Feb. 12,1917 
Studies of the Genus Phytophthora 
241 
In general 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 form appears. The kind of spore forms and the time of their 
appearance, as in the case of the macroscopic appearance of the growth, 
may be influenced by the length of time the fungus is kept in culture; 
and consequently these characters can be of little value in a systematic 
separation of the species, except when used in conjunction with more 
stable differences. The production and appearance of certain spore forms 
in some of the cultures of P. infestans will be discussed in a later paragraph. 
COMPARATIVE MORPHOLOGY 
MYCELIUM 
The mycelium of all the species is white and aerial or submerged, as 
shown by the growth on the artificial substrata previously mentioned, 
and the hyphae when young are coenocytic and filled with a dense, granu¬ 
lar protoplasm, oftentimes intermingled with larger particles, probably 
fat and glycogen. In old cultures, where the supply of nutrition is not 
so abundant, septation may often be seen; and at this stage the granular 
protoplasm becomes less dense, and many vacuoles appear. Since the 
tips of the hyphae are the young growing parts it would appear that the 
granules are translocated thither from the older basal portions of the 
hyphae. 
The appearance of the hyphae varies, depending on whether the mount 
was made from aerial or from submerged growth, the aerial mycelium 
being straight and the submerged much twisted and gnarled. 
The mycelia of the different species do not show any distinguishing 
characters when grown on oat agar. Plate 71, B, shows the mycelium 
of P. syringae on this agar, and this is the general appearance of the 
mycelium of the other forms when grown on this medium. 
On potato agar the character of the mycelia is similar to that on oat 
agar, except in the case of P. syringae , in which species the hyphae present 
a gnarled and tuberculate appearance (PI. 71, A), This character makes 
the mycelium of P. syringae readily distinguishable from the other forms 
when grown on potato agar. Ward (35) observed a similar appearance 
in the mycelium of Pythium gracile , and suggested that these structures 
may serve as reservoirs of protoplasm for the immediate use of the myce¬ 
lium or for the formation of oogonia. Butler (4) likewise observed struc¬ 
tures in the same genus and compared them to the toruloid structures 
often formed by fungi, the function of which is to carry on a vegetative 
propagation of the species. Butler’s drawings, in his Plate 1, figures 3 and 
4, bear a striking resemblance to the figure shown by the writer, to whom 
these structures appear to be more nearly homologous with sclerotia, though 
lacking the texture of the latter. The tuberculate protuberances are 
slightly darker, densely packed with protoplasm, and much more capable 
