Vol. 48 No. 1 
BULLETIN 
OF THE 
TORREY BOTANICAL CLUB 
JANUARY, 1921 
The relation of the health of the host and other factors to infection 
of Apium graveolens by Septoria Apii 
H. E. THOMAS 
Students of immunity and susceptibility have been slow to 
recognize any fundamental distinctions 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 immunity, 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 specificity in the relation of plant 
pathogens with their hosts must be reckoned with. The sapro- 
phytic 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 semisaprophyte 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 
[The BULLETIN for December (47: 555-598) was issued Deceember 28, 1920.] 
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