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 susceptibility to rust, has been established 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 (1918) 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- 



