102 Ageicultueal Expeeiment Station, Ithaca, N. T. 



of a cushion similar to an erinoid or hypertrophied condition of the 

 tissue. The epidermal cells were very much enlarged while the 

 chlorophyll bearing cells just beneath as well as some of the more 

 deeply seated cells were greatly elongated in a radial direction, and 

 strongly clavate at their outer extremity where this extended beyond 

 the lateral pressure from adjacent tissue. In many cases the epidermal 

 cells quickly separate and slough oif. The cells of the affected areas 

 possess exceedingly delicate walls so that with little disturbance they 

 would collapse. There was little protoplasm in proportion to the size 

 of the cell and a corresponding amount of cell sap. 



Several of the notable phenomena coincident with the development 

 of these cushions of abnormally turgescent tissue are exjjlained by 

 this peculiar physical derangement of the normal cell structure. The 

 frosted or whitened aspect of the cushions results from the small 

 amount of chlorophyll in proportion to the leaf surface. The amount 

 of chlorophyll in the individual cell remains the same while the grains 

 are far separated in their distribution throughout the greatly enlarged 

 cell. The curling upward of the leaves results from the greater 

 lateral pressure which exists in the cells of the lower surface of the 

 veins. 



A comparison of the normal tissues with that of the cushions will 

 serve to show the profound changes accompanying their development. 

 From serial sections made through several of these cushions on the 

 leaf veins two sections were selected from one series to illustrate the 

 changes in this part of the leaf. Camera lucida drawings were made 

 from these two sections magnified to the same scale. One of these 

 represented in figure 4 was taken from the leaf vein just before the 

 series of sections entered the cushion and practically represents the 

 normal arrangement and form of the cells of the leaf vein. Figure 

 5 represents one of the serial sections of the same leaf vein, the section 

 having been taken from the center of the cushion. Each end of the sec- 

 tion at a represents relatively the normal condition of the cells of the 

 leaf. The line of epidermal cells, e, e, is still adhering for some 

 distance, but at the summit of the cushion they have fallen away. 

 A conijiarifon with the normal epidermal cells shows how much 

 they have been enlarged. The sub epidermal cells have undergone 

 tne most profound change, being eight to ten times their normal size. 

 Some of the deeper lying cells have also elong ted radially, while others 

 have become abnormally enlarged. It will be observed that the disten- 

 tion of the cells has not only profoundly changed their normal form, 



