272 
Journal of Agricultural Research 
Vol. VIII, No. 7 
SUMMARY 
Prior to these studies no definite criteria for the identification and 
separation of the species belonging to the genus Phytophthora were 
known. With a view to supplying information for this purpose, 11 out 
of 13 described species were collected and studied from pure cultures 
grown on artificial media and from herbarium material. The results of 
these studies may be summarized as follows: 
(1) As regards rate of growth and spore forms produced, the various 
species reacted differently on the different media. 
(2) The temperature at which cultures are grown is a factor in the 
production of normal and comparable cultures. 
(3) For the purpose of testing the purity of the cultures from time to 
time two methods the details of which are described on pages 236-237 
were devised. 
(4) Continual culturing from large selected conidia does not eventu¬ 
ally produce a culture with a predominance of large individuals, or vice 
versa. 
(5) The separation and relationship of species should be made on the 
aggregate of characters, it being borne in mind that the proportionate 
value to be attached to each character must necessarily vary. 
(6) As a minor character, the macroscopic growth on a given medium 
is of some value. 
(7) The time of appearance of the spore forms from different strains 
of the same species on a given medium may vary, but eventually the 
same forms appear. 
(8) On oat agar the mycelia of the various species can not be distin¬ 
guished with any degree of certainty. On potato agar P. syringae can 
be distinguished from the remaining species by the fact that it produces 
characteristic tuberculate mycelia (PI. 71, A); and likewise P. nicotianae 
can, to a certain extent, be distinguished by the more gnarled mycelia 
and greater abundance of globoid particles of a fatty or glycogen nature 
within the threads. 
(9) Measurements of the conidia can be employed as an aid in deline¬ 
ating species provided a sufficiently large number are measuredi It is 
suggested that at least 200 should be measured and the different meas¬ 
urements grouped into classes. 
(10) For the purpose of obtaining a quantitative measure of the shape 
of the conidia the ratio of the length to the width should be ascertained 
and the ratios likewise grouped into classes. A comparison of the con¬ 
idia of P. parasitica and P. nicotianae illustrated this point. Heretofore 
the differences in shape of the conidia of these two species would be 
expressed only qualitatively, the former being called long and ellipsoidal 
and the latter short and globose. As a result of measuring the length 
and width of 400 individuals of each species and obtaining the ratios of 
the length to the width, the conidia of P . parasitica , expressed quanti- 
