114 
NEW ARKANGEMENT OF 
ever, be employed with effect, in the framing of specific 
characters. 
Within the theca, and attached to its sides and base by 
a cellular pulp intermingled with very slender filaments, is 
placed a membrane, which forms a complete lining : pro- 
ceeding from the stoma, or mouth, downwards, it is con- 
tinued to the base, when it is reflected upwards, forming a 
central column, till on a level with the mouth of the theca. 
This part has been generally denominated a Columella, 
and is very different from the placenta * in cotyledonous 
plants, to which it has been sometimes compared f. A bag 
is thus formed between the columella and that part of the 
same organ which lines the theca, in which the sporulae are 
situated. The whole is closed by a very subtile membran- 
ous expansion, stretching between the summit of the lining 
and the columella, which (although in Gymnostomum it is 
often of a stronger texture), by the maturing of the theca, 
and separation of the operculum, lacerates, and, in most 
instances, becomes evanescent J. On the open apex of the 
columella is fixed another membrane, more or less of a 
* Brown in Linn. Trans, vol, x. p. 315. 
-j* Those who, in consequence of a fancied analogy, wish to apply to the 
Mosses the terms used among the Embryonatae, or phaenogamous plants, 
might, in the language of carpologists, designate the three principal parts of 
the theca by Epicarp, Sarcocarp, and Endocarp. The sarcocarp almost al- 
ways dries up, except where it forms the interior of an apophysis, and is 
then more permanent. It is the endocarp which is turned upwards into the 
columella ; and this affords an additional argument against the columella 
being regarded in the light of a placenta or tropkosperm. 
$ In Dr Hooker's excellent figure of Gymnostomum Xanthocarpum in 
the Musci Exotici, the membrane which closes the sporular bag is admir- 
ably delineated, Tab. 153. fig, 9. It ought to be observed, that Palisot 
has described this membrance in the Mosses without a peristome, but denied 
it to the others. 
