XXVIl] 



HABIT 



!37 



The venation is open, without fusions. The only exception to this is TricJio- 

 iiuxnes {Feed) elegans Rudge, a Fern of Tropical America, which differs not 

 only in the irregular anastomosis of the veins, but also in distinct sterile and 

 fertile leaves: the former being pinnate and reticulate, but the latter entire, 

 and with open venation. It may be held as an advanced type. It is an 

 upright radial plant. The construction of the leaves is usually referable to 

 dichopodial branching (see Vol. I, Chap. v). But the dichotomy may some- 

 times be remarkably equal, giving the form of a more or less regular webbed 

 fan (Fig. 504, yJ, ^). The similarity to the venation seen in Scliizaea is marked, 

 and the whole type of leaf-architecture suggests relatively primitive relations. 

 In texture the leaves are "filmy": their translucent expanse is usually only 

 one layer of cells thick: but in T. renifonne and H. dilataUim there may be 

 three or four layers of cells, though without intercellular spaces or stomata. 



Fig. 506. Triilioinanes Gocbcliajiuin Giesenhagen. To Fig. 507. Tricboniancs radicans, longitudinal 

 the light is part of a plant of noinial size, to the left section of the apex (a/), with axillary bud 

 the same magnified. The plant is rootless, rhizoids (rt.r), and subtending leaf (/). ( x 20.) 

 serving for its attachment, which spring partly from 

 the stem, and leaf-veins, partly even from the leaf- 

 surface. Venezuela, 1890. (After von Goebel.) 



This filmy character is probably a secondarily acquired adaptation to the 

 moist conditions under which these Ferns live (Vol. I, pp. 41, 115). It is 

 accompanied by a simple construction of other parts: in particular, in certain 

 leaves pseudo-veins are present, which can hardly be anything else^than the 

 vestigial remains of true veins no longer functional (Prantl, Hyinenophyllaceae, 

 p. 24). Cognate with this is the fact that the root-systerri may be reduced, or 

 even absent in some species. Leafless branches of the rhizome covered with 

 hairs resembling those normally found on the axis and leaf of rooted species, 

 act as substitutes for true roots (Fig. 506). It may be expected that the 

 vascular system of root and leaf will be relatively simple in these Ferns as 

 compared with others, and examination shows that this surnn'se is correct. 

 Axillary branches occur very generally in the Hymenophyllaceae, but at 

 many nodes the bud remains dormant. Its position and the relation of its 

 vascular supply to that of the subtending leaf are shown in Fig. 507. The 



