298 
i 7 . CaverS. 
which further germ-discs are produced on exposure to stronger 
light, and so on. Again, if the protonema of a leafy Hepatic has 
already given rise to the young leafy shoot, exposure to weak light 
inhibits the development of the adult type of leaf, so that we get a 
slender plant with rudimentary leaves; this state of things occurs 
normally in Zoopsis, a genus allied to Cephalozia. Without going 
further into details, it may be said that all the phenomena of 
arrested development which can be induced artificially, or which 
occur occasionally, in almost any Liverwort, also occur regularly 
in nature in certain species. 
However, we shall discuss later certain questions bearing upon 
the phylogeny of the Bryophyta in general which arise from the 
phenomena of juvenile stages and of naturally occurring as well 
as artificially induced variations in the form of the gametopyte. 
There is much need for further observations on the germination of 
Hepaticae, and for work in experimental morphology in this group. 
From what has been set forth here, it would appear that the 
resemblances between the Aneuraceae and Lejeuneaceae in the 
form of the protonema cannot be regarded as indicating any real 
link between the two groups. We are therefore driven to the 
conclusion that the Acrogynae form a monophyletic series. 
A careful survey of the Acrogynae suggests that the following 
may be regarded as relatively primitive gametophytic characters:— 
leaves symmetrically bilobed more or less deeply, and either 
flat or only slightly folded ; underleaves present; archegonia 
borne on the main axis; perianth free from the involucre and 
either cylindrical or corresponding to three flat leaves carried up 
by common basal growth ; protonema filamentous. On the other 
hand, we may regard as advanced or specialised characters the 
complete absence of lobing from the leaf, or on the contrary its 
division into several deep lobes, or its division into two lobes of 
very unequal size ; the absence of underleaves; the restriction of 
the archegonia to special short branches; the fusion of the 
perianth with the involucre, lateral or dorsiventral flattening of the 
perianth, and the development of wings on its exterior; discoid 
protonema; and, finally, various elaborations such as the formation 
of pitchers, paraphylls, marsupia, etc. Many of these advanced 
and specialised characters have arisen independently in different 
lines of affinity, and are to be classed as parallel developments. 
Assuming that the Acrogynae have arisen from a Fossom - 
bronia -like type, it is probably among the Lophoziaceae or the 
