Feb. ia, 1917 
Studies of the Genus Phytophthora 
237 
poured into the flask under aseptic conditions and the culture placed at 
a temperature favorable for the germination of the conidia by means of 
swarm spores (20). When such germination took place, each conidium 
gave rise to from 20 to 40 swarm spores. Poured plates were then made 
from this solution which contained hundreds of zoospores the majority of 
which germinated readily on the agar. Instead of obtaining a culture 
from a single conidium, which is potentially a multiple of zoospores, the 
new culture was obtained from a single zoospore. 
After making a number of measurements of conidia and observing the 
great variation in size, it occurred to the writer that by continual platings 
from large selected individuals cultures could be finally obtained which 
would produce only large conidia, and vice versa. To determine this 
point, dilution plates were made, a number of single large conidia were 
marked, and the colonies resulting from one or more of these transferred 
to agar slants. When these cultures reached the normal condition in 
the production of conidia, counts were made to determine the percentage 
of large and small conidia. Dilution plates were again made and the 
large conidia marked, and the colonies resulting from this second growth 
or generation were again transferred to agar slants. When the normal 
spore-producing stage was reached, the percentage of large and small 
conidia was again determined. This procedure was repeated through 
five generations, the large conidia being selected in all cases. There was 
no material difference between the percentage of large and small conidia 
in the original culture and the culture resulting from the last selection. 
This indicated that, notwithstanding the great variation in size, the 
limits and percentages of large and small conidia remain practically 
constant and are therefore good characters to be employed in taxonomy. 
The characters to be relied on in taxonomic work must be constant 
under all conditions and not correlated with certain adaptations to hab¬ 
itat. While there are advantages in employing characters which show 
relationship between the species, this is not always possible. In the work 
here set forth the separation and the relationship of species are made 
on the aggregate of characters, it being borne in mind that the propor¬ 
tionate value to be attached to each character must necessarily vary. 
The most valuabl^characters that can be employed are doubtless those 
which are exhibited by the reproductive organs and which, as will be 
seen, furnish means of separating the genus into different groups. Other 
characters which may be conveniently used in separating the various 
species, arranged as nearly as possible in the order of their importance, 
are (1) the size and morphology of the sexual reproductive bodies, co¬ 
nidia, chlamydospores, and other structures; (2) shape of the terminal 
papillum in the conidium; (3) the ratio of the length to the width in the 
conidia; (4) variation in the mycelium; and (5) the microscopic growth, 
time of appearance of spores, and kind of spore forms produced on 
various media. 
