CONTENTS 



1 1Q 



Introduction '■^^ 



Histological Studies 123 



General structure of the outer, thickened part or integument of a 



cactus stem ^■'"^ 



Study of relative thickness of cuticle, crystal-bearing cell layer, and 



thick-walled cells of different spineless opuntias 126 



Relation of penetration of temperature to thickness of integument of 



cacti 128 



Physiological Studies 1^^ 



Effects of temperature at the melting point of ice on the tissues 130 



Protoplasmic movements *-^'- 



Determination of cooling and freezing points in cacti 131 



Comparison of cooling points, freezing points, and duration of freez- 

 ing points in different spineless cacti 132 



Summary of results of ten experiments with each of six species and 



varieties of Opuntia l*** 



Results of other workers • l^* 



Comparison of results 1^^ 



Effects of freezing the tissues 138 



Effects of cold on the protoplasm 140 



Table showing temperatures at which pieces of cactus stems were 



damaged and also killed outright, with differences in temperature.... 141 

 Behavior of plants of spineless cacti growing in the introduction gar- 

 den at the University Farm, during the winter of 1915-1916 142 



Summary 1*3 



