STRUCTURE 
23 
The manner in which the spores are collected on the backs 
of the frond is extremely curious and beautiful. They are 
contained in little cases which are known by the name of 
sporangia — singular, sporangium, which means £ a spore 
vessel.’ The sporangium is of a thin or horny texture — 
sometimes semi-transparent, and sometimes opaque — usually 
consisting of a single cell, and generally either globe-shaped, 
pear-shaped, or oval. Sometimes it is furnished with a very 
short stalk. There are two kinds of sporangia in Ferns, the 
one kind — including the great majority of known Ferns — 
being surrounded by a jointed elastic ring which passes round 
the sporangium, either in a horizontal, a vertical, or an 
oblique direction, whilst the other kind is altogether destitute 
of a ring. The collection or cluster of spore cases is often 
arranged under the protection of a scale-like covering called 
the indusium. Where this organ is present, it continues to 
cover the clusters of spore cases until the development of the 
latter bursts the membranous scale. Each separate cluster 
of spore cases whether covered or not, is called a soru s, from 
a Greek word which means £ a heap — plural sori. Some 
Ferns, however, have no indusia over their sporangia, the 
latter being naked or non-indusiate. When the sori are wliat 
is termed marginal, that is to say, when they grow along the 
extreme outer edges of the backs of the pinnules, it often 
happens that the leafy margin of the pinnule is turned back 
to cover them and to serve as an indusium. 
In the case of those Ferns whose sori are covered by 
indusia we have seen that the growth of the spore cases 
acting upon the indusia bursts the margins of the latter, 
which are then either wholly or partially thrown off. Subse- 
quently the bands or jointed rings which encircle the spore 
cases either vertically, horizontally, or obliquely, are burst by 
the elasticity of the rings, and the fine dust-like spores are 
dispersed. Sometimes the indusium takes the form of a cup 
