STRUCTURE. 
11 
to resemble in form a lance-head. Others have trian- 
gular fronds generally drawn out to a very acute angle 
at the summit. Others, again, have oval fronds, or they 
are tongue-shaped, like the Scolopendrium. In foreign 
forms some are even round or crescent-shaped. The 
venation is a very important characteristic in ferns : in 
some species the veins are simply branched, in some the 
branches cross one another so as to form a network ; 
they are composed of elongated cellular tissue, and of 
them the receptacles are formed. The fronds of ferns 
vary in size from one or two inches to five or six feet 
in length, and from one inch to two feet in breadth. 
The fronds are occasionally dotted over with glands, 
often sweet-scented, or thev are covered with hairs both 
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on the rachis and the leafy part ; in some foreign species 
a coloured powder is also present, freely scattered on 
the under part of the frond. 
The fructification of ferns includes the receptacle, 
spore-cases, and spores. The receptacle is formed of 
the substance of the veins. The spore-cases Dr. Lind- 
ley considers as “ minute leaves having the same gyrate 
mode of development as the ordinary fronds of the 
tribe, their stalk being the petiole, their annulus the 
midrib, and themselves the laminae, the edges of which 
are united.” The foot-stalk of the spore-cases is ex- 
tended round them in the form of a ring, the elasticity 
of which causes them to burst when ripe. Some species 
have the spore-cases sessile and without the ring, and 
in such cases are formed of regular valves. The pre- 
sence or absence of the ring causes the fern-family to be 
divided into two groups, annulate and exannulate. Of 
the annulate ferns a great number have the fructifica- 
tion on the back of the frond, in which cases it is called 
