450 RESEARCHES ON FUNGI 



appressorium, on the upper side of a leaf, the hyphae converge to 

 a point. The hyphae at this central point then press against the 

 cuticle, force their way through it, and at once kill the epidermal 

 cells with which they come into contact. The invading hyphae, in 

 a compact mass, then force their way in between the cuticle and the 

 epidermis, kill more epidermal cells, press against and flatten the 

 dead epidermal cells, kill the subjacent palisade cells, and break 

 through the epidermis into the intercellular spaces of the palisade 

 layer. The palisade cells become compressed by the advancing 

 wedge of hyphae. The hyphae now kill the spongy parenchy- 

 matous cells below them. Then, still in a compact mass, they grow 

 downwards through the spongy parenchyma as far as the lower 

 epidermis. Already by this time a small brown leaf-spot has been 

 formed. Hyphae now grow out laterally through the intercellular 

 spaces of the mesophyll, and thus the leaf-spot, so long as the weather 

 is sufficiently moist, continues to enlarge centrifugally. The leaf- 

 cells a short distance in front of the advancing hyphae die before 

 the hyphae reach them, a fact which indicates that the mycelium 

 excretes a toxic substance. Since the hjrphae of the gemmae and 

 of the mycelial fibrillae which spring from the sclerotia are coated 

 with calcium oxalate, Stahel has suggested that the toxic substance 

 is oxalic acid. If this supposition is correct, the leaf-spots ought 

 to give a pronounced acid reaction. Whatever the nature of the 

 toxic substance, its effect upon the leaf is such that the hyphae 

 which excrete it grow only in dead tissue. 



The lower epidermis, unlike the upper, contains numerous 

 stomata, and here the invading hyphae which grow out from an 

 appressorium enter the leaf via the stomatic clefts. Stahel proved 

 by experiment that the gemmae on the under side of a leaf infect 

 the leaf more quickly and more easily than gemmae on the upper 

 side. Nevertheless, under natural conditions, as might be expected 

 from the fact that the gemmae settle more easily on the upper 

 side of leaves than on the under side, most infections take place 

 from the upper side. Stahel investigated 213 leaf-spots and found 

 that 175 (82 per cent.) had the infecting gemma on the upper side 

 of the leaf-spot and 38 (18 per cent.) on the lower side. 



In his infection experiments, Stahel observed that the leaf-spots 



