II] THE XEROPHYTIC HABIT 41 



often called "the Filmy Ferns." But there are also filmy species o{ Asplenium 

 and of Todea, and even of so leathery a genus as Danaea. Specimens of 

 D. trichomanoides were, discovered by Spruce, growing in a wet habitat near 

 Tarapoto in Peru (1856). This species, and D. crispa, and some other species 

 in less degree, have pellucid leaves almost equalling the Hymenophyllaceae 

 in their texture. The filmy character is in fact an adaptive hygrophytic 

 condition, which has been assumed homoplastically in several quite distinct 

 groups of shade-loving Ferns. It is secondary and derivative, and cannot 

 rightly be accepted as any indication of a primitive and Moss-like nature, 

 though this was once the accepted belief. A similar texture is approached 

 in the leaves of some shade-loving species of Selaginella. 



Notwithstanding the hygrophytic character of the Class as a whole, 

 and their general preference for a shaded habit, as well as the fact that they 

 may, like the Filmy Ferns, show sometimes extra adaptation to these con- 

 ditions, many Ferns are able to endure exposure to strong insolation, and 

 even to drought provided it be temporary. Others grow epiphytically, a 

 habit which is congenial enough under shade in tropical rain-forests: but 

 elsewhere the epiphyte, having no access to the soil, must needs have some 

 means of economising its water-supply. The requirements are in either case 

 met by such modifications of structure as are characteristic of other xero- 

 phytes, and are specially seen in Flowering Plants. There is a small Flora 

 of British Ferns of rocks, and even of dry wall-tops, including such species 

 as Asplenmni riUa-niuraria, and A. Ceterach: or of tree-trunks and rocks, 

 such as Polypodimn vidgare. These may sometimes be found with their firm 

 leathery leaves dried to crispness in summer: and yet they tolerate those 

 stations. In A. CeteracJi the dense covering of scales is an efficient protection 

 in addition to the texture of the leaf (Fig. 47). Abroad there are certain 

 genera, such as Notholaena, Cheilanthes, and Pellaea, which are still more 

 typically xerophytic, having stiff, attenuated, and often highly divided leaves 

 of small area, with hard polished leaf-stalks, and a waxy glandular covering 

 to their surface. Others, such as NipJiobohis and Elaphoglossum, bear a 

 covering of protective scales over the whole shoot: sometimes the protective 

 armour is specially developed on the rhizome, as is seen in Phlebodium 

 auremn (Fig. 48). Sometimes the scales are borne chiefly on the lower 

 surface of the blade, the petiole, and the rhizome, while the upper surface 

 is almost clear, as in Polypodimn {Lepicystis) incamiin. This rock-growing 

 Fern is a most successful xerophyte. It may be seen shrivelled for weeks 

 without rain under a tropical sun, but when moistened it swells again, and 

 continues growing as Mosses do. In other cases isolated species even of 

 peculiarly hygrophytic genera may be specially protected, as in the case 

 of HynienophyUuni sericeuni, whose long pendent leaves are densely covered 

 with a felt of ferruginous hairs. Others may show a xerophytic folding of 



