179 
Phytogeny of M archant tales. 
of the stalk, may obviously be regarded as confirming the inter¬ 
pretation of the Composite receptacle, found in all Marchantiaceas 
except Clevea and Plagiochasma, as a branch system, which shows 
increasing complexity in the higher forms of the Marchantia-series. 
The development of a single archegonium in each involucre can no 
longer be taken as an indication that the carpocephalum differs 
in nature from the Composite type, but at the same time the inter¬ 
affinities of the Marchantiaceae are difficult to disentangle, and it 
is probable that the presence of a single archegonium in the 
involucre is (apart from the cases of Clevea and Plagiochasma ) due 
simply to reduction and is not a primitive character. 
As already pointed out, the evolution of the stalked carpo¬ 
cephalum is closely correlated with the facilitation of spore-dispersal, 
which has also been the chief factor in the evolution of the sporo- 
gonium. In the Ricciaceae the spores can only escape by the decay 
of the surrounding thallus tissue in which the sporogonium is 
embedded ( Riccia ), or that of the envelope projecting above the 
general level of the thallus ( Tessellina ), in addition to the disorgani¬ 
sation of the thin capsule-wall and calyptra. Tessellina marks a 
great advance upon Riccia, in that the sporogonium is raised above, 
instead of being deeply sunk in, the thallus, and the spores no 
longer have to wait until the decay of the whole thallus sets them 
free. With this shifting of the sporogonium to the surface we may 
correlate its differentiation, first seen in Corsinia, into foot and 
capsule, and the sterilisation (already indicated in Tesellina ) of some 
of the archesporial tissue of the capsule. The sterile archesporial 
cells have the primary function of supplying food to the developing 
sporogenous cells, and in Corsinia this is their sole function. In 
Boschia, however, the sterile cells lose their solid contents before 
the spores have ripened, and then (having acquired annular and 
spiral fibrous thickenings) serve for the conduction of water and 
salts to the developing spores, becoming elongated in order to 
perform this second function more efficiently. Finally, in the 
higher Marchantiales the elaters become greatly elongated and 
assume a third function after the dehiscence of the capsule, namely 
that of assisting hygroscopically in the dispersal of the spores; 
along with this we get the provision of mechanisms for the dehiscence 
of the capsule by means of a lid or of valves, or both, often accom¬ 
panied by the formation of thickening fibres in the cells of the 
capsule-wall (already indicated in Boschia). 
In Riccia the developing sporogenous cells readily absorb food 
