46 



Journal of Agricultural Research 



Vol. XVI, No. 2 



The first uredinia began to burst through the epidermis 144 hours 

 after inoculation, except on those plants which had been under the bell 

 jar for 12 hours. On these plants the first and only uredinium appeared 

 10 days after inoculation. At this time the rust was well developed on 

 all the plants that had been under for 48 hours, whereas those that were 

 kept under 24, 72, and 96 hours showed the maximum infection only 

 two days later. 



Although all the fully developed uredinia were in every case approxi- 

 mately the same size, color, and shape, the virulence of the attack varied 

 considerably. The plants kept under the bell jar for 48 hours produced 

 the greatest number of uredinia per leaf; those which had been under 24 

 and 72 hours, respectively, somewhat fewer, and those which had been 

 under 96 hours, still fewer; while only one uredinium appeared on the 

 single infected leaf of the plants kept under for 12 hours. In the present 

 work, therefore, all inoculated plants were kept for 48 hours in the moist 

 chamber, then removed to their respective places on the greenhouse 

 benches. 



It was found that the superficial layer of each uredinium contains 

 larger spores, and when this layer is removed, the remaining spores are 

 considerably smaller. But if the uredinium is allowed to produce a new 

 crop of spores, those on the surface again attain the same dimensions as 

 the original ones. For this reason precaution was taken to measure 

 spores from uredinia in the same stage of development. 



Table 1.— Results of ineasuring varying numbers of uredinios pores of Puccinia graminis 

 trilici and P. graminis avenae 



Experi- 

 ment. 

 No. 



Source of urediniospores. 



Triticum aestivum . 



do 



do 



do 



do 



Avena saliva 



do 



do 



do 



Num- 

 ber of 

 spores 

 meas- 

 ured. 



Spore dimensions. 



Range of length. 



25 



50 



100 



200 



400 



25 



5° 

 roo 

 200 



26. 88-38. 72 

 26. 88-40. 32 

 26. 56-40. 32 

 25. 60-40. 32 

 23. 68-40. 32 

 23- 36-35- 20 

 23- 36-35- 20 

 23- 04-35- 20 

 21. 12-36. 48 



Range of width. 



M 

 17. 92-22 

 17. 92-23 



16. 96-23 

 16 32-23 

 16 32-23 



17. 28-22 

 17. 28-22 

 16. 96-22 

 16 32-23 



.08 



.04 

 -36 

 -36 

 •36 

 .08 

 .08 

 .08 

 .04 



Modes. 



032. 00X19. 52 



^33. 60X19-84 



32. 96X19. 84 



32. 96X19. 84 



32. 96X19. 84 



O27. 52X19- 52 

 29. 12X19. 52 

 29. 12X19. 52 



o Modes doubtful but showing tendency to form as indicated. 



b Mode of length doubtful but that of width definitely established. 



c Modes indeterminate. 



As to the number of spores to be measured from a given group, it was 

 found that 100 gave equally as good results as 200 or 400, while when 

 less than 100 were used the results were not always representative or 

 conclusive. Table I gives the results of measuring different numbers of 



