﻿MR. JOHN SCOTT ON THE TREE FERNS OF BRITISH SIKKIM. 21 



that the production of adventitious buds in the genus Lycopodium is by no means a 

 frequent occurrence ; and I have as yet observed it on two species only, L. nummularis 

 folium and a handsome new species akin to Z. FJilegmaria, which has been lately intro- 

 duced to the Gardens from Sumatra. 



Defoliation. —HYiQ fall of the frond is in general gradual ; and in Sikkim one only of the 

 species is truly deciduous. This, as I have previously remarked, is the Alsophila comosa^ 

 which casts its leaves about the beginning of August, and so remains for from four to 

 five weeks, when another cycle of fronds is rapidly unfolded. The fall of the leaf in 

 this plant during August is singular and anomalous. Through all those wide forest^ 

 stretches every tree and bush in the height of their rainy season is in vcrdui:e, while this 

 tree fern, like some imperfectly acclimatized intruder, stands bare and leafless. The 

 fall of the leaf in this case therefore cannot be attributed to the ordinarily assigned 

 causes, such as change in the season, nor the other less or more potent influence of 

 axillary buds. It is, indeed, difficult to conjecture as to the cause of defoliation in 

 this case. Its congener, AlsopUla latehrosa, is in full verdure, as are all their more 

 humble congeners. I have carefully examined the base of the fronds of this plant, and 



compared them with those of other non-deciduous species, without findin"- 



any 



planatory difference. In all the species we find the parts much as H. Von Mohl 

 described them as having place amongst phjenogams. A "separating layer" of cortical 

 tissue, originating in the axillary side of the leaf, extends downwards, and ultimately 

 forms a complete line of demarcation between the stem and distant parts of the leaf 

 With this process of vegetation, corresponding to the maturity of the frond, all further 

 development of the distant parts of the frond ceases (and we may suppose that it will 

 tend to direct the sap-current into newer and younger channels), the separating layer 

 becomes hard and woody, and forms a clean detaching surface. This " separating layer " 

 appears as a very thin membrane in the earliest stages of development of the frond, and 

 forms a distinct line between the distal and proximal parts of the leaf *. A similar 

 membrane is, I believe, more or less distinctly present in all leaves, forming a line of 

 demarcation between the axial and lateral organs. In many cases, as in nearly all 

 monocotyledons, and generally in aU lax-textured petioles, a slight constriction in the 

 nascent leaves is the only indication we find of this organ, which becomes absorbed in 

 the more rapidly developing longitudinal tissues. In dicotyledons, on the other hand 

 with a more equable development of the tissues, the separating layer is very generally 

 present, increases in thickness with the transverse tension of the leaf, and forms a 

 distinct articulating surface, whence, as they individually reach maturity, they drop from 

 '^- axis. Such are the phenomena presented in the fall of the leaf of A. comosa and a 

 few other ferns, though here, as in endogens generally, the majority undergo decay from 

 above downward to the hardened bases of the stipes, which often retain their attach- 

 ment to the stem for several years. 



the 



which 



Edinburgh, 



and 



1866, by Dr. M"^Xab. It is there shown that the incipient leaf is divided into proximal __ 



- hypophyU" and "epiphyU" of Dr. M=Is^ab, the latter giving rise to the lamina, and the former to^the 7\xZ'Z 



portio 



organ B 



J 



