1Z 
a ring. The ring when present is either 
(I) 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 
sort 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 txvolucre (L. tnvolucrum, 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. 
