MOULD FUNGI AS EXCITING AGENTS OF DISEASE. 629 



Facultative saprophytes, which as a rule complete their 

 cycle of development as parasites, but which can exist 

 under certain circumstances as saprophytes, and can pass 

 through some portion of their developmental cycle as 

 such. 



2. Facultative parasites. These are species which Facultative 

 usually grow as saprophytes, but which, however, at 



times find the conditions necessary for their existence in 

 living organisms, and can then multiply there. 



3. Obligatory saprophytes, which cannot obtain any 

 foothold in the living vegetable or animal organism, but 

 can only grow on dead substrata. 



The mould, yeast, and fission fungi behave so differently in 

 their roles as exciting agents of disease that they must be dis- 

 cussed separately. It is not the purpose of the present work 

 to give an accurate description of the parasitic diseases 

 caused by the mould fungi ; they will only be noticed in so 

 far as they furnish facts for some general conclusions. For 

 a more detailed description of the diseases of plants we must 

 refer to the works of de Bary, Hartig, and Frank.* 



The natural mode of origin and spread of the diseases 

 caused by the bacteria will be discussed in detail in the 

 sixth and seventh chapters, while in this place we shall only 

 speak of the action and fate of bacteria which have already 

 penetrated into the body. 



A. The Mould Fungi as Exciting Agents of Disease, (a) Parasitic 



mould fungi. 



The mould fungi are dangerous chiefly to the higher Mould fungi 



i mi 11 i XT j.- r ii which attack 



plants. They are able to spread in the tissue of the plants, 

 plant, their germinating tubes and mycelial threads 

 penetrating into natural openings or through accidental 

 injuries of the epidermis, and then running between the 

 cells ; they frequently also bore through the walls of the 

 cells, and in that case have probably the power of 

 secreting a ferment which dissolves the cellulose. 



The mode of action of the parasitic moulds is very Mode of 



penetration 



* De Bary, Vgl Morph. u. Biol. der Pilze, Leipzig, 1884, P. 384, ff. and action. 

 Hartig, Lehrb. d. Baumkrankheiten Vortruge im iirztl. ver. zu Miinchen, 

 1881. Frank, Die Pfankenkrankheiten , Schenk's Ilandb. d. Botanik. 

 Breslau. 



