INJURIES INDUCED BY PLANTS 47 



appear to be necessary that we should regard the sound wood of 

 a growing tree as being alive, even although only a portion of its 

 cells may exhibit the phenomena of life.* In many cases it is 

 difficult to decide whether wood e.g. the duramen in many trees 

 was actually living when attacked by the fungus-mycelium, 

 or whether its parenchymatous cells were then dead. But apart 

 from those doubtful cases, in which it is difficult to decide at 

 once whether a fungus is existing as a parasite or as a sapro- 

 phyte, there are many fungi which occupy a position somewhere 

 between those which are strictly saprophytic and those which are 

 strictly parasitic. Numerous fungi are in a position to complete 

 the whole course of their development as saprophytes, though, 

 under certain circumstances, they may also live in a purely 

 parasitic manner. Agaricus melleus and the genus Nectria may 

 serve as examples. Such fungi are designated facultative 

 parasites. Other Fungi, which, as a rule, go through the whole 

 course of their development as parasites, but which are capable 

 of growing as saprophytes, at least during certain stages of their 

 existence, are designated facultative saprophytes. To this group 

 belong, for instance, PhytophtJwra ornnivora and Cercospora 

 acerina. We have thus to distinguish four groups: I. Obligate 

 saprophytes. 2. Facultative parasites. 3. Facultative sapro- 

 phytes. 4. Pure that is to say, strictly obligate parasites, 

 which can only grow parasitically, e.g. the group Uredinea. 



The spread of an infectious disease may take place in two 

 distinct ways, either by infection caused by the mycelium, or 

 by infection caused by the spores, including the gonidia. 



Infection by the mycelium is met with in nature most frequently 

 in the case of those parasites which grow below ground. This is 

 to be explained by the fact that the varying amount of moisture 

 in the air admits of the development of the mycelium above- 

 ground only in exceptional cases, as, for example, in Herpotrichia 

 and TrichospJiceria. 



In the case of infection by the mycelium, it is, to a certain ex- 

 tent, the same individual fungus that spreads from root to root and 



* [In many such cases the destructive fungus may be looked upon as a 

 Saprophyte, when viewed with regard to the immediate seat of its action 

 e.g.) wood but as a parasite with regard to the tree as a whole, whose life is 

 destroyed in consequence of the secondary results of the damage. ED.] 



