16 THE BRITISH FERNS. 
the rest. The correct appreciation of these terms is necessary to 
the comprehension of descriptions of the plants. The peculiarities 
of the division of the frond indicated by these terms, are much 
employed in distinguishing the species of Ferns, and are moderately 
constant and reliable. 
The parts of pinnated or bipinnated fronds are, in some species, 
joined to their respective rachides by a natural joint, as the stipes 
is, in some species to the stem, the firmer tissues not being con- 
tinuous at that point, so that the parts, whether pinn® or pinnules, 
readily detach themselves after maturity is past. When so jointed 
the parts are said to be articulated with the rachis. 
In the majority of Ferns, the mature fronds are alike fertile, and 
are similar in their appearance ; but in certain species, the habit is 
to produce some of the fronds wholly barren, and others wholly 
fertile; in these instances the fertile fronds are more or less con- 
tracted. This habit is exemplified, among our native species, in 
the Allosorus crispus, and Blechnum Spicant. 
The fronds of Ferns are variable in their duration like the leaves 
of other plants. In some species they are persistent, so that either 
absolutely, or with very slight shelter, the plants become evergreen. 
The species possessing this habit are the most valuable for the 
cultivator. In other species the fronds are fragile, and of short 
duration, produced only during the warmer portion of the year, 
and shrinking before the first breath of winter. Among these 
latter, however, are comprised some of the most delicately beautiful 
species. 
Fern fronds are traversed by ribs or fibres, which serve to give 
them their elasticity. These ribs are what are called the Veins. 
The venation thus forms a framework, on which the herbaceous 
portions are, as it were, spread out to the influences of light and air ; 
and, consisting of bundles of woody fibre traversed by the nourishing 
vessels, it constitutes the vascular system of the plant. The stipes 

