XXVII] ANATOMY 239 



The Hymenophyllaceae are all Ferns of relatively small size: some of 

 them are very minute, for instance the rootless T. Goebelimium, with leaves 

 only 3-4 mm. in length (Fig. 506). They are mostly inhabitants of wet 

 forest, often occupying decaying stumps or rocks on the forest-floor, or 

 spreading epiphyticalh^ up the stems and branches of trees, according to 

 the hygrophytic conditions, to which they are very susceptible (see Forrest 

 Shreve, Bot. Gaz. Vol. li, p. 184, 1911). 



Anatomy 

 The most obvious structural peculiarity of the Hymenophyllaceae is the 

 "filmy" texture of their leaf-expanses. In accordance with this, and with 

 the moist conditions under which they live, the vascular system, though not 

 generally rudimentary, is nevertheless relatively simple. The stem is 

 uniformly protostelic, and an undivided leaf-trace of simple outline is given 

 off, naturally without any foliar gap, while the vascular supply to the axillary 

 bud, if present, is united basally with the leaf-supply (Fig. 507). The stele 

 varies considerably in structure in different species: in Hymenophylhun there 

 is less variation than in Trichoinanes. It is delimited by endodermis, which 

 is lined by pericycle that extends some- 

 times to several layers, and surrounds a 

 continuous phloem : the centre is occupied 

 by a xylem-core, which is more or less 

 parenchymatous towards the centre (Fig. 

 509). The protoxylems are sometimes 

 peripheral, but usually mesarch, and are 

 decurrent from the leaf-trace. There is 

 rather wide variation according to size 

 and habit. Fig. 509 is taken from T. 

 scandens, a rather large species with wir}' 

 stem. In cQvtSimsTpeciQsoi Hy7iie7iophj'/hi ui 

 with large rhizomes, such as H. scabruiii ^. „, • r , , r 



^ . ^ig. 50Q. 1 ransverse section of the stele of 



and dilatatuni, the protOXylem lies centrally Trichoma nes scandens. /.v = protoxyleni : 



embedded in parenchyma, the metaxylem ^ = ^"^l°^«"^^is- (After Boodle.) 

 forming a ring round it (Fig. 5 10). The ring is often separated by parenchyma 

 into two bands, their definition having relation to the origin of the root-supply. 

 The lower of the two plates of xylem seen in H. dilatatum may be absent in 

 smaller t)'pes. The phloem, that completely encircles the xylem in larger 

 forms, may also disappear on the lower side in smaller types, and thus the 

 stele may become collateral [T. nmscoides). Finally, in T. microphyllum the 

 xylem may be represented only by a single tracheid, while in T. Motley? 

 there is no xylem, and the stele consists of only a few cells of conjunctive 

 parenchyma. These are plainl}- reduced types. The petiole in all cases 



