28 



This black layer consists in reality of a dense 

 network of the mycelium of a fungus. The fungus, 

 however, is not a parasite, and consequently does not 

 extract anything from the leaf it is upon, but subsists 

 entirely upon " honey dew " deposited by aphides or 

 "green fly," hence, if the latter are kept in check, you 

 will not be troubled with sooty mould. 



