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 month 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 Gi/mnosfoniinn 

 pi/riforme. But in none is it so remarkable, so far as I have examined, as in 

 Bartramla pomiformis. In this plant the columella is borne on a pedicel even 

 longer tiian 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 the 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 



