172 



ON THE DEVELOPMENT OF THE THECA. 



" The distance of the columella from the theea 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, as far as I have examined, as in Bartramia pomi- 

 formis. 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 columella 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 living mem- 

 brane 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 membrane, 

 which passes outward from the base of the columella to the theca, on the inner surface 

 of which it is reflected upwards to the stoma. The membrane is attached to the 

 stoma, all round frequently by a distinct process ; and after forming this attachment, 

 it passes horizontally inwards, and becomes again continuous with the columella at 

 its apex. Until about the period of maturity, or a little earlier, the columella is 

 continuous from the base of the theca, up to the arch of the operculum, when a 

 transverse line (indicating a tendency to separation) appears above the point of its 

 connexion with the columellar membrane. Most commonly this separation does 

 actually take place, and the upper portion falls with the operculum. This portion 

 was first described, and named very appropriately, by Greville and Arnott, the 

 opercular membrane. The opercular membrane, when mature, either remains 

 attached to the columella, falls with the operculum, or (as in the genus Poly- 

 trichum) shrivels from below upwards, and remains attached to the apices of the 

 teeth of the peristome, in the form of a horizontal membrane or tympanum." 



" In an early stage the inner layer of the operculum separates in the form of a 

 distinct membrane, which ultimately dividing longitudinally into a definite number 

 of processes, or teeth, forms the peristome. In some rare instances this membrane 

 never breaks up into teeth, as in the genus Diphyscium ; whilst in one instance, 

 Buxbaumia, it is double ; the external splitting into ciliae, and the internal 

 remaining entire. At the same time that this membrane is formed from the 

 opercular, the opercular membrane forms another, immediately within the first, 

 by a separation of its exterior series of cells. This also, more or less, divides 

 longitudinally into a determinate number of teeth, thus forming the inner peri- 

 stome. The number of teeth forming each of these peristomes has been ascertained 

 by mycologists to be either four or a multiple of that number 



( To be concluded in the next number.) 



* This fact appears to have been first ascertained by Dr. Brown, who has published some admirable obser- 

 vations on this subject in the Linnean Transactions, vol. xii., p. 557. 



