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P. Cavers. 
each involucre ; (3) the Composite type, with a two-furrowed stalk 
(except Lunularia) and several archegonia in each involucre. The 
first type of carpocephalum was regarded by Leitgeb as a purely 
dorsal outgrowth of the thallus ; the second as representing a single 
branch, the growing-point of which is used up in the development 
of the carpocephalum without undergoing dichotomy; and the third 
as being composed of a branch-system, the growing-point being not 
only used up but undergoing repeated dichotomy during the 
formation of the carpocephalum, this dichotomy occurring at an 
early stage and affecting the whole carpocephalum and not merely 
the stalk. As we have seen, however, the facts recently discovered 
have broken down the distinction between Leitgeb’s second and 
third types, and we are probably justified in regarding the carpoce¬ 
phalum which terminates the apical growth of the thallus as being 
in all cases formed by repeated dichotomy of the apex, resulting in 
from two to sixteen growing-points, each of which may produce 
either a single archegonium or a series of archegonia. In Clevea 
and Plagiocliasma the carpocephalum when fully developed has 
much the same structure as in the other Marchantiaceae which have 
a single archegonium in each involucre, except for the absence of 
rhizoids from the receptacle and of a groove from the stalk, and it 
can hardly be doubted that we have even here a difference in degree 
rather than in kind as compared with the carpocephalum of other 
genera. That is to say, the difference between the “ dorsal out¬ 
growth ” carpocephalum of these two genera and the “ branch 
system ” carpocephalum of the other Marchantiaceae depends upon 
the behaviour of the growing-point in the early stages of development. 
Exormotheca presents what is perhaps the simplest type of a 
definitely “Composite” carpocephalum, which here consists of a 
branch system with two growing-points. The apex undergoes a 
single dichotomy, and each of the growing-points produces a series 
of archegonia, the latter being developed in centrifugal succession, 
in each series, with reference to the centre of the dorsal side of the 
receptacle. This Composite receptacle differs from the Clevea type 
chiefly in the hastening of the events that take place in the laying 
down of the young carpocephalum. The growing-point now enters 
into the composition of the receptacle from the very first, undergoing 
dichotomy and becoming used up in the development of the arche¬ 
gonia, so that the formation of a receptacle necessarily terminates 
the apical growth of the thallus on which it is borne. The presence 
of ventral scales and of rhizoids, the latter passing into the furrow 
