A crogynous Jlingermanniales. 289 
capsule-wall has well-developed fibrous thickenings in its cells, 
which are usually arranged in two layers. An apical cap or lens¬ 
like thickening of the capsule-wall, such as occurs in many Mar- 
chantiales and Anacrogynae, is apparently never found in Acrogynae, 
but the number of layers composing the capsule-wall is often more 
than two, and may be as many as eight. 
The capsule-wall and the seta are remarkably constant in 
structure in the various genera and groups of genera, thus affording 
characters of considerable value in classification, and in some cases 
affording a useful clue to the probable inter-relationships of the 
orders. For instance the genera Porella and Radula, forming the 
two orders Radulaceae and Porellaceas, approach the Lejeuneaceae 
even more closely in the structure of the capsule-wall than in the 
vegetative characters which led the older systematists ( e.g ., in the 
Synopsis Hepaticarum) to unite these two genera into the tribe 
“ Platyphylleae” and place this next to the “ Jubuleae” (Lejeuneaceae). 
Again, the genus Pleurozia (Physiotium) was placed with genera 
like Lepidozia and Ccilypogeia in the “ Trichomanoideae ” of the 
Synopsis, while in Engler and Prantl it is given ordinal rank and 
brought close to the Lejeuneaceae. Pleurozia differs from Radula , 
Porella, and the Lejeuneaceae in that the capsule-wall consists of 
about eight instead of two layers of cells, but in other genera with 
a many-layered capsule-wall all the layers except the outermost one 
consist of flattened cells, while in Pleurozia the innermost layer has 
precisely the same type of thickening as the inner of the two layers 
in the Lejeuneaceae. That is to say, the capsule-wall of Pleurozia 
has evidently been derived from the Lejeunea and Frullania type by 
the interpolation of several layers of narrow flat cells between the 
outer and the inner layer of more or less cubical cells, and the 
structure of the capsule supports the evidence drawn from other 
characters as to the probable relationship between Pleurozia and 
the Lejeuneaceae. 
In certain genera of Acrogynae, the gametophyte shows a 
remarkable degree of dimorphism, in that the germinating spore 
produces a vegetative thalloid plant-body on which arise leafy 
shoots bearing the sexual organs. In two of these genera ( Metz - 
geriopsis and Pteropsiella) the vegetative body is a flattened thallus 
resembling that of an Aneura or a Metzgeria, while in Proto.cephalozia 
it is a branching filamentous structure closely resembling the 
protonema of a typical Moss. In each case, the sexual organs are 
borne on leafy branches which have the typical Acrogynous 
