and on the Sexes of Mosses. 469 
according to circumstances, the seta gradually enlarges in diameter at the 
apex, and imperceptibly assumes the form of the theca. A section of the 
dilated apex, if made at an early period, will exhibit a central portion and a 
cortical layer, only differing from the structure of the seta itself by being more 
distinctly defined. As the theca advances towards maturity, the cortical layer 
gradually recedes from the central axis, but is still connected with it by little 
transverse fibres, or rather strings of cells, which pass from one surface to the 
other without apparent arrangement. The axis, or columella as it is now 
termed, is supported on a pedicel which is continuous with the central tissue 
of the seta; whilst the outer layer, or true theca, is an expansion of the external 
layer of the seta. Surrounding the theca, near the apex, is a faint line, which 
indicates the situation of a transverse dehiscence to take place at the perfect 
maturity of the theca. The portion above this line varies considerably in 
figure, and is called the lid, or operculum. The ring or orifice of the theca, 
formed by the fall of the operculum, is called the mouth or stoma. It is 
necessary to name these parts in this stage of the development, to explain 
clearly the succeeding steps of the process. 
The distance of the columella from the theca varies in each species; in 
many being but trifling, whilst in some it is considerable, as in Gymnostomum 
pyriforme. But in none is it so remarkable, so far as I have examined, as in 
Bartramia pomiformis. In this plant the columella is borne on a pedicel even 
longer than itself, and only occupies a small space in the upper and middle 
part of the theca. A section of the columella, in this stage, exhibits a trace 
of division into an external layer and a central axis. This external layer is 
gradually pushed outwards (until it comes in contact with the theca) by the 
formation of the sporules, between it and the axis to which the name colu- 
mella is with greater strictness applied. The layer itself has received the 
name of internal or lining membrane of the theca; but as I have ascertained 
the presence of a distinct and very important lining membrane to that part, 
it will be more convenient to assign the name of columellar membrane to 
this, as to tbe columella it assuredly most naturally belongs. The cavity in 
which the sporules are developed is closed on all sides, being bounded at the 
centre by the columella, and at the circumference by the columellar mem- 
brane, which passes outward from the base of the columella to the theca, on 
