showed no injury 

 from cold. 



The lowest temperatures recorded on the University grounds 

 during the months of January and February, 1916, were — -5° C. 

 (23° F.) on the night of January 11-12, and —5.6° C. (21.9° F.) on 

 the night of February 1-2. There are no temperature records at the 

 University Farm for these dates, but it is safe to assume that the 

 minimum temperatures there were one or two degrees lower than 

 those given above. During these cold nights plants of Opuntia ficus 

 indica and Opuntia sp. Burbank Special were slightly injured. 



SUMMARY 



1. In this study it has been found that the species of spineless 

 cacti having relatively thick integuments are more resistant to low 

 temperatures than those having somewhat thinner integuments, as 

 the penetration of low temperatures through a thick integument is 

 slower than through a thinner one. The term integument is used 

 here to include the cuticle, epidermis, crystal-bearing layer, and 

 several layers of thick-walled cells lying immediately below. The 

 thick integument may be an accidental rather than a developed 

 character. Since our lowest winter temperatures are often of short 

 duration, a cactus plant having a thick integument may pass 

 through such a period practically uninjured, even though the tem- 

 perature may be low enough to be fatal to the protoplasm. A 

 thick integument protects a cactus plant against sudden and severe 

 temperature changes at any season. 



2. The freezing point of the cell-sap of the cactus plant is very 

 little below 0° C. (32° F.), the freezing point of pure water. This 

 suggests that the soluble substances in the cell-sap of cacti are 

 Tery diffuse. The ordinary cactus contains as much as 90 per 

 cent water. 



3. The collecting and freezing in the intercellular spaces of the 

 water from the cells is not in itself particularly harmful to the 

 plant. Neither is the protoplasm poisoned with the. concentration 

 of the cell-sap solution as a result of at least a part of the water 

 being withdrawn and frozen. 



4. The protoplasm of these plants can withstand without injury 

 a certain low critical temperature, but a temperature below this 



