o74 REPRODUCTIVE ORGANS OF MUSCI. 
of the sporangium is filled with free spores, which are small 
cells with two coats and markings resembling those of pollen- 
cells. The sporangium is either indehiscent ; or it opens by 
four vertical slits so as to form four valves, as in the sub-order 
Andreeez ; or more commonly by a transverse slit close to its 
apex, like fruits with circumscissile dehiscence, by which a 
kind of lid is produced, called the operculum (jigs. 826, 0, and 
827) ; which is either persistent or deciduous. The sporangium 
is sometimes much dilated at the base, where it joins the seta ; 
this swelling is called an apophysis, or, if it only occurs on one 
side, a struma. 
The wall of the sporangium is commonly described as con- 
sisting of three cellular layers, the outer of which forms the 
operculum, just described, and the inner two layers the peristome. 
iRiver Fig. 828. : Fig. 829 
Fig. 827. Pottia truncata, showing the separation of the operculum from 
the sporangium. From Henfrey. Fig. 828. Sporangium, wu, of the 
Hair-moss, deprived of its calyptra and operculum. p. Peristome. e. Epi- 
phragma or tympanum.—/ig. 829. Transverse section of asporangium 
of the Hair-moss, showing the columella surrounded by free spores. 
At the dehiscence of the sporangium the stoma or mouth is 
entire, smooth, or unfurnished with any processes (fig. 827) ; or 
it is surrounded by one or two fringes of teeth, called collectively 
the peristome (fig. 826, »), which, as just stated, are formed from 
the two inner layers of the wall of the sporangium. These teeth 
are always four or some multiple of that number. Sometimes a 
membrane from the inner wall is stretched across the mouth of 
the sporangium, and forms what has been called the epiphragma 
or tympanum (fig. 828, e). When the mouth is naked, the 
Mosses in which such a sporangium is found are called gymno- 
stomous or naked-mouthed ; when the mouth is surrounded by a 
single row of teeth, they are said to be aploperistomous ; or, 
when with two rows, they are diploperistomous. The different 
appearances presented by the teeth, as well as their number 
and degree of cohesion, form important distinctive characters 
