Hymenophyllum tunbridgense. The generic name is derived from two Greek words—hymen, “a membrane,” and phyllon, “a leaf”; tunbridgense has reference to the fact that the species was first of all noticed at Tunbridge Wells. The Tunbridge Filmy Fern.
The fronds, rarely more than two or three inches long, are of a very dark green colour, and rise from a slender, creeping rhizome which produces immense quantities of fine roots. The texture of the fronds is of a delicate nature, and the veins are strongly marked; in colour the leaves are dark green. The outline of the fronds is roughly ovate; that is, it tends to be broadest towards the centre. The pinnæ branch alternately from either side of the rachis, and these are usually very distinctly lobed. If examined with a magnifying glass it will be seen that the edges of the pinnules are bordered with bristling points.
In the case of the Tunbridge Filmy Fern, the sporangia are gathered in a little cup formed by the margin of the leaf; these are present at the end of the veins which branch out from the mid-veins of the pinnæ.
Hymenophyllum tunbridgense. The Tunbridge Filmy Fern.
The Tunbridge Filmy Fern has a very wide distribution in this country. As a rule the plant grows intermingled with moss, and on this account it is often passed by without recognition. Almost everywhere where there are wet and especially water-splashed rocks one may expect to find this species. The Tunbridge Fern can only be cultivated in the same manner as that indicated in the case of Trichomanes. The plant is evergreen.
Hymenophyllum unilaterale. The specific name is, of course, a Latin word meaning “one-sided”; the application of the term is explained in the following description. (In some books this species has been called H. Wilsoni, out of compliment to a Mr. Wilson, who is said to have noticed the species first of all.) The One-Sided Filmy Fern.
Some botanists have considered that the One-Sided Filmy Fern is merely a variety of the former species, though it is generally considered to be a distinct type. The two plants often grow mixed up together and superficially look very much alike; it is only after a close examination that the differences become apparent. In a general way it will be found that in the case of the One-Sided Filmy Fern the fronds are somewhat more narrow than those of the Tunbridge Filmy Fern. The chief point of distinction is indicated in the popular name:—if the pinnæ are closely examined it will be seen that the upper portion is much more divided than the lower side. Another distinctive point to which attention should be drawn is that in this species the pinnæ show a marked tendency to curve backwards.
A study of the fertile leaf of the One-Sided Filmy Fern will also give us an additional point in the identification. With a magnifying glass it is plainly seen that the margin of the cup-shaped receptacle into which the sporangia are gathered has not the toothed border to be found in the case of the Tunbridge Filmy Fern.
As has been indicated, we may look for the One-Sided Filmy Fern in exactly similar situations to those which suit the Tunbridge Filmy Fern. The former species is said to be more common in Scotland and Ireland than the latter. The One-Sided Filmy Fern is, of course, an evergreen.