STKUCTUBK. 
regularly when they reach maturity. In a few instances 
they are sessile, and without the ring, opening by regular 
valves. The ferns have in consequence been sometimes 
divided into annulate and exannulate groups. When the 
fructification is borne on the under surface, or what is 
called the back of the frond, it is said to be dorsal, and 
such ferns are said to be dorsiferous ; but when it is pro- 
truded from the edge of the frond, it is said to be mar- 
ginal. The spore-cases in all annulate ferns are col- 
lected together into groups of various outline called sort, 
which sometimes form distinct spots, and in other cases 
take a linear or oblong arrangement. In the marginal- 
fruited group the spore-cases are collected around the 
extremities of the veins which are extended beyond the 
margin. The exannulate ferns have their spore-cases 
collected upon the sides or surface of contracted fronds. 
The spore-cases in certain of the annulate groups spring 
from the surface of the fronds without any covering, and 
in other groups rise as it were from beneath the cuticle, 
which, in this case, is pushed up in the form of an 
investing membrane, called the indusium, or sometimes 
involucre. Hence are formed two classes, the indusiate 
and the non-indusiate ferns. The term involucre seems 
more properly confined to those ferns whose spore cases 
have no covering, but a membrane interposed between 
them and the frond. 
The spore-cases arise directly from the veins, either on 
the under surface or the margin of the fronds. The 
seem to have been for a long time considered as special 
organs, not having any very clear analogy with any- 
thing that occurs in flowering plants. Dr. Lindley, 
nowever, suggests that they may be considered as minute 
leaves, having the same gyrate mode of development as 
the ordinary leaves or fronds of the tribe ; their stalk 
being the petiole, the annulus the midrib, and the case 
itself the lamina with the edges united. This view ap- 
pears to have originated in a persuasion that there was 
no special organ in ferns to perform a function which in 
