168 BOTANICAL GAZETTE [SEPTEMBER 
occasional formation of homothallic spores in the germ sporangia, 
and to consider as typical the condition shown in the diagram. For . 
a more detailed account of the zygospore germination in Phycomyces, 
as well as for the characters of the homothallic form into which this 
heterothallic species has been transformed, one must refer to the 
paper on zygospore germinations already cited. In the type, per- 
haps somewhat arbitrarily selected for discussion, the germinations 
are mixed—both male and female spores being produced in a single 
germ sporangium. The mycelia in this species are unisexual, the 
zygospores and germ tubes are bisexual, and the spores in the germ 
sporangia are unisexual, If the germ tube be forced to form a 
mycelium without the intervention of sporangiospores, a bisexual, 
7. e., homothallic, mycelium results, which may produce typical 
homothallic zygospores. Phycomyces as discussed, therefore, is 
heterothallic, homophytic, heterosporic, and homosporangic. 
In the bryophytes, Marchantia polymorpha is the only form which 
has been investigated in regard to the sexual condition of its sporo- 
phyte (cf. p. 170). Its gametophyte shows a differentiation into male 
and female thalli, and the germination of the zygote produces a 
sporophyte which bears a sporangium containing both male and 
female spores. Marchantia, therefore, like Phycomyces is hetero- 
thallic, homophytic, heterosporic, and homosporangic. 
Selaginella, as a representative of the heterosporous ferns, follows 
in the main the Phycomyces type. It differs from Phycomyces and 
Marchantia, however, in that it is heterosporangic—the male and 
female spores being separated in microsporangia and macrosporangia. 
The spores themselves, moreover, are morphologically of two kinds, 
the female or macrospores being conspicuously larger than the male 
or microspores. This morphological differentiation of the spores 
and sporangia is known only in the heterosporous ferns and in the 
flowering plants, and is accompanied by a reduction in the size of 
the gametophyte. Among the homosporous ferns, prothalli are often 
found with only archegonia or antheridia, and investigators have 
been able to suppress the formation of one or the other in certain 
species where archegonia and antheridia occur normally side by side 
on the same prothallus, The writer is aware, however, of no form 
