1906] BLAKESLEL—DIFFERENTIATION OF SEX 297 
they arise fom differentiated branches which are only distantly con- 
nected with the hyphae which bear the oogonia, and in Saprolegnia 
dioica and S. anisospora we have forms which have been described 
as dioecious. Cultural investigation alone can determine whether 
these latter forms are in fact heterothallic. It is perhaps significant 
that in this group forms have been found which have remained 
sterile under cultivation (cj. HoRN, 11. p. 232). It is not improbable 
that they may represent unmated strains of heterothallic species. 
Of especial interest will be an investigation for the possible occur- 
rence of two sexual races in groups such as the desmids, the flagel- 
lates, and the infusoria, where the whole vegetative organism func- 
tions directly as the gamete. 
Among the cryptogams, with the exception of the mucors and Mar- 
chantia, the sexual relations of the offspring from a single zygote in 
heterothallic forms, the zygotes of which give rise to more than a 
single individual, have never been investigated. The condition in 
the bryophytes has been already discussed under Marchantia. In 
the thallophytes writers see an alternation of generations variously 
expressed or suggested in the interpolation of carpospores between 
the fertilized zygote and the young plant. Whether in the hetero- 
thallic oedogoniums, to mention but a single example, the four 
Carpozoospores formed at the germination of the oospore are always 
all of the same sex, like the germ spores in Mucor Mucedo, or may 
be some male and some female, like the germ spores in Phycomyces, 
can be decided only by an investigation of the individual thalli 
which they produce. If species in the Saprolegniaceae and Per- 
Onosporeaceae are found to be_heterothallic, these forms will 
likewise furnish a fruitful field for investigation. 
The discussion in the foregoing pages is based for the most part 
upon investigations done or already in progress in the Botanical 
Institute in Halle. The writer wishes to express his grateful appre- 
ciation to Professor Kies for the facilities of the laboratory and 
for his unfailing sympathy in the researches undertaken. 
Paris, April, 1906. 
