ANATOMY OF OPUNTIA LINDHEIMERI ENGELM. 297 



from the same initial layer. Figure 8, which is taken from the 

 margin of one of the discolored areas, shows that the stone cork 

 is merely a secondary modification of the thin-walled cork. Not 

 infrequently this tissue is several cell layers in thickness (fig. 6). 

 The cell layers are not always continuous but they may be one 

 cell thick in some places and two or three cells thick in others 

 immediately adjacent (fig. 7). 



The stone cork cells of cacti, which were previously misrepre- 

 sented both by Schleiden and myself, are thickened along the 

 outer and lateral walls, leaving a portion of the inner wall very 

 thin. Since the secondary thickening is not uniformly refractive, 

 the cell walls appear to be stratified. In case the tissue is more 

 than one cell layer in thickness, minute pores or canals extend 

 from the lumen of the lower cell to that of the one above (fig. 6) . 

 In single cell layers the walls are not pitted. The thickened wall 

 of the upper cell in a series is never porous. The position of these 

 canals or pits indicates that the active phellogen is always on the 

 inside of the zone of stone cork. 



