298 
Journal of Agricultural Research 
Vol. XXXI, No. 3 
The similarity of the chemical picture of disease in the spinach 
and in the celery points very strongly to a parasitic agency being at 
work in the spinach. But it must be borne in mind that it is not 
known at all what effects these various derangements of normal 
nutrition produce on plant growth, and exactly as some of these 
conditions produce spots, lesions, overgrowths, etc., which simulate 
the diseases brought about by parasites, so a disturbed physiology 
from other than parasitic agency may give a chemical aspect which 
simulates more or less closely the condition which accompanies 
these two diseases caused by parasitic fungi. It would seem that 
the effects which environmental conditions produce would be largely 
of a.repressive nature, depressing, for example, the total nitrogen 
production, and such a condition might also be expected with 
hypoplastic diseases. Necrotic diseases of the types studied, on the 
other hand, are destructive, reducing that which has already been 
produced. 
With such a criterion, the analyses given when compared with 
those of spinach go to emphasize that in the case of an infectious 
' disease, such as spinach mosaic is known to be, one may be dealing 
with a parasite whose effect upon the host is not dissimilar to known 
parasites, but the similarity of chemical relations alone does not 
permit judgment in this matter. As far as the cabbage disease is 
concerned, its infectious nature has not been shown and the evidence 
at hand does not warrant judgment on purely chemical grounds, as 
to its parasitic nature. Surely the similarity of chemical analyses 
is not proof that the cabbage disease is a mosaic disease. 
The interpretation of disease production on the basis of the 
parasite simply establishing a food and water relation with the host 
opens the question whether disease production by parasites in.general 
is to be interpreted on such simple grounds. Is the living together 
* of host arid invader in all types of parasitism, especially the higher 
types, merely a food and water relation and is the matter of speciali¬ 
zation of a parasite Upon a species or upon related species determined 
; by the ability of the parasite to invade, grow, and to wrest food from 
the reserve stuffs of the host? 
The data here presented deal with organisms of not a very high 
specialization in parasitism. These organisms simply produce a 
necrotic condition in the host.' There is no stimulation of the host 
nor any evidence of tolerance by the tissues invaded. Whether 
’ toxic substances are produced by the invaders is not known. These 
two orgariisrns, though readily grown in pure cultures on ordinary 
laboratory media of complex nature or on synthetic media of simple 
salts and sugar, have not been found to be omnivorous in their para¬ 
sitic habit. Both, on the coritrary, are extremely selective in their 
parasitism, attacking only the celerylike plants (Apium graveolens). 
The analyses seem to show that the striking thing in the disease 
production is the mere feeding on the host tissue, the disintegrating 
of protein in the processes of metabolism. It would seem plausible 
to suggest that with the necrosis-producing parasites at least, the 
ability to appropriate food is a very important factor in parasitism. 
'The fungus must first be able to invade and tolerate the conditions 
Within the host, riot being walled off or killed by acid or some cellular 
product. After these barriers are passed, the degree of disease 
production may largely be correlated with the ability to attack the 
