12 PKACTICAL BOTANY. 



6. The air-space, traversed obliquely by filaments 

 of thin-walled cells, suspending a central mass con- 

 sisting of 



7. The spore-sac, which will be seen to be composed 

 of two or three layers of thin-walled cells. 



8. A layer of spore-mother cells, recognised by 

 their dense protoplasmic contents. 



9. The central mass of thin-walled tissue of the 

 columella, which extends upwards into the concave 

 operculum. 



By careful observation of longitudinal sections under 

 a high power the following points may be verified : 



1. That stomata occur in the epidermis towards 

 the base of the sporogonium. 



This may be confirmed by cutting tangential sections from the 

 base of the sporogonium, in which the stomata, with their two 

 guard-cells, may be clearly seen in surface view. 



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 



