Cold-Reststance in Spineless Cacti 



125 



lie, in an early stage, in a straight radial direction and take their ori- 

 gin from the periblem, which direction may be observed here and 

 there in Fig. 1. At a later age it is impossible to find out how the 

 tissues were formed, as the cells of the green parenchyma sometimes 

 divide longitudinally, and the thick- walled cells are so crowded that 

 they no longer lie inradial rows. 



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



weeks old, showing regular character of cells. (Highly magnified). 



It is hardly necessary to mention that the thick cuticle, the 

 crystal-bearing layer, and its adherent thick-celled tissues, together 

 with the slimy contents of the green parenchyma, are of great 

 importance to the plant, as these protect the stem against a high 

 transpiration rate, which would be disastrous for the species in a 

 dry and hot climate. It would not be worth while to describe the 

 thickness of the layers of that part which forms the integument of 

 the stem (i. e., cuticle, epidermis, crystal-bearing layer, and thick- 

 celled layer) if it were not observed that this integument is thickest 

 in those species which are resistant to cold. More than this, 

 physiological experiments demonstrated that the time of penetra- 

 tion of a certain temperature varies greatly when integuments 

 of different species of cacti are used. 



