Filmy Ferns

Filmy Ferns (Hymenophyllaceæ), an order of Ferns (q.v.), including only three genera, but as many as two hundred species. They grow in large, spreading, mossy masses on rocks and tree-stems in moist places, are chiefly tropical, rare in the southern hemisphere, still rarer in the northern. The only British species of the large genus Hymenophyllum (H. tunbridgense), despite its apparently local name, is one of the most widely distributed of plants, being found not only over Europe, but in South America, at the Cape, and in New Zealand. Trichomanes has ninety species, mostly tropical, only one ranging as far as southern Europe. Although chiefly distinguished by systematists on account of the minute structure of their sporangia, they are more remarkable for the exceptionally simple structure of their moss-like leaves, which consist usually of only a single layer of uniform parenchymatous cells. More rarely several layers are present, but there is never a distinct epidermis; moreover, as the plants grow in very wet places—often, for instance, in the spray of waterfalls—and as the leaves are so hygroscopic as to be constantly saturated, even when not absolutely covered with moisture, the usual air-spaces, and with them the stomata of ordinary leaves, are not developed. From this saturation with water instead of air the thin, wet, filmy foliage is thus peculiarly transparent, and so seems singularly dark-green by reflected light. To the exceptional yet beautiful appearance thus accounted for their interest to fern-growers is due.