MOSSES AND LIVERWORTS 119 
knowledge goes at present, the gulf between Alge and 
Archegoniates is a deep one. 
The dependence of all Archegoniates upon water for 
fertilization, and especially the presence of ciliated 
spermatozoids, are strong arguments for the derivation 
of the group from aquatic ancestors, but at present this 
is about all that can positively be asserted. 
Among the Archegoniates themselves, the relation- 
ships are much more obvious. Undoubtedly the lowest 
forms are the Hepatice, shown both by comparison with 
the alge and with the other Archegoniates, and probably 
these are to be considered as the primitive forms from 
which the others have sprung. 
Among the Hepatic, the lower Jungermanniacee, 
such as Metzgeria (Fig. 27, C), seem, on the whole, to be 
the simplest, although the sporophyte even in the low- 
est ones is more perfect than in Riccia, which has the 
lowest type of sporophyte found among the Archegoni- 
ates. Assuming that the lower thallose- Jungermanni- 
acez are the most primitive of Hepatics, we have seen 
that, from this type, several others have been developed. 
Tn one line (Marchantiaceew) differentiation has resulted 
in the specialization of tissues, the plant retaining its 
primitive thallose form (Fig. 27, A, B). In the leafy 
liverworts, the tissues have remained very simple and 
the differentiation has been purely external, resulting 
in a definite axis or stem bearing three rows of leaves 
(Fig. 27,D,E). A third line of development has given 
rise to the complex leafy gametophyte of the true 
mosses. 
In the simpler Hepatic the sporophyte is small 
and exclusively devoted to spore-production, e.g. Riccia. 
