266 
Journal of Agricultural Research 
Vol. VIII, No. 7 
The biometrical methods therefore furnish a means of extending the 
descriptive method and of expressing quantitatively what investigators 
have heretofore attempted to express qualitatively. 
The constants here obtained, offering a more ready comparison than 
the mass of individual measurements previously given, are (a) the mean 
or the point about which the individuals of the group cluster; (b) the 
median, the point on either side of which exactly half of the individuals 
fall; (c) the mode—that is, the class containing the greatest number of 
individuals; (d) standard deviation, expressing the degree of variation 
in the group; (e) skewness, which is a measure of “the degree of sym¬ 
metry of the distribution of the individuals composing the group.” 
In the calculation of the constants the ordinary biometrical methods 
as given by E. Davenport (n) were followed, and for the moments 
Sheppard’s corrections (33) were employed. In calculating the mode, 
median, skewness, and probable errors the formulae given by Pearl and 
Surface (22) were used. 
The writer is aware that the peculiar substrata on which some of the 
cultures were grown may affect the constants. It may be possible to 
give in a subsequent paper the results of measurements obtained under 
normal conditions—that is, directly from the host plants. The constants, 
together with their probable errors for the conidia of the various species, 
are shown in Table IV. In the examination of the table the following 
points should be noted: The species P. cactorum isolated from Panax 
sp. and Phyllocactus sp. show a remarkable uniformity. P. fagi is 
closely related to P. cactorum , the mean for length being slightly greater 
in the latter. The skewness for both length and width, as shown by 
P. fagi , is more closely related to the strain of P. cactorum obtained 
from Panax sp. than from Phyllocactus sp. P. infestans and P. phaseoli can 
not be distinguished by differences in size of the conidia. P. jairophae 
and P.nicotianae are here shown to differ as regards both length and width. 
As previously pointed out, there are slight differences in the mycelium 
of the two species, the latter being somewhat broader and more gnarled 
and containing a larger amount of a substance of a fatty or glycogen 
nature. P. infestans and P. erythroseptica , both of which are parasitic 
on the potato (,Solanum tuberosum ), show great differences in size. 
The constants obtained for the standard deviation, as seen, show that 
the deviation from the mean for length varies from 4.31 ±0.102 juin the 
case of P. infestans to as high as 11.10±0.264 ix in the case of P. syringae, 
and for width the deviation varies from 3.03 ±0.072 \x in the case of 
P. infestans to 8.27 ±0.197 /x in the case of P. jairophae . Skewness in the 
case of length is positive in all cases, with the exception of P. jairophae , 
P. erythroseptica r and P. nicotianae . In the case of width it is positive 
in all cases with the exception of P. cactorum (Panax sp.) and P. syringae . 
