ACTION OF THP: FUNGUS ON THE LEAF TISSUES. 45 



Tho Inidlv infested leaves hei-oine y-reatlv increased in thickness and 

 breadth and the weight is often iiiucli increased above the normal, the 

 tissues become stiti'ened in a coriaceous or cartihiginous manner, the 

 cell walls become greatly thickened, and the cells become more com- 

 pressed. The cells of the palisade tissue are increased in size and 

 num})er, and suffer an entire loss of chlorophyll, as in the case of the 

 cortical parenchyma of the branch. The walls of the epidermal cells 

 become considerably thickened and the nuiltiplication of the paren- 

 chyma cells on either side of the midrib causes a pronounced gather- 

 ing and distortion of these tissues. As the midrib does not elongate 

 in proportion to the increased extent of the parenchyma, it acts as a 

 gathering string passing through the leaf from end to end, and the 

 parenchyma becomes folded upon itself. The increase in the number 

 of cells occurs more extensively among the palisade tissue of the upi)er 

 half of the leaf than among the cells of the spongy parenchyma of the 

 under leaf surface; hence the majority of badly diseased leaves are 

 convex above and concave below, though this appeaianc(> is often lost 

 sight of, owing to the number and variety of folds wliich the leaf bhide 

 assumes. 



