Notes and Brief Articles 



269 



the logs, especially if they are to be used for sawing into lumber. 

 A large log with a defective center might be made to furnish a 

 large amount of first-class veneer, but could not to advantage be 

 sawed into lumber. 



The relation of the health of the host and other factors to in- 

 fection of Apium graveolens by Septoria Apii is discussed at 

 length by H. E. Thomas in the Torrey Bulletin for January, 

 192 1. According to the author, "students of immunity and sus- 

 ceptibility have been slow to recognize any fundamental distinc- 

 tions in the relations of host and parasite in the great group of 

 organisms which cause disease in plants and animals, and yet the 

 concepts of saprophyte, semi-saprophyte, and obligate parasite 

 have been current at least since the time of DeBary. Under the 

 influence perhaps chiefly of Ehrlich's side chain theory of im- 

 munity, degrees of resistance have been regarded on the one hand 

 as inversely parallel to the virulence of the attacking organism, 

 and on the other hand as directly parallel to the vigor of the host. 

 In plant pathology this view has been particularly prominent in 

 the literature of the facultative parasites. With the development 

 of the science of immunity, the animal pathologist has gone so 

 far as to regard the interactions of host and parasite as specific 

 in each case. It is becoming increasingly apparent that the speci- 

 ficity in the relation of plant pathogens with their hosts must be 

 reckoned with. The saprophytic fungus may be able to live on 

 dead tissue from a wide range of plants, sometimes showing little 

 preference for any one of them. The semi-saprophyte may or 

 may not be more limited in its food range on dead material and 

 attacks from one to a considerable number of living plants with 

 varying degrees of virulence and with variable results to the 

 hosts. The obligate parasite is usually still more restricted in its 

 host range and is much more closely adapted to the living host, 

 having completely lost the ability to grow on dead tissue, even 

 that of its most common host. In the more highly specialized 

 forms the relation may become specific to such a degree that a 

 comparatively slight change in either host or fungus will com- 

 pletely change the virulence of the parasite or the effect on the 



