431 



of cork took place in the way described in the preceding general section. 

 It was proved, however, to be a secondary phenomenon. The disease first 

 manifested itself in the pale green color of the mesophyll usually near the 

 finely anastomosing veins. This appeared more frequently in the palisade 

 than in the spong}' parenchyma. In isolated cases, instead of becoming 

 pale, the cell contents discolored to a brownish tone which was accom- 

 panied by the suberization of the walls. The epidermis, in its changes, 

 followed the mesophyll groups and small dead tissue centers were produced 

 which did not change any further. 



In the group of cells forming the transparent places in the leaf because 

 of the dissolution of the chlorophyll, an enlargement was seen on account 



Fig. 72. Potato leaf perforated as a result of a morbid formation of cork. 



of which the non-participating epidermis was pushed outward. A cork 

 formation now set in among the enlarged mesophyll cells ; then these places 

 broke out. By the advance of these processes backward into the flesh of 

 the leaf, the cork centers were depressed to complete perforation. This 

 can be understood easily since young leaves are aft'ected. In their growth, 

 these stretch all the tissues ; since the tissues containing cork cannot stretch 

 with the other tissue, they must tear. 



The process, therefore, is, in principle, that found on the stems of 

 cacti. 



In other plants also, which show perforations of the leaves, the im- 

 poverishment and enlargement of different cell groups may be recognized 

 as the early stages and, on this account, naturally belong to the phenomena 



