iz TECHNICAL BULLETIN 17 



a normal one. In general contour the 3OO-spore curve and the 500- 

 spore curve approach the looo-spore curve, altho the first is somewhat 

 more irregular. The slight rise at the lower extreme indicates that 

 the short spores tend to group themselves about a mode of their own. 

 It is possible that improvement in the method of sampling might in- 

 crease the accuracy of the results obtained from a smaller population. 

 In the present study about 100 spores were measured from one mount. 

 The spores were distributed as evenly as possible in the drop of water 

 and each spore was measured in passing systematically over the slide 

 from the upper left to the lower right hand corner. An attempt was 

 made to make the mount so that two or three spores would come into 

 the field at once. For all other conditions, 500 spores were measured. 



Results obtained in the study of the morphology of spores de- 

 veloped on potato dextrose agar at various temperatures are interesting. 

 Table III shows very little difference in the means of spores de- 

 veloped at 1 8 and 24. From the comparisons in Table IV it is seen 

 that these differences are insignificant. If, however, we examine the 

 coefficients of variability, we find that there is a significant difference 

 in the amount of relative variation in the length of spores. This fact 

 is very clearly brought out in the curves in Figure 2. The degree of 

 variation is not increased by a temperature 4 degrees lower (14 C.)> 

 but the mean length of the spores is slightly increased. This may be 

 due to the fact that at a lower temperature the black outer wall on 

 the spores and mycelium is laid down much more slowly, so that the 

 spores have a longer time in which to form. This is further sub- 

 stantiated by the fact that at 32 the spores are very much shorter. 

 The amount of relative variation is practically the same as at the 

 lower extreme. These differences in length of spores produced at 

 various temperatures are graphically represented by the curves in 

 Figure 2. 



The most striking difference in spore morphology was obtained 

 by comparing the spores produced on different media. As the fresh leaf 

 and the autoclaved head cultures were incubated at 24 C, we may 

 compare these results with those pbtained from the agar culture at 

 24 C. Comparing first the spores from the head and from the agar, 

 we find that the former are slightly longer. The amount of relative 

 variation in the two is practically the same. On the fresh leaves, 

 however, the spores are very much longer and decidedly more uniform. 

 These differences are illustrated in the curves in Figure 3. 



