Lang —On the Morphology of Cyathodium . 423 
of air-chambers, although their original protective function had been lost, is 
readily comprehensible. 
C. foetidissimum has been shown to resemble Targionia in the position 
of the archegonia and the development of the involucre. The fact that 
the archegonia in C. cavernarum occur on what is the actual upper face 
of the thallus can be placed in relation to the greater reduction of the 
basal tissue, which would make the displacement, as in Targionia and C. 
foetidissimum , difficult. The antheridiophores, which appear to correspond 
to what in Targionia are clearly modified branches, are in both species of 
Cyathodium further reduced and specialized. The sexual organs themselves 
-do not appear to be modified. 
l When the reduction exhibited by the spore-producing generation in 
Cyathodium is considered it is questionable whether any adaptive explana^ 
tion can be given. It is not obvious that the change of habitat would 
render the reduction in size of the capsule or foot advantageous, or, to put 
it in another form, that under the new conditions the larger sporogonium 
with its greater spore output would have ceased to be of value. A clearer 
understanding of the facts appears to be reached, if we regard the changes 
in the sporogonium as imposed upon it by the modifications of the thallus, 
upon which it is produced and by which it is nourished. The reduction in 
thickness of the basal tissue renders any considerable accumulation of 
reserve material, at the expense of which the growth of a large sporogonium 
could -take place, impossible, and the conditions of assimilation of an extreme 
shade form make it unlikely that the place of such reserves could be taken 
by the continuous production of the necessary amount of constructive 
substance. Under these conditions the reduction in size of the sporogonium 
can be readily understood, and its degree corresponds to that of the vegeta^ 
tive organs. The reduction is evident not only in the capsule, but in the 
sterile tissue of the foot. A large bulbous foot, such as that found in 
Targionia and all the other Marchantiaceae, would indeed be impossible 
in connexion with the small mass of underlying tissue in Cyathodium. The 
fact that the terminal cells of the cylindrical foot have their surface greatly 
increased by the formation of processes after the stimulus of fertilization 
and of the growth of the embryo have led to the increase in thickness of the 
base of the calyptra indicates the need for more rapid absorption than at 
first was possible. 
On the grounds of the facts of structure, unfortunately without the as- 
sistance of experimental data, I am therefore inclined to regard Cyathodmm 
as a truly reduced form, derived from a form not unlike Targionia by 
adaptation of the gametophyte to life in shady and damp situations and 
by changes in the sporogonium induced by the alterations in the gameto- 
phyte upon which it is borne. 
Apart from questions of genetic relationship, one point of interest may 
