226 



AMERICAN JOURNAL OF BOTANY 



[Vol. 9 



of the number of rust pustules on the upper surface of each leaf, of the 

 length of each leaf in inches, of the extreme length of the entire plant, and 

 of the number of stools it had produced. The plant was then dried, and 

 its dry weight was obtained. The dry weight of the plant was adopted 

 as the index of its relative vigor of growth, because it makes possible more 

 accurate seriation of the variables on this value than an index such as the 

 height of the plant or the total leaf length. 



As an index of the degree of infection of the plant the value adopted 

 was the number of pustules on an average unit area of the most severely 

 infected leaf calculated by dividing the number of rust pustules on the 

 leaf by the length of the leaf in inches, and by its width at the base in six- 

 teenths of an inch. This value was found to have a positive correlation 

 (r = .7803 =b .0167 for the 250 variables of experiments I, II, and III) 

 with the value that at first thought would seem most desirable: namely, 

 the total number of rust pustules counted on the leaves of the plant, divided 

 by the total leaf length in inches, and by the largest leaf width in sixteenths 

 of an inch and is preferable for adoption in work of this kind not only 

 because it is easier to obtain, but also because it avoids the error introduced 

 by the development of new leaf surface during the incubation period. The 

 most highly infected leaf on the plant was usually the lowest leaf in good 

 condition. In tables 6-10 both values are given. 



Experiment I 



66 oat plants were grown in soil in 2-inch pots, divided into three groups 

 on the basis of the number of plants grown to a pot. The soil was a rich 

 garden loam. The seed was sown July 6, 1920, three grains being put 

 into the soil for every plant desired, and the seedlings were later thinned out 

 to the number of plants desired. The pots were kept on a bench in the 

 greenhouse until August n, when they were taken out of doors and set 

 near a patch of rusty oats, subjecting the plants to natural conditions of 

 inoculation and infection. The experiment was concluded on August 24. 

 The data on this experiment are given in table 6. 



TABLE 6 



Experiment II 



70 plants were grown in 3-inch pots, divided into three groups on the basis 

 of the number of plants grown to a pot. Soil, method of seeding, and dates 

 * Plus 2 destroyed. 



