May, 1922] 
RAINES — VEGETATIVE VIGOR OF THE HOST 
The data presented by Thomas (1921) on the parallel relation between 
health of the host and infection in the case of the leaf blight of celery, and 
observations of similar occurrences in other diseases caused by non-obligate 
parasites like the late blight of the potato (Jones, 1905) and the crown gall 
of the beet (Levine, 1921) indicate that phases in which a symbiotic tend- 
ency comes to the fore may occur in diseases of a predominantly destruc- 
tive type caused by facultative parasites, and suggests the generalization 
that the host-parasite relation in any given instance is not constant but 
may vary with the state and condition of the organisms and with the prog- 
ress of the relation. It is important to recognize that there may occur 
mutualistic phases and stages in host-parasite relations of a violent and 
destructive type, just as there are destructive phases in parasitisms of a 
predominantly symbiotic tendency such as those of the mildews, the rusts, 
and the smuts. 
Conclusion 
The inquiry initiated by the occurrence in rust literature of statements 
of a relation between host vigor and susceptibility other than the inverse 
relation commonly conceived as existing between these variables can be 
considered as having brought forward evidence indicating that through 
most of the course of certain infectious diseases such as the rust diseases 
of the cereal grains, and in certain phases of other diseases like the leaf 
spot of celery and the crown gall of the beet, the vegetative vigor of the 
host and the virulence of the disease may be in direct relation. The demon- 
stration of such a relation in diseases of large importance suggests, in turn, 
emendation of current pathological concepts of the relation between host 
vigor and pathogen activity into a form more in accord with our knowledge 
of parasitological phenomena in general. A more catholic point of view 
in pathologic thought, recognizing that, for longer or shorter phases in the 
course of a disease, the relation between host and parasite may be highly 
mutualistic, would be of material value as a working concept in the study 
of disease and in defining the practical problem of disease prevention and 
control. 
The work presented in this paper was done in the Botanical Laboratory 
of Columbia University, under Professor R. A. Harper, to whom the writer 
is greatly indebted for pointing out the problem and for constant con- 
sultation and advice during the progress of the investigations. Acknowl- 
edgment is also made of indebtedness to Dr. Michael Levine for taking the 
photographs of the soil-culture experiments, and to Dr. H. E. Thomas for 
helpful advice in devising the method used in the dosage studies on the 
corn rust. 
LITERATURE CITED 
Adami, J. G. The principles of pathology. Vol. i. Philadelphia and New York, 1910. 
Arthur, J. C. Problems in the study of plant rusts. Bull. Torr. Bot. Club 30: 1-18. 1903. 
Bailey, L. H. Cyclopedia of horticulture. New York, 1914. 
