ILLUSTRATIONS 



PLATES 



Plate I. Plant of Opuntia Castillae in the cactus garden. University grounds, 

 uninjured with a temperature of 12 degrees F., but seriously 

 frozen with a temperature of 6 degrees F ..Frontispiece 



Plate II. Plant of Opuntia sp. Burbank Special in the introduction garden, 

 University Farm, injured with temperatures of 20 degrees F. 

 Plant of rather rapid growth Frontispiece 



FIGURES 



Fig. 1 .-Cross-section of a piece of joint of Opuntia castillae about two weeks 



old, showing regular character of cells 125 



Fig. 2.-Cross-section of a piece of joint of Opuntia castillae about two 



years old 126 



Fig. 3.- Cross-section of a piece of joint of Opuntia sp. Burbank Special 



about two years old 127 



Fig. 4.-Cross-section of a piece of joint of Opuntia Ellisiana about two 



3'ears old 127 



Fig. 5.-Apparatus used for determining the rate of penetration of temper- 

 ature through the integuments of various spineless cacti; and 

 also for making a. comparative study of the cooling points, freez- 

 ing points and duration of freezing points of difTerent spineless 

 cacti 129 



Fig^ 6.-Curve showing cooling point, freezing point and duratiori of freez- 

 ing point of Opuntia castillae 132 



Fig. 7.-Curve showing cooling point, freezing point and duration of freez- 

 ing point of Opuntia Ellisiana , 133 



Fig. 8.-Curve showing cooling point, freezing point and duration of freez- 

 ing point of Opuntia sp. Burbank Special 133 



Fig. 9.-Curves showing the freezing of dead tissues of spineless cacti, in- 

 cluding Opuntia sp. Burbank Special, Opuntia fusicaulis, and 0. 

 ficus indica 137 



Fig. lO.-Apparatus used for studying the effects of low temperatures and 



of freezing on the protoplasm of cacti 139 



