Apr., 1922] RAINES VEGETATIVE VIGOR OF THE HOST 19! 



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 (1919, 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 

 (1914, 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 



