THE STRUCTURE OF FERNS. 
19 
tongue, in both of which the fronds in the undeveloped 
state are folded straight. 
The substance of the fronds is traversed by veins va- 
riously arranged ; in some species forming straight, nearly 
parallel lines, in others joined together like network. The 
manner in which the veins are disposed is called the mna- 
tion^ and the nature of this venation affords useful data in 
the division of the ferns into family groups. It is from 
some determinate part of these veins that the clusters of 
fructification proceed, that part to which they are attached 
being called the receptacle. 
A correct appreciation of the condition and position of 
the receptacle with reference to the veins, is of considerable 
importance in the study of the genera and species — that is 
to say, the family groups and the individual kinds. In 
some, though few, of the native kinds, it is projected be- 
yond the margin, and the little cases of seeds are collected 
around its free extremity. More commonly, however, the 
veins stop within the margins, and the seed-cases grow in 
round or elongated clusters, situate in some cases at their 
ends, in others along their sides, and protruded through 
the skin of the lower surface of the fronds. 
No flowers are produced, but the plants bear, generally, 
c 2 
