Physiology of the Saprolegniaceae . 385. 
Summary of Results. 
(1) It is possible, by the method of culture outlined, to isolate species 
of Saprolegnia relatively quickly by means of a single zoospore. 
(2) The study of the various species of Saprolegnia verifies the work 
of Klebs on S', mixta as to the effect of nutrition on the differentiation of 
the reproductive and vegetative processes. 
(3) Not all the species of Saprolegnia produce sexual organs under the 
conditions favourable for those of S. mixta and S. hypogyna and are there- 
fore physiologically as well as morphologically distinct. 
(4) Saprolegnia hypogyna , which has morphologically no true anther- 
idia, can be made to develop such under proper nutrient conditions ; K 3 P 0 4 , 
KN 0 3 , Na 2 HP 0 4 , Ca 3 (P 0 4 ) 2 , and Ca(N 0 3 ) 2 , added to haemoglobin solu- 
tions, are, in different degree, effective to this end. 
(5) The antheridial filaments arise in either a diclinous or an andro- 
gynous manner in all the species examined under varying culture con- 
ditions, and hence their origin is of no special importance as a diagnostic 
character. No sign of heterothallic species was detected, unless the absence 
of sexual organs in some species points that way. It is believed that these 
species have not yet been placed under the proper conditions for the forma- 
tion of sexual organs. 
(6) The variations were so extensive that nearly all the characters 
used for diagnostic purposes in this genus were affected. At first thought 
this seems to indicate that no separation can be made of the different 
species of Saprolegnia by means of constant characters. We can, however, 
use the resources of the chemist and physicist, and, by stating the exact 
conditions of culture, establish a standard to which all forms may be 
referred. A species thus defined can be identified by placing it in these 
conditions. The assumption here needed seems to be perfectly justified ; 
for it has not been proved as yet that characters which have been developed 
under cultivation will be retained by the plant when it is returned to the 
original environment. 
(7) In spite of the great similarities of certain very constant forms 
and their overlapping under different conditions, it is concluded that they 
are entirely distinct forms, at least physiologically, if not always morpho- 
logically. This result, combined with what Maurizio and De Bary found 
in this family of fungi, forces one to assume that there are a great many 
simple forms- — elementary species in the sense of De Vries — within the 
genus Saprolegnia. 
In conclusion, I may say that this paper adds something more of 
evidence towards the doctrine that sex in plants is determinable by external 
conditions. It is admitted that the evidence is not conclusive ; for the 
structure and the development of the plants studied are of such a nature as 
