l86 Transactions British Mycological Society. 



the hyphae which disorganise the sclerotium in this region. 

 Probably, during the wet periods noted by Miiller to be pre- 

 requisite for the rupturing of the wall, water passes into this 

 cavity which is bounded by the non-elastic sclerotium. The break 

 naturally occurs where the cavity is widest and the pressure 

 correspondingly greatest (fig. 4). 



The actual morphology of the ascospore and the aiscus has 

 been fully described by Miiller. 



Summary. 



The foregoing points may be summarised as follows: 



1. Infection takes place by means of the germ tube which 

 grows from the ascospore through a stoma on the under surface 

 of the leaf. 



2. The fungus branches and penetrates the cells of the leaf 

 chiefly on the palisade layer but finally concentrates in the cells 

 of the upper epidermis. 



3. Subsequent development of the fungus takes place actually 

 within the cells of the upper epidermis. The cells are consequently 

 killed and the walls ruptured and pushed apart. 



4. Black sclerotium formation is first apparent in the thickened 

 outer walls of the upper epidermal cells and is later increased 

 by the development of a plectenchj^matous layer beneath these 

 walls. 



5. Splitting of the apothecia is accompanied by the develop- 

 ment of a special mechanism whereby the sclerotium is broken 

 down in the region of the split. 



REFERENCE*. 



MuLLER, K. Zur Biologic der Schwarzfleckenkrankheit der Ahornbaume 

 hervorgerufen durch den Pilz Rhytisma acerinum. Centr. f. Bakt. Abt. 2, 

 XXXVI, p. 67. 1913. 



EXPLANATION OF PLATE VII. 

 Transverse sections through Rhytisma spot. 

 Fig. I . Early yellow spot stage. 



2. Early conidial stage. 



3. Late conidial stage. 



4. Apothecial stage. 



a. =upper epidermis, b. =paUsade tissue, c. =cuticle. d. =spongy mesophyll. 

 c. = lower epidermis. /.= ascus layer. A. = hyphae. ^. =comdiophores with 

 conidia. 5. = sclerotium. 



* A full list of references will be given in a further account of Rhytisma. 



