182 
F. Cavers. 
Whether or not the simple structure of the Riccia sporophyte 
is due to reduction, correlated with the sunken position of the 
spore-fruit in the thallus tissue, remains an open question. On the 
whole it seems probable that Riccia has been derived from a form 
not unlike Splicerocarpus by elaboration of the thallus as an 
adaptation to life in more exposed and less hygrophilous surroundings, 
leading in several species to marked xerophily—shown by the large 
overlapping purple ventral scales and the formation of perennating 
tubers. But there is a somewhat wide gulf between Riccia and 
the Sphaerocarpales, despite the close agreement in the structure 
and mode of branching of the apical growing-point, the development 
of the sexual organs, and the embryogeny; for in Geothallus and 
Riella, and even in Splicerocarpus itself, we find a marked tendency 
towards the differentiation of a leafy shoot which is characteristic 
of the Jungermanniales. Campbell suggested that the three great 
phyla of Hepaticae (Marchantiales, Jungermanniales, and Antho- 
cerotales) may be derived from a primitive form (Lotsy’s “ Sphaero- 
riccia”) which combined the simple thallus of Splicerocarpus with a 
Riccia- like sporophyte. And, as pointed out by Tansley (63), the 
Protobryophytes probably arose, by restriction of spore-production 
to the fertilised egg, from algal ancestors “ which possessed arche- 
gonia as their female organs, and zoosporangia or aplanosporangia 
as asexual organs, with no regular alternation of generations, but 
with the power of bearing sporangia or sexual organs according to 
temporary conditions, just as is the case in so many green algae.” 
Assuming that the sporogonium of Riccia is primitive, it is of great 
interest as showing the first step in the progressive sterilisation of 
a simple spore-fruit formed by segmentation of the fertilised egg. 
As already stated, the sporogonium does not reach a very high 
grade of development in the Marchantiales. This arrest of the 
sterilisation process is evidently correlated with the fact that in the 
Marchantiaceae the stalked carpocephalum carries up the sporogonia 
into the air and thus renders unnecessary the development of a long 
sporogonial seta or of more elaborate mechanisms for spore dispersal, 
such as are found in the Jungermanniales. Only in the peculiar 
genus Mouoclea is the capsule elevated on a long seta, as in the 
Jungermanniales; as already stated, this may be correlated with 
the extremely hygrophilous mode of life of this genus. In the 
Corsiniaceae and Targioniaceae there is no special means of raising 
the sporogonium above the general level of the thallus, but these 
plants do not grow in such moist situations, Targionia being in fact 
