Anacrogynous Jungermanniales. 213 
characters of the capsule, which is spherical (ovoid in Blasia) and 
when ripe, dehisces by four valves extending from apex to base 
(except Fossombroiiia and Petalophyllnm , where the wall breaks up 
into irregular plates). The capsule-wall consists typically of two 
layers of cells (four in Blasia and Treub'ui), well-developed fibrous 
thickenings being usually present in either the outer or the inner 
cells, or in both. There is usually a basal columella or elaterophore 
but sometimes this is represented only by a few elongated elater- 
like cells at the base of the capsule-cavity. In addition to the 
calyptra, there is usually a tubular envelope around the sporo- 
gonium, in addition to the calyptra, but only in Calyculavia is there 
an involucre in addition to a perianth. Except in Pellia and Calycu- 
laria, there is a definite differentiation of the plant-body into stem 
and leaves. 
The simplest type of vegetative structure is found in Pellia , in 
which the thick median portion of the thallus passes rather 
gradually into the thinner lateral wings. In P. epiphylla many of 
the cells have broad ribbon-like thickenings on the walls; usually 
each cell has only a single band, and in a longitudinal section of the 
Fig. 39. Diagrammatic longitudinal sections through the calyptra and 
involucre of (I) Pellia epiphylla, (II) P. Neesiana, (III) P. calycina. 
thallus the bands are seen to be arranged in lines running from the 
upper to the lower side. These bands are also found in the thallus 
of P. Neesiana, but are absent from P. calycina ; both these species 
differ from P. epiphylla in being dioecious. Though Pellia is so 
largely studied as a type in elementary teaching, the descriptions 
given in text-books are mostly very inaccurate in various points; 
this is partly due to the fact that the three species just mentioned— 
the only species of the genus, and all occurring in Britain—are 
frequently confused with each other. The antheridia are sunk 
separately in cavities, scattered over the upper side of the thick 
median region of thallus. The archegonia are developed in groups, 
