264 
Journal of Agricultural Research 
Vol. VIII, No. 7 
potato foliage were made to oat agar, and when examined 10 days later 
in a dozen test tubes oospore-like bodies, similar to those previously found 
by Clinton and Pethybridge, were found in each tube. Another transfer 
to oat agar from these test tubes produced a supply of oospore-like bodies, * 
but subsequent transfers again gave negative results. 
In a communication to the writer under date of June 25, 1915, Pethy¬ 
bridge states that “with P. injesians, however, the formation of sexual 
organs appears to stop after a time. I have not seen any in my cultures 
for many months now. The cultures, however, are still quite virulently 
pathogenic to potato tubers. ” The small number of oospores produced, 
together with the variability in their production, may explain the failure 
of so many investigators working with this species to observe these bodies 
even when grown on a medium favorable for their production. 
In the case of P. infestans , which has been placed in the phaseoli group, 
Clinton (8) was unable to follow the different steps in the development 
of the oospores, owing to their scarcity, and claims that the oogonium is 
much more prominent than the antheridium, “since the latter is so fre¬ 
quently missing. ” Pethybridge, however, states unhesitatingly that the 
oospores of P. infestans follow the same course of development as in P. 
erythroseptica , and for this reason they are placed in the phaseoli group. 
In all the examinations made by the writer, however, only a few cases 
were found which might or might not be recognized as antheridia, and 
with the evidence at hand it does not seem to him that it has been con¬ 
clusively proved that the oospores of P. infestans belong to the phaseoli 
group. On account of the scarcity of antheridia and indefinite knowledge 
as to their nature it would seem perhaps more logical at present to place 
them in the faberi group or the phaseoli group, bearing in mind that their 
relationship is not yet entirely solved. These are the reasons why the 
phaseoli group is not designated “ infestans group, ” and they likewise con¬ 
stitute an additional argument against the subdivision of the genus at 
present into two genera. 
Measurements oe oospores. —Four hundred measurements were made 
of the diameter of the oospores of the various species, and, except in the 
case of P. syringae , cultures of which were made on sterilized carrot 
plugs, all were made from normal oat-agar cultures. Table III gives 
the results of these measurements, the first column showing the classes 
in microns and the other columns the number of oospores out of a total 
of 400 falling into each class. As it was impossible to obtain a sufficiently 
large number of oospores in the case of P. infestans , this species is not 
included in the table. The diameter of the oospores, as will be seen, 
varies from 17.5 to 49.5 fi . The greatest number of oospores of P. cac - 
torum from Phyllocactus sp. and Panax sp. and of P. phaseoli fell in the 
same class—27.5 It should be emphasized, however, that in the case 
of P. phaseoli , unlike that of P. cactorum , a larger percentage of the 
