Cytology of Pythium ultimum , n. sp . 301 
wall, curved ; sometimes especially in luxuriant cultures, two 
to an oogonium, and then often of diclinous origin and 
frequently straight. 
Oospores , one in each oogonium, spherical, the diameter 
varying between 18*3 \ x and 14*7 /x (average of fourteen measure- 
ments = 16-3 /x) with a smooth thick two-layered wall of 
yellowish colour, enclosing finely granular cytoplasm, a 
central reserve globule and one lateral nucleus ; germinating 
at once or after a period of rest extending to seven months 
and always by one or more germ-tubes. 
Zoospores are never developed. 
Fischer (’ 92 ) in Rabenhorst’s Kryptogamen Flora places 
Pythium in the Peronosporaceae. He further divides the 
Peronosporaceae into the Planoblastae and Siphoblastae. Of 
course the genus Pythium as originally defined was piano- 
blastic, but, as we have seen, Pythium ultimum, while agreeing 
with its fellows in other respects, is siphoblastic. This sub- 
division of the Peronosporaceae is not based on a sound 
principle, for even if we regard Pythium ultimum as an 
exception, similar remarks apply to the old genus, Perono - 
spora. The subdivision proposed involves the separation of 
like from like. 
Schroter (’ 93 ) in the Pflanzenfamilien places Pythium in a 
special group — the Pythiaceae— and this has much to recom- 
mend it, as Pythium differs very considerably from the other 
members of the Peronosporaceae. The Pythiaceae are, how- 
ever, separated from the Peronosporaceae, and included with 
the Saprolegniaceae in a group, the Saprolegniineae. Schroter 
was probably greatly impressed with the similarity in the 
‘ habit ’ and the c mode of life ’ of certain species of Pythium 
and many members of the Saprolegniaceae. It must be 
remembered, however, that none of the Saprolegniaceae pro- 
duces a periplasm, and this difference in character in itself is 
quite sufficient to justify us in preferring Fischer’s classification. 
It may be added that I have found the cell-wall in Saproleg- 
niaceae to consist invariably of typical cellulose. In Pythium 
