Relationships of Bryophyta to other Phyla. 85 
thallus and the substratum. In Dumortiera also, the sporogonium 
is apparently exempt from the reduction that has overtaken the 
gametophyte. In any case, however, it can hardly he maintained 
that the entire series of Marchantiales displays reduction caused 
by adaptation to moist or shaded habitats, for most of these plants 
are more or less decidedly xerophilous as compared with the 
Liverworts as a whole, and the markedly hygrophilous forms are 
pretty evenly distributed through the different divisions of the series. 
It might he urged that reduction of the Marchantia sporogonium 
to the Riccia type has been caused by the hemming-in of the 
developing sporophyte by the tissues of the parent plant, leading 
to precocious formation of the spores and almost complete reduction 
of vegetative growth ; just as the germination of the spores of 
certain Ferns within the sporangium causes precocious formation 
of the sexual organs and reduction of the prothallus. If logically 
followed out, this view would have to be applied to the Bryophyta 
as a whole, and it would lead to the conclusion that the Bryophyte 
sporogonium was originally a free-living plant, which later became 
enclosed in the tissue of the gametophyte ; this retention might be 
supposed to have coincided with the evolution of the archegonium 
and with the passage from life in water to life on land, and therefore 
to have occurred in the algal ancestors of the Bryophyta. Many 
objections may be urged against this hemmmg-in or retention theory. 
To begin with, it may well be that the hemming-in of the Riccia 
sporogonium is the effect and not the cause of its simple structure, 
and that the Riccia sporogonium is embedded in the thallus because 
it has not developed the sterile basal tissue which enables the 
sporogonium of higher forms to raise its spore-producing upper 
portion above the thallus. Moreover, the well-developed sporogonia 
of the highest Marchantiales are hardly less embedded in the 
thallus than are those of such lowly types as Corsiuia. The 
sporogonium of Targionia , which is sessile on the thallus, is quite 
as highly specialised as that of Marchantia, which is raised high 
above the thallus on the stalked carpocephalum. 
The assumption that the simple structure of the Riccia sporo¬ 
gonium is primitive leads naturally to the conclusion that—whatever 
views may be held as to the relationship between the alternating 
generations in other phyla—the Bryophyte sporogonium had an 
antithetic origin, and that it has never been anything except a 
sporogonium. This tempts one to discuss the question whether 
the sporogonium may have given rise to the free-living sporophyte 
