﻿14 MR. JOHN SCOTT ON THE TREE FERNS OF BRITISH SIKKTM. 



fio-. 3). The plant grew amongst moist rocks, comparatively free from all other 

 arborescent vegetation, and unsheltered from sun and winds. 



The distribution of the vascular woody bundles in the stipes of tree ferns has also very 

 evident relations to mechanical conditions. In the sections of the diirerent species 

 represented in PL VII., the woody bundles form rings concentric with the periphery of 

 the stipes, with either rafter-like strutts passing off at an acute angle from the upper 

 sides towards the interior, or somewhat irregularly distributed towards the upper 

 periphery, but in either case increasing the strength of the upper planes. Now in the 

 large horizontal spreading fronds of ferns it is evident that the greatest pressure will 

 fall upon the upper side, so that the above arrangement perfectly accords with the 

 mechanical conditions. In the sarmentose and rhizomatous species similar relations 

 are obvious, the development of woody tissue in these organs having generally an 

 inverse relation to the fronds which they support ; and they are very markedly influenced 

 by habitat. Thus Polypodium phymatodes, when growing in its indigenous habitats, 

 has a much-hardened parenchymatous stem, studded in the central parts with slender 

 woody bundles, and a ring of larger bundles near the periphery. In culture, under 

 shade, it becomes softer and less tenacious in all its parts. So also the sarmentum of 

 Acrostichtmi scandens, which has a a central pith-like substance surrounded by a layer 

 of woody matter and an excentric ring of unequal-sized woody bundles (the lar 

 corresponding to the sides applied to the shoots around which it winds, and of course 

 that of greatest pressure), with a more slender peripheral ring of the same imbedded in 

 an extremely tenacious parenchyma, which again becomes comparatively soft in 

 individuals grown under shade and shelter. I know of no better illustration of this 



o 



hypothesis than that afforded by the Bipterls WalUcUi. This species has a hypogfBOUs 

 freely ramifying rhizome, rarely more than half an inch in diameter, with lari>e stout- 

 textured flabeUiform fronds (posed in vertical planes), sometimes 30 inches in length 

 and 40 inches broad, supported on slender tapering stipes upwards of 4 feet in height. 

 To support and counteract the strain of such a frond demands considerable strength in 

 both stipe and rhizome, so that what they decidedly want in thickness must be sup- 

 plemented by density and tenacity : and so we find it to be. The rhizome is separated 

 into four distinct paits— first, a central medullary parenchyma, encased in a thin ring of 

 woody tissue, which is again surrounded by a thick layer of woody-textured parenchyma 

 (corresponding to, though of course essentiaUy distinct from the wood of Dicotyledons) ; 

 and enclosmg all is a hard scaly rind. The fronds are uniserial on the upper side of the 

 rhizome, and somewhat distant, while the slender wiry roots are indifferently produced 

 from Its whole surface, though most abhndantly from the underside. The species affects 

 social habits, and is usually found in large patches, thus affording in a partial manner 

 a mutual protection and support. It is evident, however, that even under the most 

 favourable natural environments, the support of such large and heavy fronds (which 



even m the dry state weigh three ounces and upwards), with the root-fixments to 



woodv 1 hT ir^'; T'*^ .''' ^"' i'^P-rfeotly secured by an ordinary amount of 



Ta 1 1" Lh f r '''f''^''^'' '-^ '^'^--''^ ■' -d I do regard the remarkable increase 

 a direct result of frond-tension. That the rliizome and stipes should not have undergone 



