426 
CECIL YAMPOLSKY 
Three weeks after germination the seedhngs, which represented about 
fifteen percent of the seed sown, produced female flowers. No male plants 
were produced. I am not ready to report on the subsequent behavior of 
the seedlings. 
Discussion 
Sex Determination and the Alternation of Generations 
In the discussion of the time at which sex is determined in plants it 
must be borne in mind that there are two generations to be considered, the 
gametophytic haploid and sporophytic diploid generations. This alterna- 
tion of generations occurs regularly (except in plants that reproduce parthe- 
nogenetically only), so that a consideration of sex determination must 
take both generations into account. This makes possible the existence of 
sex differentiation at two distinct stages in the life history. In the seed 
plants, which we have been considering, sex differentiations are always 
present in the gametophytes (female, embryo-sac, and male, pollen tube), 
while the sporophyte may be of any grade from male through hermaphrodite 
to female. From the standpoint of the sex of the gametophyte, if we at- 
tempt to assume a segregation of sex determiners in the reduction division, 
we have the curious condition that the already sex-determined individuals 
may give rise to sporophytes of graded sexes ranging from males through 
hermaphrodites to females. The assumption, therefore, must be made 
that after fusion a redetermination occurs at one or several stages which 
brings about the graded sex forms. 
In the dioecious liverworts, where antithetic alternation of generations 
is conspicuous, sex determination may occur at the reduction division. No 
one has yet observed sex differentiation in the sporophyte. Strasburger 
(1909a) finds in Sphaerocarpos, in which the spores remain united in tetrads, 
that in the vast majority of cases, two spores of the tetrad give rise to fe- 
male prothallia and two spores give rise to male prothallia. 
Allen (191 7) in a brief note reports the presence of sex chromosomes in 
the gametophyte of Sphaerocarpos. Although it is difficult to count the 
chromosomes, he says that there are eight. In the female gametophyte one 
of the chromosomes is larger and thicker than the rest, while in the male 
gametophyte there is one that is smaller than the other seven. This is a 
case in which the differential chromosome condition is found in the game- 
tophyte, while in animals it is, of course, associated with the diploid genera- 
tion. The sporophyte of Sphaerocarpos is more or less globular in shape, 
the foot being in the form of a bulb. We must assume that the sporo- 
phyte contains the double number of chromosomes, namely 16, two of which 
are the larger one of the female and the smaller one of the male. The sig- 
nificance of these two chromosomes is not yet understood, especially since 
the sporophyte of Sphaerocarpos is not sexually differentiated. 
It is interesting to note that sex determination in the monoecious fiver- 
