174 
F. Cavers. 
the elaboration of the pores, which may remain surrounded by a 
single ring of cells with thickened radial walls, or may have several 
concentric rings of cells, or superposed tiers of cells forming a 
barrel-like structure. The elaboration of the thallus has clearly 
taken place independently in several distinct lines, and is only in a 
very general way correlated with the advance made by the members 
of each line in other respects. 
Much the same may be said of the sporogonium. The early 
stages in differentiation are, to so speak, hurried through in the 
lower Marchantiales, and then the sporogonium makes practically 
no further advance. From the sterilisation of a single peripheral 
layer ( Riccia ) we pass to that of certain archesporial cells—serving 
at first for nutrition ( Corsinia) and then acquiring the additional 
function of assisting in spore-dispersal by becoming thickened 
locally (Boschia shows the beginnings of this)—and to the differen¬ 
tiation of foot, seta, and capsule. The capsule-wall remains one 
cell thick, except for the apical cap found in Targioniaceae and 
Marchantiaceas, and (except in Ricciaceae, Corsinia, the Operculatae, 
and Lunularia) acquires fibrous thickenings. Except in a very 
general way, the sporogonium affords very little guidance to the 
phytogeny of the Marchantiales. Taken along with other characters, 
however, it helps in indicating the probable relationships of the 
groups. 
The sexual receptacles, on the other hand, show a wide range 
of organisation, starting from Riccia, where the individual sexual 
organs are scattered over the median portion of the thallus, each 
antheridium and archegonium sunk in its own cavity, and leading to 
the complex stalked gametophores of Marchantia. The evolution of 
the male receptacle need not be traced in detail, but it may be 
noted that once the antheridia have been aggregated into receptacles 
and the latter have by a further step been sharply defined from the 
ordinary thallus tissue, the further advance of the male receptacle 
has, so to speak, lagged behind that of the female receptacle. Only 
in the highest members of the phylum does it become a definite 
branch-system with several growing-points, raised above the thallus 
on a grooved stalk. In the forms intermediate between these highly 
organised types and the Ricciaceae, the male receptacle is at the 
most a mere ridge or cushion borne dorsally on the thallus or 
situated terminally on a branch whose growing-point is used up in 
the development of the receptacle. 
It is not difficult, on biological grounds, to understand why the 
