REPORT OP COMMITTEE ON PUBLICATION 



145 



The cells of the spongy parenchyme are first to exhibit the indications 

 of the disease, followed shortly by the palisade cells. The vascular tissue 

 is never affected but the mesophyll cells immediately adjacent are first to be 

 injured. This is particularly noticeable during dry weather. The epidermal 

 cells may finally die, due rather to the death of the contiguous cells than to 

 the direct effects of the mycelium. 



Coincident with the loss of color the cell contents become plasmolysed, 

 the wall collapses and finally 'the entire cell becomes brown. This effect 

 seems to occur first in the palisade cells bordering the veins rather than in 

 the spongy parenchyme, as one would expect, thus producing the peculiar 

 network of brown lines so prominent on the upper surface within the yellow 

 area. Gradually all the cells throughout the older portion of the spot are 

 killed, causing the dead brown center. The collapsing of the diseased cells 

 reduces the thickness of the leaf within the lesion to approximately one- 

 half to two-thirds that of the normal. 



The cortex, collenchyma, epidermis, medullary rays and pith of the stems 

 are affected. Istvanffi (1913) states that the phloem and rarely the xylem 

 may also be involved. The writer has never seen any evidence of the disease 

 in these tissues. Farlow (1876) also states that all parts of the stem are 

 affected except the wood. In the writer's opinion this difference can only 

 be interpreted as further proof of the greater susceptibility of the European 

 varieties. 



Plate 5. Sections of healthy and diseased shoots. 



