SPOROGONIA 353 



2. That immediately above the upper limit of the 

 air-space there is a band of brick-shaped cells, elongated 

 transversely, and with pitted walls : these are not derived 

 from the epidermis but from the subjacent layers, and 

 serve to connect the peristome with the outer wall of 

 the theca. Closely above this band the tissues are 

 again more delicate, and it is here that the rupture of 

 the capsule takes place, by which the operculum is 

 set free. Above this more delicate zone is 



3. The slightly projecting lower lip of the operculum, 

 at which point the epidermal cells are thickened, and 

 of peculiar form, constituting the annulus. Beneath 

 the operculum, which consists of the superficial layer 

 of epidermis together with two or three layers of 

 thin-walled tissue, lies 



4. A layer of cells with peculiar yellow or brown 

 thickening of the walls : this gives rise to the 

 peristome : further details will be given below. 



Returning to the spore-forming layer, it may be 

 verified under a high power 



i. That it consists at first of a single layer of cells 

 (the archesporium) : to ascertain this for certain it 

 may be necessary to cut sections from sporogonia of 

 various ages, and to compare them. 



ii. That the cells of this layer divide repeatedly ; and 

 ultimately, by division of each of the resulting spore- 

 mother cells into four, the spores are produced. 



Mount some mature spores in water: they will be 

 seen to be of spherical form, with smooth walls, and 

 granular, oily contents. 



XIV. In order better to understand the structure and 

 origin of the peristome, cut off transversely the whole 



A A 



