142 Bulletin 79 



From the above table it is observed that Opuntia castillae and 

 O. Ellisiana are resistant to lower temperatures than any of the 

 other species experimented with, being injured at -14° and -16° C. 

 (6.8° and 3.2° F.), respectively, and that Opuntia ficus indica from 

 both Malta and Sicily and Opuntia sp. Burbank Special are least 

 resistant to cold. These species show injury at temperatures of 

 —5° and— 6° C. (23° and 21.2° F.), respectively. Another species, 

 Opuntia fusicatilis, was injured at a temperature of — 8° C. (17.6° F.). 

 As will be shown later, these results coincide with observations made 

 under field conditions during the winter of 1915-16. 



In this work it must be remembered that only pieces of cactus 

 stems were used and that for the most part these were protected 

 by the thick integument of the cactus on but two sides, above and 

 below, while the cut surfaces were naturally without the protection 

 of the integument. Since experiments, the results of which are given 

 on page 10, indicate that a considerable length of time is required 

 for the low temperatures to penetrate the integument, it is to be 

 inferred that uncut joints would resist the same low temperature 

 for a greater length of time. This, however, represents only 

 difference in the length of time required for the penetration of the 

 low temperature. It would not enable the protoplasm to endure 

 a greater degree of cold. 



BEHAVIOR OF PLANTS OF SPINELESS CACTI GROWING IN THE INTRO- 

 DUCTION GARDEN AT THE UNIVERSITY FARM DURING THE WINTER 



OF 1915-1916 



The first frost of any note during the winter of 1915-16 occurred 

 on the night of December 16-17, when the temperature dropped 

 to —1.50 C. (29.3 F.). None of the species of cacti was injured. 

 A little later there were several cold nights when the temperature 

 dropped as low as —4° and —6° C. (24.8° and 21.2° F.). With 

 these temperatures some injury was don^ to plants of Opuntia ficus 

 indica and Opuntia sp. Burbank Special. This was particularly 

 true of the younger and partly immature joints, which lost their 

 color and turgor and began to shrink. 



On December 28, the temperature dropped to — 7.20° C. 

 (19.04° F.) on the University grounds, the corresponding tempera- 

 ture at the University Farm being one or two degrees lower. 

 With this, considerable damage was done to plants oi Opuntia fi,cus 

 indica and Opuntia sp. Burbank Special growing at the University 

 Farm. The ends, also, of the less mature joints of Opuntia fusicaulis 



