THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 271 



tube and enters the leaf through the epidermis. It traverses the intercellular 

 spaces of the palisade cells and also the spongy mesophyll. 



In the epidermal cells the mycelium penetrates the cross walls repeatedly, 

 so that the outer surface of the epidermal cells gradually becomes split away 

 from the rest of the leaf. Meanwhile black material becomes deposited in 

 between the epidermal cells, and the infected area changes colour from yellow 

 to black. 



tt 



# 



Fig. 262. — Rhytisma acerijium. Stromata on leaf of 

 Acer pseiidoplcitanus. 



Asexual Reproduction 



At a fairly early stage in development it is possible to see the elaboration 

 of the future asexual reproductive area by the modification of the part between 

 the upper and lower walls of the epidermal cells (Fig. 263). Not only does the 

 deposit of black material become denser along the lower surface but the 

 mycelium gradually forms a continuous tissue in the bases of the cells and 

 produces vertically growing hyphae, which extend upwards towards the 

 upper walls of the epidermal cells. These elongating hyphae are reallv 

 conidiophores and later abstrict conidiospores. 



Meanwhile the mycelium from which the conidiophores arise becomes more 

 dense and forms a closely interwoven tissue of small uninucleate cells termed 

 a plectenchyma. Not only does this form the entire floor of the cavitv, 

 but circular ramparts grow up, dividing the area into a number of cavities 

 which are for a time covered by the upper walls of the original epidermal 



