1906] BLAKESLEE—DIFFERENTIATION OF SEX 173 
but that to obtain archegonia, they must be exposed to a still greater 
illumination. BucHTIEN (7) has shown that in Equisetum external 
conditions have a similar influence upon the apparent sex of the 
prothalli. 
As yet attempts to influence arbitrarily the sex in unisexual plants 
have entirely failed. Even though it remain impossible to change 
the sex in the thalli of Marchantia, it may be found that, by experi- 
menting on the sporophyte where we must assume the sex is unsegre- 
gated, one may-be in a position to bring about the exclusive pro- 
duction of either male or female spores in a given sporangium. Such 
a result if accomplished would be analogous to the suppression of 
one set of sexual organs on the prothalli of ferns. 
The behavior of the gametophyte of homothallic ferns and that 
of the sporophyte of such heterophytic flowering plants as Melan- 
drium album (19) shows that, abnormally in certain forms and nor- 
mally in others, only one sex may make its appearance. The con- 
clusion suggested by an assemblage of facts, especially from the 
animal kingdom, is generally accepted that in so-called unisexual 
forms one sex is dominant and finds expression in the formation of 
gametes or spores of the given sex, while the opposite sex exists in 
a latent condition. However probable such a conclusion may appear 
for the majority of forms investigated, it must be admitted as at 
least a possibility that in certain plants or in certain stages a single 
Sex May exist in a pure condition. The fact that besides the occa- 
sional production of unisexual germ tubes the zygote of Phycomyces 
gives rise typically to germ tubes in which the differentiation of sex 
has not taken place is proof neither for nor against the purity of the 
male and female thalli, and suggests that the not infrequent occur- 
rence among heterophytic flowering plants of individuals with male 
and female flowers is as much an indication that both pure and 
mixed conditions may exist in the sporophyte of these plants as a 
Proof that in heterophytic plants the opposite sex always exists in 
a latent condition. The germinations of the zygotes of Phycomyces 
and Marchantia suggest the possibility that the sex may be pure in 
the gametophyte while mixed in the sporophyte. The observations 
On unisexual plants, however, have been as yet confined almost 
entirely to the sporophytic stage, and little is known as to how strict 
