56 STRUCTURAL BOTANY 



by a massive stalk (see Fig. 28, i). The sporangia spring 

 from the sides of the placenta, and are all roofed in 

 by the indusium. Each sporangium consists of a long 

 slender stalk, made up of two or three rows of cells, 

 bearing the terminal spore-case or capsule (see Fig. 28, 

 B and E). A club-shaped hair, secreting resin, is usually 

 borne on the stalk. The capsule is not spherical but 

 much flattened, resembling the case of a watch in form ; 

 its wall when mature consists of a single layer of cells ; 

 its interior is occupied by the spores, forty-eight to sixty- 

 four in number, which are of a brown colour when ripe. 



The sides of the capsule are formed of cells with thin 

 membranes, but around its edge runs a single row of 

 larger cells with peculiarly thickened walls of a rich 

 brown colour, forming a very conspicuous feature under 

 the microscope, when the sporangium is ripe. This 

 special row of cells is called the ring or annulus (see 

 Fig. 28, J5, 0, E). The annulus starts from the stalk at 

 one side, passes over the crest of the capsule, and extends 

 about half-way down on the other side. Here it 

 suddenly comes to an end. The cells of the wall 

 immediately below the termination of the annulus are 

 broad and flat; this is the place where the capsule 

 ultimately opens (see Fig. 28, B and (7, st). In the 

 annulus both the inner and the radial cell-walls are 

 much thickened ; the free outer walls of the cells, however, 

 remain thin. The function of the annulus is to cause 

 the dehiscence of .the sporangium when ripe. 



Each sporangium arises from a single superficial cell of 

 the placenta ; in this respect it differs from the sporangia 

 hitherto considered, namely, the pollen-sacs and ovules 

 of flowering plants, and the two kinds of sporangia in 

 Selaginella. The great majority of Ferns are distin- 



