no 



Journal of Agricultural Research 



Vol. XIX. No. 3 



successfully in describing the size of similarly variable bodies, first by 

 Rosenbaum {i8) for Phytophthora and more recently by Gaumann {6) 

 for Peronospora. This method seems especially valuable in the case of 

 such variable structures as the conidia of the Peronosporaceae, since by 

 means of it data gathered from large numbers of individuals may be so 

 presented that the range of variation in size which is encountered, as 

 well as the size class which predominates in the species, is at once appar- 

 ent. Also it furnishes a most accurate method for comparing the sizes 

 of such bodies in different species. 



Table I- — Measurements and ratios of length to width of 400 conidia of Sclerospora 

 philippinensis arranged in classes. 



For these reasons this method seems well adapted to depict the size 

 of the conidia of the Sclerospora of Philippine maize. Accordingly the 

 measurements of 400 conidia are given in tabular form and are also 

 plotted as curves (fig. 1,2). The ratio of length to diameter in classes 

 is given in figure 3. These show clearly that while spores are encountered 

 with such widely differing dimensions as 18 /x long by 12 /x in diameter, 

 and 51 by 23 fx, the size which predominates is 34 /i by 18 /x, and by far 

 the greater number of spores encountered are from 27 to 39 /x in length 

 by 1 7 to 2 1 /x in diameter. Although these measurements are of conidia 

 produced on maize, they have been compared and found to agree with 

 similar measurements of conidia from teosinte and sorghum. On com- 

 paring like tabulations of dimensions of fresh conidia with those from 

 material mounted in glycerin or dried, the wTiter finds constant slight 

 differences, particularly in width. Therefore, these 400 measurements 

 were made on four occasions at the beginning of the period of maximum 

 conidia production (2 to 3 a. m.) from fresh material mounted in dew 



