12 



a ring. The ring when present is either 



(1) vertical if it passes vertically from the 

 stalk over the apex of the sporangium, or 



(2) horizontal when it passes horizontally 

 round the sporangium at about its middle, or 



(3) apical when it is at the apex, or (4) oblique 

 when it passes obliquely round the sporangium. 

 When the sporangium reaches maturity, the elas- 

 ticity of the annulus gives way, the case opens in 

 a direction transverse to the ring and the spores 

 are dispersed in the shape of fine dust. Exan- 

 nulate sporangia burst by means of a simple 

 slit or pore. Groups of sporangia are termed 

 sori when the individual sporangia are distinct, 

 and synangia when they are coherent. The 

 sporangia in the latter case are exannulate. 

 Sori vary in shape and may be linear, round, 

 oblong, oval, etc. 



3. Indusium. Sori are either naked or pro- 

 vided with hairs and scales or covered in the first 

 instance at least with a membranous or rather 

 leathery covering called an indusium (a 

 shirt) or involucre (L. involucntm, cover). The 

 presence or absence of the indusium and its form 

 when present are of importance in classification. 

 The indusium is attached to the receptacle 

 in various ways. 



