Toit] CURRENT LITERATURE 73 
The determination of sex.—In a recent paper on the determination of sex, 
STRASBURGER” adds to his already extensive contributions to this difficult 
subject. As in previous papers, he maintains that the problem is phylogenetic, 
and that there is a striking parallelism between the animal and plant kingdoms 
in the evolution of sex. In both kingdoms the original differentiation appears 
only in the haploid generation, but with the differentiation of sex in this genera- 
tion came fertilization and the formation of a diploid generation, which, in 
both animals and plants, became the dominant one. 
The point at which the separation of sexes takes place | in various plant 
groups is noted briefly; the statements, in most cases, depending upon facts 
already known, rather than upon cytological or other evidence in connection 
with this particular paper. 
In monoecious Chlorophyceae the thallus is bisexual and the sexes are 
separated at the formation of oogonia and antheridia; at fertilization the two 
sexes are united; the reduction of chromosomes takes place during the first 
two divisions of the zygote, but is not accompanied by any separation of sexes, 
the product of the zygote being bisexual. In dioecious Chlorophyceae the 
separation of sexes occurs at the reduction division, so that the products of | 
the zygote are unisexual. Thus the separation of sex tendencies appeared 
first in connection with the reduction divisions. 
In monoecious bryophytes there is no separation of sexes at the reduction 
divisions, the separation occurring later, at the formation of antheridia and 
archegonia; but in dioecious forms the separation occurs at the reduction 
division. That the separation of sex tendencies as well as their union at ferti- 
lization is decisive, is shown by the fact that protonema from vegetative cells 
of a sporophyte of a dioecious moss produces leafy plants bearing both anther- 
idia and archegoni 
In hcmngiiaaae pteridophytes there is no separation of sexes at the reduc- 
tion divisions, the spores being bisexual and the sex tendencies being separated 
later in the gametophytes arising from the spores. The division which many 
homosporous pteridophytes show in their gametophytes is due merely to external 
factors, the gametophytes being really monoecious. In heterosporous forms 
the separation of sexes does not occur at the reduction divisions, but much 
earlier, during the divisions leading to the formation of spore mother cells, 
so that the spore mother cells are already all male or all female, all the spores 
of a microsporangium producing male prothallia and all those of a megaspo- 
rangium producing female prothallia. The two sex tendencies are united in the 
sporophyte, which can then produce both microsporangia and megasporangia. 
Through the heterospory of the sporophyte the dioecism of the gametophyte 
became firmly established. 
In seed plants the sexes are recognized by the external ‘‘sex organs” of 
™ STRASBURGER, EDUARD, Ueber geschlechtbestimmende Ursachen. Jahrb. Wiss. 
Bot. 48:427-520. pls. 9, 10. 1910 
