I 94 
THE FERN WORLD 
eighth of an inch from each other, are the lines of fructifica- 
tion. Each line — running in the direction of the veins and 
midway between the mid-rib and the edge of the frond — 
consists of twin sori, covered when young with a thin whitish 
cuticle which is the indusium. As the spore cases ripen, the 
indnsinm splits in the centre and is thrown hack. The sori 
set face to face, then become confluent, and being brown in 
colour look at a distance like brown caterpillars. The fruc- 
tification is usually confined to the upper two-thirds of the 
frond. The width of the latter varies from an inch, or even 
less, to two or three inches, according to the length and 
development of the plant. The stipes is sometimes of a dark 
purplish colour, and the same colour is frequently continued 
along the rachis. Both stipes and rachis are furnished — the 
former often abundantly — with rust-coloured scales. If these 
are examined with a powerful magnifying glass, it will he 
seen that their substance is beautifully reticulated. The 
rootlets of the Hartstongue are long, fibrous, and abundant. 
It is of course the normal form of the plant which has been 
described. This form, however, is subject to the most 
extraordinary variation. Nearly five hundred distinct 
varieties — which assume almost every conceivable shape — 
have actually been named and described in the British Islands 
alone ; and no doubt there are many undiscovered variations 
which would still further swell the number. When once 
the eye is familiarized with the form and habit of the Harts- 
tongue, however, there will be no difficulty in recognizing 
the varieties. Merely to enumerate and describe them would 
fill a small volume. The species vulgare is the only one 
which we possess of the genus Scolopendrium, which includes 
Ferns having on the hacks of their fronds elongated clusters 
of sporangia, running parallel with and close to each other, 
and covered by indusia, which when the spores are ripened 
split down the centre longitudinally and between the twin sori. 
