244 
Journal of Agricultural Research 
Vol. VIII, No. 7 
the contents of the conidia show any distinguishing characters (Pl. 
73 , 74 )- 
Measurements or conidia. —Measuring spcre forms to delineate 
species in the various fungus groups is a well-established practice. In 
forms less variable than those of Phytophthora spp. a small number of 
such measurements might be sufficient to determine the average size. 
The conidia of species of Phytophthora, however, are so variable that a 
small number of measurements are likely to be misleading. This is 
brought out by the following figures, which show the variation in the 
measurements of the same species as given by 12 different workers. 
Name of worker. 
Size of conidia. 
Size of oospores. 
Cohn and Lebert... 
45 by CC to 68.. 
20 by 70. 
Hartig. 
2 C tO 40 bv 2C to AO. 
Schenk 1 . 
20 (oogonium). 
16 to 24. 2 
24 to 30. 
24 (oogonium). 
30 to 45. 
None found. 
26 to 28. 
Bary. 
C c to 40 by co to 60 to 03. 
Schroeter. 
7 C to 40 by co to 60. 
Osterwalder. 
14.64 to 24.4 by iiq.c6 . 
Himmelbauer. 
Not given. 
Zimmerman. 
17 to co by 2 c to 60. 
Hori. 
co to co bv co to 60. 
Abnormal, 29 by 85.5. 
Bubak. 
15 to 25 bv 15 to 120. 
Not given. 
Do. 
21 to 36. 
Van Hook. 
30 to 42 by 40 to 58.. 
Writer. 
1 c to 20 bv 18 to C 4 .. 
1 "Uie kleinsten derselben sind 5, die grossten 36 Theilstriehe meines zeissechen Mikrometers lan2 und 
4 bis 25 Theilstriehe breit.” 
2 De Bary states that the oospore is in general three-fourths to four-fifths the diameter of the oogonia, 
and gives 24 to 30 n as the diameter of the latter, so that the diameter of the oospore given above was ob¬ 
tained accordingly. 
As the tneasurements given above, if correct, would warrant the 
establishment of more than one species, it seemed advisable to make a 
large number of measurements of all the forms in culture and present 
them in their entirety. Accordingly measurements of conidia were made 
from normal oat-agar cultures of relatively the same age—that is, cul¬ 
tures which had just begun to form conidia, except in the case of four 
species: (i) P. erythroseptica , which was obtained from normal cultures 
grown on sterilized flies; (2) P. parasitica , and (3) P. nicoiianae , ob¬ 
tained from normal cultures grown on Lima-bean agar, and (4) P. 
syringae from sterilized carrots or bean pods. The scarcity of conidia 
of these four species on oat agar made the use of special media necessary. 
It is realized that error may have resulted from the use of different 
substrata for growing the various forms; but in preliminary trials with 
a number of different artificial substrata the size of the spore forms 
remained practically constant on all. 
The measurements, of which there were two sets of 200 conidia each, 
are presented in Table II. These measurements were made at different 
times and show the variation that may reasonably be expected. The 
