236 
Journal of Agricultural Research 
Vol. VIII, No. 7 
ing to the directions given by G. P. Clinton (7). Unless otherwise stated, 
therefore, this was the media used in all cases where measurements and 
drawings were made. 
In order to obtain a definite spore form, it was necessary in some cases 
to employ special substrata, such as cooked carrot, potato cylinders, 
corn-meal agar, and sterilized flies. As artificial media were most gen¬ 
erally employed, it seemed important to determine whether these influ¬ 
enced the morphology and size of the various structures to any appre¬ 
ciable extent. Preliminary trials were made with P. infestans on potato 
foliage, but no appreciable differences between the spores produced on 
this and on those grown on artificial media could be detected. The 
morphology and size of the various structures of a number of other forms 
as they occur on the original host are now being studied, and in a later 
paper these studies will be compared with those recorded here. 
The observations and measurements were made at the time of the 
first appearance of conidia and oospores, at which stage the culture most 
nearly approaches normal. Soon after this, if the cultures are kept at 
growing temperatures, the spore forms begin to show various abnormal¬ 
ities, chief among which may be mentioned various proliferations and 
the formation of secondary conidia. 
The temperature at which cultures are grown is a factor in the pro¬ 
duction of normal and comparable cultures, low temperatures tending to 
diminish the size of the spore forms and the higher temperatures tending 
to diminish their production. The cultures from which these studies 
were made were kept at room temperature varying from 18 0 to 20° C. 
No records were kept of the number of times they were transferred. In 
making the transfers it was found that, if the substrata and temperature 
conditions were the same, the spore forms appearing were also the same, 
no matter whether mycelia, conidia, or sexual bodies were used in the 
transfer. 
The purity of the cultures was tested as often as seemed necessary. 
Several methods were employed in the tests. If the presence of bacteria 
was suspected or if the sole object was to determine the presence of 
bacteria, transfers were made from the cultures to nutrient beef agar, a 
medium on which species of Phytophthora make no growth, but which 
is especially favorable to the growth of bacteria. In the case of forms 
which produced an abundance of conidia, dilution plates were made 
and the cultures recovered from colonies resulting from single conidia. 
This was a long and tedious process, however, as generally only a small 
percentage of the conidia germinate on the poured plates. More recently 
a modification of the above method was used. Where there was a doubt 
as to purity, the culture was transferred to an Erlenmeyer flask con¬ 
taining media known to produce an abundance of conidia. When the 
conidia appeared and were in a normal condition, sterilized water was 
