236 PHYTOPATHOLOGY [VOL. 6 



Pinus radiata. 



(50) 11-24 x 22-37 (17-20 x 26-30)/*. 

 Pinus radiata (aecial infection). 



(148) 11-26 x 19-41 (17-22 x 24-30);*. 

 Pinus attenuata. 



(28) 19-30 x 26-41 (20-22 x 26-30)/z. 

 Pinus sabiniana. 



(50) 15-24 x 20-37 (17-20 x 26-30) M . 

 Pinus virginiana (eastern form). 



(50) 15-24 x 22-37 (19-22 x 26-31) M . 



The values for the extreme measurements in this list vary so much 

 that a composite formula would read: 11 to 30 by 16 to 43/z. It is plain 

 that the extremes cannot be used for comparison. 



If we consider the standard values only, it appears that the widths 

 vary very little from the values 17 to 22 //. The lengths, however, permit 

 us to distinguish two well-defined groups, one wth the values 22 to 26 /* 

 and another with the values 26 to 30 /z. The slight variations from these 

 values lie well within the unavoidable margin of error in measuring and 

 in rounding off the figures obtained. The first group includes all the 

 specimens on Pinus contorta (murrayana from the Rocky Mountains), 

 P. contorta (Sierra Nevada), and P. jeffreyi, that is, high elevation forms; 

 the second group, the material from Pinus ponderosa, P. sabiniana, and 

 P. radiata, that is, forms from the middle and lower slopes of the Sierra 

 Nevada and from the coast little above sea level. To the latter group 

 must be added Peridermium cerebrum from Pinus virginiana. Fur- 

 ther inoculation experiments will have to decide the question whether 

 or not these groups actually correspond to two different fungi. Obviously 

 too great an importance must not be accorded to spore measurements 

 unless they are well supported by other structural and biological char- 

 acters. Morphologically all the aeciospores examined resemble each 

 other very closely. On the other hand, the remarkable regularity of 

 standard values as shown, for instance, in nine specimens from Pinus 

 contorta on the basis of five hundred measurements, suggests the use- 

 fulness of such spore measurements for purposes of identification. That 

 our material from Pinus contorta has nothing to do with Peridermium 

 filamentosum which is very common in the same localities, is shown by a 

 comparison with the formula for aeciospores of the latter fungus (numerical 

 basis 100) : 



(100) 17-39 x 22-78 (22-26 x 30-33)/*. 



The fact that the standard values for Peridermium cerebrum on Pinus 

 virginiana are practically the same as those for P. harknessii on Pinus 



