Cold Resistance in Spineless Cacti 



INTRODUCTION 



The differences in the frost resistance of cacti, the importance 

 of these plants in parts of Arizona as emergency and supplemental 

 stock feed, and the perennial interest in spineless cacti as possible 

 forage plants, led to the outhning of this Adams fund project. 

 This study has for its object the determination of the causes of hardi- 

 ness in cacti and is designed to throw light on the question as to 

 whether resistance to cold in these plants lies in the character and 

 structure of the plant body or in the character of the cell sap and 

 the protoplasm. An attempt has been made to determine the limits 

 of resistance in several species and varieties of spineless cacti. These 

 plants have been studied both in the field and in the laboratory 

 with reference to the relation between their morphology and physi- 

 ology and cold resistance. The behavior of their protoplasm at 

 low temperatures has been carefully noted. No attempt has been 

 made to determine the critical temperature which the plant can 

 endure without injury to its tissues, but the minimum freezing tem- 

 perature which has been found to damage the plant has been noted, 

 also the temperature at which the plant is killed quickly. The 

 results obtained in this study by Mr. Uphof check closely with 

 field observations made by the writer on Opuntia castillae over 

 a period of fifteen years. 



This experiment was also suggested partly through the failure 

 of Burbank spineless cacti to grow successfully under Arizona con- 

 ditions and to endure our winters, as noted by the writer in Bulletin 

 67 of this Station. This observation, together with the fact that our 

 State has a rich and varied cactus flora, led to numerous speculations 

 concerning the factors of hardiness in cacti. Though this study 

 has been outlined for some time, the lal^oratory work was begun by 

 Mr. Uphof, in cooperation with the writer, only last summer. 

 Recent studies in the native cacti of our State have led to the 

 observation that the various species almost invariably have a 

 limited altitudinal distribution. This means, in brief, that the 

 different species are confined to areas similar in temperature and 

 rainfall conditions. In a few instances low winter temperatures 

 are known to be the chief factor in limiting the distribution of certain 

 species, as, for example, Cereus Thiirheri, the organ-pipe cactus, 

 Cereus giganieus, the giant cactus, and Cereus Schottii. The explana- 



