﻿MR. JOHN SCOTT ON THE TREE FERNS OF BRITISH SIKKIM. 27 



porary cessation of the vegetative forces, "brought about by the vicissitudes of climate, or 

 at least specific periodicities of rest and action in the vegetable economy. 



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I may now briefly remark on the probable causes which may initiate the secondary 

 growth or *' separating layer " of the leaf. This I am disposed to attribute to mechanical 

 strains and an acquired habit of disarticulating at certain periods. In support of tbis 

 view I may note its occurrence at the point where the strain must evidently be greatest, 

 near the junction of the leaf with the stem, or where the flexibility of the petiole is lost 

 in the comparatively inflexible stem ; and in accordance with this or the greater down- 

 ward strain of the generality of leaves, we find the separating layer originating in the 

 axillary side of the leaf, thence growing downwards at right angles to the plane of the 

 leaf: possibly an examination of phyllodia might show a simultaneous growth from 

 either side. The commencement of the above organ is early apparent in the constriction 

 of the petiole adjoining the '*pulvinus." Several illustrations of a disformiiig character 

 might, as I am aware, be adduced ; but it is to be remarked that many such are only 

 apparently so, and depend on more or less evident modifying agents, as, for example, 

 the loose thickened tissue and great sheathing petioles of many monocotyledons, which, 

 equalizing the strain, leave no traces of constriction or " separating layers." 



14. Circination, protective in the unfolding leaves, assists in the economy of fertiliza- 

 tion when characterizing the inflorescence, and in both cases is the result of an unequal 



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development of the cells on opposite sides of the rachis. 



15. The lacuncB at the base of the fronds of tree ferns have been compared by Lindley 

 to the perforations in the rhizome of NymphcBa; but these, as shown by Trecul, and 

 as I, from personal observations, can confirm, are simply the base of decaying adven- 

 titious roots. Martins, guided by analogy, suggested their affinity with the antheridial 

 organs of other Cryptogams. The subsequent discovery of the antheridial organs has 

 displaced this view, though their analogy with those of Isoetes and. Zi/copadium renders it 

 by no means improbable that they may really be abortive representatives of those organs. 

 They have also relations to those accessory organs described by Karl Mueller as occurring 

 in the axils of the leaves of Lycopods, though here, again, we are alike in utter ignorance, 

 as admitted by Mueller, of the import of these organs. After a full consideration of the 

 above views, I am rather disposed, as I have above stated, to regard them as the aborted 

 costas of a sessile strongly auricled frond, in which view I am supported by a line of 

 very similar perforations on the lower parts of the rachis, in the plane of the primary 

 divisions of the frond, of which parts they are clearly abortions. 



16. The scars of the frond in the fully developed caudex are arranged with great 

 regularity, and, from their varying angular divergence in closely allied species, afford 

 good discriminating characters. 



17. The prolificness and bifurcation of the caudex occurs frequently in several species, 

 tliough we rarely find both characters strongly pronounced in a single species. In pro- 

 liferous species the buds are most frequently on the exterior base of the stipes, also in 

 various extra-axillary and lateral positions ; but in no case have I observed them truly 

 axillary. Adventitious buds are in certain cases with difficulty, as I have above ex- 

 plained, distinguishable from normal bifurcations of the axis. This is especially the case 



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