214 
F. Cavers. 
each group becoming sunk in a pocket-like cavity which opens in 
front, the archegonia being directed horizontally and accompanied 
by short mucilage-hairs. The margin of this pocket shows differences 
characteristic of the three species. In P. epiphylla and P. Neesiana 
it forms a short involucre beyond which the calyptra of the ripe 
sporogonium extends for some distance, whilst in P.calycina it forms 
a long bell-like envelope enclosing the calyptra. In P. epiphylla the 
involucre or pocket-margin grows out only on the posterior side, 
forming a hood-like scale, but in P. Neesiana it grows out all round 
as a short collar-like sheath (Fig. 39). 
In the young capsules of Pellia, after the capsule-wall has 
become differentiated as an outer layer of large cells and (usually) 
two inner layers of small cells, but before one can distinguish 
between the spore-mother-cell and those which will later form the 
free elaters mingled with the spores, the inner tissue (endothecium) 
shows a fairly sharp separation into two portions—a sterile basal 
portion extending from the bottom of the capsule and spreading 
out fanwise as seen in longitudinal section, and a fertile portion 
which extends over the top of the columella-like basal tissue. The 
columella, or elaterophore, consists of elongated cells, forming a 
bundle of fixed elaters. In P. calycina (23, 36) the fixed elaters of 
this bundle (about a hundred in number) are much longer than the 
free elaters and often reach more than half-way up the capsule ; 
like the free elaters which are mingled with the spores, they 
usually have a double spiral band. In P. epiphylla the fixed elaters 
are fewer in number (thirty to fifty), shorter, thicker, and furnished 
with from one to four spiral bands. P. Neesiana resembles P. 
epiphylla in the structure of the capsule; in these two species the 
inner layers of the capsule wall hear ring-like fibres, which are 
lacking in P. calycina. When the capsule opens the four valves 
move outwards and downwards, exposing the mass of spores and 
loose elaters, which are held together for a time by the tuft of fixed 
elaters, the mass then becoming loosened by the hygroscopic 
movements of the elaters and gradually shaken out. 
The basal tuft of fixed elaters in the capsule of Pellia arises 
in the same way as the apical sterile tissue forming the elaterophore 
of Aneura and Metzgeria . The differences in the mature structure 
of the columella and in its behaviour when the capsule dehisces, 
are clearly connected with the difference in position. In Aneura 
the apical columella remains coherent until the capsule dehisces, 
when it splits into four portions, one of which remains attached to 
