Wilson: North American Peronosporales 79 
low osmotic pressure is necessary to the proper development of 
P. infestans and that it is “ limited to certain combinations of 
chemicals as sources of carbon, nitrogen, and energy. The only 
really* efficient single carrier of these which was found is aspar- 
agin, and the availability of this substance seems to be dependent 
upon the presence of other chemicals” (pp. 51, 52). His most 
successful formula is as follows: Potassium phosphate 0.25 gm., 
potassium chlorid 0.05 gm., potassium nitrate 0.5 gm., magnesium 
sulphate 0.1 gm., calcium carbonate 0.025 gm., asparagin 0.5 gm., 
water 1 liter. 
In the course of his extensive studies on the germination of the 
conidia of P. infestans in relation to various substrata Garbowski 
(1913) devoted considerable attention to the subject of synthetic 
media with the result that he recommends Knop’s solution with 
the addition of glucose (0.2 gm. to 50 c.c.). 
Taxonomic Considerations 
From the discussion of the various species of the genus it is 
evident that there are two distinct types of sexual organs present 
in species which have been referred to Phytophthora. When De 
Bary described the oospore formation in P. omnivora his account 
showed nothing which did not agree with the process as we know 
it in Peronospora. Recent investigations have confirmed this on 
P. Fagi, P. Cactornm , and P. Syringac, while the description of 
P. Nicotianae indicates that it belongs to the same group of 
species. These species have been designated by Pethybridge as 
the C act or urn-group. In P. Faberi the sexual reproduction is 
unknown, while in the remaining species of the genus the sexual 
organs are of the peculiar type described by Pethybridge and by 
Dastur. The group of species producing this type of gametes 
has been called in like manner the infestans-group. Here we find 
a mode of sexual reproduction which is unique among the 
Phycomycetes. So distinct is this . method of oospore formation 
that Pethybridge proposes to separate the species which possess it 
into a new family, calling it Phytophthoraceae. While the re- 
maining species are retained in the family Peronosporaceae under 
the generic name Nozemxa. While the process of odgenesis is so 
poorly understood at present, yet it is apparent from the peculiar 
