ANATOMY OF OPUNTIA LINDHEIMERI ENGELM. 295 



by a tangential division of the epidermal cells (fig. 5). The 

 inner layer of daughter cells then becomes the initial phellogenetic 

 layer. It is, therefore, by the successive division of this initial 

 layer that the cork tissue above becomes progressively thicker. 



There often appear, on the segments, rusty brown areas be- 

 ginning usually in the areolae of the spines. Sorauer 4 is of the 

 opinion that these may be due to unfavorable atmospheric in- 

 fluences. This condition may also be due, in part at least, as 

 I have previously pointed out, to the presence of a fungus, Hen- 

 dersonia opuntiae. These areas may remain superficial so that 

 the epidermis alone is concerned in initiating cork formation. 

 At other times more deeply seated layers may be involved, in- 

 dicating that any part of the epidermal and fundamental tissue 

 systems may generate cork. One of the diseased areas of the 

 latter type is represented diagrammatically in figure 1. A thick 

 layer of cork formed by the epidermis, hypoderma, collenchyma 

 and palisade parenchyma separates the discolored area from the 

 surrounding tissue. The margin of one of these areas is shown 

 in detail in figure 10. When this pathological condition occurs, 

 the stomata in the affected area are barred by the formation of 

 cork. As far as can be learned no one has properly illustrated or 

 described the manner in which this is brought about. Schleiden's 

 illustrations and my own do not give any idea of the process. 

 As indicated by the dotted line in figure 3, the initiation r.f this 

 process consists in a division of the accessory guard cells parallel 

 to their inner wall. The lowermost cell becomes the phellogen- 

 etic cell for the series of successive divisions which follow. As 

 growth continues, a slight protrusion and enlargement results 

 in the closure of the small space that is always present between 

 normal accessory guard cells. An advanced stage in this process 

 of barring is exemplified in figure 4, from which the manner in 

 which this phenomenon is brought about is readily apparent. 

 Often too, the crystal-bearing hypodermal cells protrude into the 

 substomatal opening, as shown in figure 2, closing it partially or 

 entirely. 



Contrary to my previous account, the thick walled tissue, in- 



4 Sorauer, P., Handbuch der Pflanzenkrankheiten 1 : 427-430, 1909. 



