Jan. 13, 1919 Morphology of Urediniospores of P. graminis 47 



spores from the same plant taken on the same day from uredinia on the 

 same leaves. Stemrust of both wheat and oats was tried with similar 

 results. As noted from Table I, the modes of a population of 100, 200, 

 or 400 spores are the same, but the limits of variation are less in a popu- 

 lation of 100 than in those of 200 or 400. It will also be seen from the 

 table that at least 100 spores should be used. In the present investiga- 

 tion 200 spore measurements were made for each experimental group 

 until January i, 1916, which constituted about half of all measurements 

 made. Beginning with this date 100 spore measurements for length 

 and 100 for width were made, instead of 200 for each. 



As a comparative basis of dimensions in this work, the biometric mode 

 is used in preference to the arithmetic mean. The mode represents the 

 group containing the largest number of individuals of a certain size, thus 

 indicating that this size is the prevailing one in a given spore population. 

 Comparative calculations made show that, as a general rule, arithmetic 

 means usually fluctuate around the biometric modes, as seen from 

 Table II; and consequently there is, on the whole, but little difference 

 between the two bases of recording. It will be seen that in most cases 

 the figures are almost identical ; in two cases they are the same ; and only 

 in one case is there a difference of 0.24 /t, which may be considered negli- 

 gible, since two consecutive measurements of the same group of spores 

 may give even greater variation. In this experiment 100 spores were 

 measured in each case. 



Table 1 1 . — Correlation of bioinetric •modes with arithemeiic means of urediniospore dimen- 

 sions of Pitccinia graminis tritici on wheat 



Number of generations rust was confined to wheat. ) Biometric modes. 



33.28X19-84 



7 1 32. 00X19. 84 



10 32. 64X20. 16 



Arithmetic means. 



M 

 33.36X19. 69 

 31.58X19-84 

 32. 64X20. 02 



The apparatus used for meteorological observations is fully described 

 in the discussions of the particular experiments performed. General 

 notes on the behavior of the various cultures were taken at the close of 

 each urediniospore generation before transfers were made to new plants, 

 on the average every two or three weeks. The preliminary spore meas- 

 urements were made of the original rusts found on the grasses in the 

 field, the subsequent measurements were made on the first following 

 generation and once or twice more during the period the rust was kept 

 in culture. For color determination Ridgway's * chromotaxia was used. 

 The Zeiss screw micrometer was used for measuring the urediniospores. 



1 Rtogway, Robert, color standards and color nomenclature. 43 p., 53 col. pi. Washing- 

 ton, D. C. 



