Apr,, 1922] 
RAINES — 
VEGETATIVE VIGOR OF THE HOST 
191 
some particular class of substances within the host is strongest in the case 
of the carbon compounds, because of the relatively large amounts of carbon 
needed by the growing plant and because of the facility and exactness with 
which the growth and vitality of the host plant can be experimentally con- 
trolled through this phase of its metabolism. 
Stakman and his co-workers have studied the light relations of Puccinia 
graminis tritici inoculated on seedling plants of wheat. Stakman and 
Piemeisel (1917, p. 487) state that a considerable amount of sunlight is 
necessary for the best development of the rust. They found that during 
periods of cloudy weather the incubation period may be lengthened a week 
or more, and that the rust does not develop so abundantly as during bright 
weather. Shaded plants invariably were more weakly infected than the 
others. Partially etiolated plants were infected with difficulty, and the 
rust developed very weakly on them. No rust developed on etiolated 
plants. Stakman and Levine (191 9, p. 71) found that the rust developed 
considerably better in fairly high intensities of light than under conditions 
of less favorable illumination. The size of the urediniospores responded in 
a similar manner. They summarize their observations on the light relations 
of the rust as indicating that 
... In as much as the photosynthetic activities of the host plant are affected by the 
light intensity, in so much does the structure and function of the rust depend on the same 
factor. 
Water Relations 
While many observations have been made on the relation of moisture 
conditions of soil and air to rust virulence on plants in the field, they are 
subject to criticism in that they do not distinguish between the effect of the 
moisture conditions on uredospore germination and penetration, and the 
effect on the vigor of the host and the progress of the rust infection in its 
tissues. Abundant moisture is always favorable to uredospore germination 
and infection, but the effect on the physiology of the host is specific for the 
plant. Abundance of moisture will favor the growth of a mesophyte, but 
it will have a depressing effect on the vigor of a plant of xerophytic tend- 
encies. Limiting ourselves, therefore, to observations when inoculation 
was artificially effected under conditions of maximum atmospheric humidity, 
the evidence permits the inference that the moisture conditions of at- 
mosphere and soil most favorable for the growth of the host plant are 
likewise optimum for the growth and sporulation of the rust. 
The most complete and suggestive experiment is reported by Stakman 
(iqi4, p. 35). Wheat plants of both drought-resisting varieties and or- 
dinary mesophytic types were employed, and they were grown in two series. 
The soil in one series was kept very wet, while that in the other series was 
kept as dry as possible without endangering the life of the plants. On 
inoculation with rust, the drought-resisting forms exhibited better infection 
