4 
INTRODUCTION. 
and either form an angle with the midrib, when they are said to be oblique (Pig. 51), or run 
parallel with the margin (Pig. 38), or with the midrib (Pig. 44). The sori are in some of 
these groups entirely exjiosed on the surface of the frond, and in others are covered by a scale 
or membrane of the same form as the sorus, and which completely invests the receptacle in 
the earlier stages of development. This cover is the indusium proper, and takes some modifi- 
cation of either a plane, vaulted, or cup-shaped fonn. The entire margin or lobules of a frond 
are sometimes changed in texture, and form an accessory indusium, which is more or less con- 
joined with the proper indusium, and thus constitutes a marginal cyst containing the spore- 
cases and opening exteriorly : it may be vertical (Pig. 76) or reflexed (Pig. 79) ; urn-shaped 
(Pig. 76), calyciform (Pig. 83), or two-lipped (Pig. 78). Sometimes the fertile fronds are 
contracted, and inclose the sori by their revolute margins, which thus constitute an universal 
indusium (Pig. 3, 58, &c.). 
The further differences of structure, and the various modifications which occur and serve to 
distinguish the groups from one another, will be explained further on. One of the most 
striking of these differences is the paniculate arrangement of the sori, which occurs among the 
Osmundaceae, and some of the Schizgeaceie and Ophioglossaceae. The fructification of the Marat- 
tiacese is also of a very peculiar nature. 
As we have in most cases adopted Mr. Smith’s views of genera and species, we have also, in 
these succinct explanations of terms and of structure, endeavoured to assimilate our remarks 
with the definitions which accompany his paper, in Hooker’’ s Journal of Botany, in order to secure 
the advantages of uniformity in terminology. — M. 
