26 o 
Journal of Agricultural Research 
Vol. VIII, No. 7 
media. Sterilized carrots were employed fox P. syringae . A slight 
modification of oat agar as originally used—that is, Quaker-oat agar as 
used by Pethybridge and containing 7.5 gm. of agar to 1,000 c. c. of 
media—was employed for the remaining two. 
The abundance of oogonia and oospores is so variable that this is a 
poor character for use in the separation of species. There is one excep¬ 
tion, however—that is, P. phaseoli as compared with P. infestans. Al¬ 
though the former is very closely related to the latter, there is a striking 
contrast between them in the abundance of oospores produced on oat 
agar, a large number constantly developing in the case of the former and 
few or none in the case of the latter. The entire absence of oospores in P. 
infestans , or the scarcity in the development of them, has been observed 
by all previous investigators working with this species. Although it is not 
advisable to rely on this character alone in separating these species, it is 
useful for this purpose when taken in connection with more important 
characters. 
Except in rare cases in which they arise intercalarily, the oogonia 
originate as terminal enlargements of the mycelial threads, as shown in 
Plates 76 and 77. In the former case it is often difficult, unless antheridia 
are present, to determine whether they are true oogonia or chlamydo- 
spores, as found in P. faberi (PI. 77, A) and other species already men¬ 
tioned. In certain forms the antheridium is formed first, and in others 
the oogonium; but the priority of development of these two organs is 
determined with such difficulty that it can not be used with safety in 
the separation of the species. 
The position of the antheridium varies in the different species, and 
this variation is constant on all media and can thus be used in separating 
the genus into groups. These groups may be conveniently designated 
according to the specific name of one of the species belonging to each 
group, preferably the oldest described form, as first suggested by Pethy¬ 
bridge (24). The writer has established a third group, which embraces 
the forms in which the antheridia are entirely absent or in which the 
relation of the antheridium to the oogonium is unknown. This group is 
analogous to the Fungi Imperfecti. It will be seen, therefore, that there 
are three groups, which are designated the “cactorum group,” the “pha¬ 
seoli group,” and the “faberi group.” 
Cactorum group. —In this group the antheridium, which is elliptical 
to reniform, is always distinct and is generally on the side of the oogonium. 
In some stages of development of the antheridium and oogonium a definite 
fertilization tube can be observed. After fertilization the antheridium 
disappears entirely or becomes transparent and the contents much 
reduced. P. cactorum (PI. 77, C), P. fagi (PI. 77, Z)), and P. syringae 
(PI. 76, A) are included in this group, and according to Pethybridge 
probably P. nicotianae also. Fertilization in this group consists in a part 
