PLANT PATHOLOGY—ZJONES. 413 
VI.—THE RELATIONS OF PARASITE TO HOST AND ENVIRONMENT. 
Although parasitology in relation to plant pathology dates from 
but little later than in animal pathology, and the relations involved 
would seem simpler than with the animal parasite, yet the fact remains 
that we are far behind the animal pathologists in understanding these 
relations. Some of the reasons for this are evident. The preeminent 
value of human life among the animals has focused attention upon 
human pathology. Even where attention has been given to the path- 
ology of the lower animals, the students have as a rule approached 
the subject from the viewpoint of human pathology, and have been 
eager to apply to this any suggestions from comparative work on the 
lower forms. The result has been intensity and concentration of 
research upon the diseases of this one organism, man. 
In plant pathology the natural tendency has been exactly the oppo- 
site. From the beginning the phytopathologist has included in his 
range of interests all the diseases of all plants known to him. The 
numbers of disease-inducing parasites is so enormous that it has con- 
sumed his professional energies simply to catalogue them. Concen- 
tration when attempted has been secured by narrowing one’s interests 
within the parasitic group rather than within the host group. 
I believe that we need to have, far more than heretofore, special- 
ization by hosts in our phytopathological studies. Whether one is to 
probe deeper into problems of relations of environment to parasitism 
or into matters of predisposition and variations, either with host as to 
susceptibility or parasite as to its biological forms, attention should 
be focused long and intensively upon the one host. Experience has 
convinced me that one can not understand the diseases of a cultivated 
plant like the potato, for example, except as he understands them in 
relation not only to the normal physiology and morphology of the 
plant, but in relation to its history and its variations under culture. 
Progress requires that we have specialists on types of host plant as 
well as of parasite. 
And passing to the cellular relations of host and parasite, how 
little we know! The very simplicity of the plant’s organization 
makes the pathological reactions harder to investigate than with the 
animal. In the plant the unit in the more fundamental pathological 
relations is not the organism but the cell, an object so minute as to 
make the study of the chemical interrelations highly difficult. We 
recognize the cell membrane as the first barrier to be overcome by the 
invader and we believe the cytolytic enzyme the first weapon in the 
attack. Yet, save with certain soft rot diseases, we know little that 
is definite about these enzymes in their action. We see evidence of 
other disturbing effects of parasite upon host cells, even in advance 
of actual invasion. Sometimes these are inhibitory or fatal, some- 
