May, 1922] RAINES — VEGETATIVE VIGOR OF THE HOST 233 
action of the chemical. This aspect of the soil-culture experiments may 
be considered as in agreement with the suggestion arrived at in the biblio- 
graphical review that it is questionable whether a direct relation between 
any environmental factor, either physical or chemical, of the nature of a 
nutrient or a stimulus, and susceptibiHty to rust, has been estabhshed in the 
case of the cereal grains. 
Vegetative Vigor of the Host as a Susceptibility- and Resistance-factor 
in Infectious Diseases 
Increased susceptibility with increased vigor of the host, in plant 
diseases, is not confined to the rusts. Marchal (1902) found that infection 
of lettuce by Bremia lactucae was favored by nitrogen and phosphates and 
retarded by an excess of potash. Jones (1905, p. 38) mentions that high 
fertilization, especially with nitrogenous manures, lowers the powers of 
the potato plant to resist blight and rot. McCue (1913, p. 18) observed 
that tomato plants treated with phosphatic fertilizers developed less leaf 
blight than control plants, while plants on nitrogen and potash plots which 
at the same time gave the highest yields, indicating greatest vigor of growth, 
were more heavily infected than the controls. Peltier (191 8) has observed 
with the citrus canker, and Fromme and Murray (1919, p. 227) with the 
angular leaf spot of tobacco (''the development of the organism within the 
tobacco leaf is apparently dependent to a marked degree on those pre- 
disposing factors which promote a rapid, vigorous growth of the host"), 
that infection is heavier under conditions which favor the growth of the 
host. Thomas (1921) obtained evidence of increased resistance to leaf 
spot {Septoria Apii) of celery plants the vitality of which was depressed 
as a result of infestation of the root system by nematodes; and of decreased 
resistance in plants richly fed. And Levine (1921) has observed that crown 
gall on beets developed more rapidly and to larger size on roots grown in a 
highly manured soil. 
While the claim that increased vigor of the host means greater sus- 
ceptibility to an infection may appear somewhat anomalous from the point 
of view of current theories regarding the infectious diseases, observations 
such as form the subject of the present paper are readily understood when 
we consider the infectious diseases in the light of the larger class of biological 
phenomena of which they are an artificially selected group — -namely, 
parasitism, commensalism, and symbiosis, the class of biological phenomena 
in which one organism lives within, and derives its sustenance from, the 
tissues of another living organism. In each of the four main groups of 
parasitic organisms — the bacteria, the protozoa, the worms, and the fungi— 
a series of intergradations are to be observed in the physiological interrela- 
tions of host and parasite, from the unceasing and violent struggle that 
continues until the destruction of one or other of the principals, to a relation 
of a more benign type characterized by great subordination and even tend- 
