230 



AMERICAN JOURNAL OF BOTANY 



[Vol. 9 



the dosage for the variables compared in any experiment was uniform and 

 that the variation in the amount of rust observed on the different groups 

 of plants in the experiment is due to differences in the reactions of the plant 

 tissue to the infection to which they were subject. 



The explanation of the apparent reversal of the result in soil-culture 

 experiment V as compared with the others is probably to be found in the 

 age of the plants and in the length of time they were exposed to infection. 

 The experiments are compared in table 12. 



TABLE 12 



Experiment V differs from the other four experiments in that (i) when 

 set out of doors to be inoculated the plants were from 7 to 1 1 days younger. 

 Even at the conclusion of the experiment these plants had only half the 

 dry weight of the plants of experiment IV and were evidently much less 

 mature. (2) When the experiment was concluded the plants had been 

 out of doors and subject to infection 6 days longer. If we allow an incuba- 

 tion period of 10 days for the rust, then the rust present on the plants of 

 experiment V at the conclusion of the experiment represents inoculation 

 through a period of time twice as long as in the case of the other experiments. 

 (3) The amount of rust on the plants at the conclusion of the experiment was 

 several times greater in experiment V than in any of the other experiments. 



The last-mentioned fact immediately brings into view an aspect of the 

 method of experimentation used tending to limit the value of the pustule 

 count as a criterion of the frequency of penetration and infection by the 

 uredospore germ tube. It is probable that only in cases of very sparse 

 infection is there a pustule for every focus of infection, and that only in 

 cases of very sparse infection is the number of pustules counted an accurate 

 index of the number of infections which have taken place. With abundance 

 of infection there appears a tendency for the coalescence of foci of infection, 

 for two or more mycelia the result of contiguous infections to coalesce and 

 produce only one pustule; and this tendency would be highly accentuated 

 on the more vigorously growing host plants where the parasite finds a 

 favorable nidus and develops more luxuriantly. In experiment V the 

 error introduced by the coalescence of mycelia may well have masked a 



