22 
BRITISH FERNS . 
forming sometimes good characters for the distinc¬ 
tion of species. These veins are never netted , as they 
are in the majority of flowering plants, but they are 
often forked, or dichotomous. It is upon the veins 
of the under part of the frond that the organs of 
fructification are placed; and this part is called the 
receptacle. 
The organs of fructification consist of a number of 
little capsules, called seed-cases, spore-cases, sporangia, 
or thecae. These are collected together in little 
clusters, which are called sori. The sori are placed 
upon the receptacle. When the sori are placed on 
the under part of the frond, they are called dorsal; 
but when on the edge of the frond, marginal . They 
are never placed on the upper surface of the frond 
alone. Sometimes the whole frond is converted into 
a receptacle, as in the case of the Flowering Fern, the 
Adder’s-tongue, and some others. 
The spore-cases are mostly small, slightly ovate 
bodies, with a single cavity. They are surrounded 
with a ring, one end of which is fixed to the re¬ 
ceptacle. When the spores contained in the inside 
have reached maturity, the elasticity of the ring 
causes the spore-case to burst transversely, and the 
spores are scattered about in the form of fine dust. 
In Osmunda , Botrychmm , and Ophioglossum , the spore- 
cases have not the elastic ring, but are composed of 
two valves* 
