Feb. i2 t 1917 
Studies of the Genus Phytophthora 
249 
In P. cactorum isolated from Phyllocacius sp. and from Panax sp. the 
greatest number of individuals as regards length and width fall in the 
same class, although the actual number of species in all the classes 
varies. The measurements of conidia of P. fagi show that it is closely 
related to P. cactorum. There ____ 
* - - --- - - 
is also a striking relationship 1 
between P. infestans and P. ***) 
phase oli sss ^ 
As shown by the curves in s 
figures 1 to 6 , the classes con¬ 
taining the preatest number 
of individuals in the different \ 
V 
species show the least varia- 
firvn the' width nf the 
T 
t 
1 
conidia. The length of the \ 
\ 
\ 
conidia therefore may be ea3 
\ 
dUUjJlCU a! UlUaXU^ LU UivlUC 1 
the species into two groups: ^ 
\ 
Those in which the predomi- | 
A 
> 
\ 
naung class is more ciia.ii 34*0 ^ ~~ 
H in length, and which include ^ 
\ 
^35 
Kg 
s: 
'Hi 
the forms P. faberi , P. fa - §*** 
s. 
'St 
trophae , . arecae , jP . syrmgae y 
and P. erythroseptica , and , 
V 
s. 
those in which the predomi- * 
''' 
\ 
# 
nating class is 34.5 jx or less, 
and which includes the forms _ 
* 
/ 
/ 
4 
f/\ 
\r m ‘ 
- 
— 
-**' 
SAo 
P. cactorum , P. fagi , P. nico - n 
r ' 
/ 
/ 
$ 
tianae , P. infestans , and P. 
phaseoli. 
/ 
* 
y %4 
1 "j 
Differences in length or / 
width alone in different 
species do not give an accu- — JL 
'I O 4 SOF" 
rate idea as to differences in I 
form or shape ot the conidia. '"*0 /& so so so so to so so /& //? &0/30/4 
The form or shape IS a con- Fig. 3.—Graph showing the variation in length of conidia 
Stant character and should Phytophthora nicotianae, P. cactorum (Panax sp.),P. arecc 
. , and P.syringae. 
be employed m differential 
ing the species. P. parasitica , for example, can be readily dis¬ 
tinguished from P. nicotianae by the fact that the conidia of the former 
are long and elliptical, while those of the latter are ovate to globose, 
as shown in Plates 73 and 74. Similar differences exist in the conidia 
of other species also; but while such differences can be readily detected 
under the microscope, it has not been possible heretofore to express 
