DISTRIBUTION OF LEAVES OX THE STEM. 



131 



resembling a spiral stairway. Upon this supposition, the leaves 

 would be the relics, or rather the advantageous results, of the 

 segmentation of such a frond-like 

 expansion, the segments separated 

 through the development of the 

 stem in length and firmness, and 

 modified in the various adaptations 

 to the conditions of higher vege- 

 table life ; even as leaves themselves 

 are modified into tendrils, bud- 

 scales, petals, or other usefully specialized structures. The type 

 on this conception would be a frond, consisting of 

 an elongating axis with a continuous leaf-blade on 

 one side, and this taking a spirally twisted form. 

 But the frond of Fucaceous Algae, Hepaticse, and the 

 like, is two-bladed. While a one-bladed frond, or 

 with one blade suppressed, might be the original of 

 alternate-leaved spirals, the two-bladed frond, simi- 

 larly broken up, would give rise to the opposite or 

 other varieties of verticillate arrangement. 1 



248. Fascicled Leaves need to be mentioned here, 

 in order that they may be excluded from phyllotaxy. 

 The}' are simply a cluster or tuft of leaves, belonging 

 to more than one node, and left in a crowded con- 

 dition because the internodcs do not lengthen. They 

 may belong either to the alternate or the verticillate 

 series. In Barberry and in the Larch (Fig. 247), 

 they are evidently alternate ; and they may be inferred, 

 to be so in Pines (Fig. 248), or even may be seen to 

 be so in the bud-scales which form the sheath sur- 

 rounding the base of the 2, 3, or 5 foliage-leaves. 

 In Junipers, the leaves of the fascicles are in the 

 verticillate order. 



1 This is the conception of the late Chauncey Wright. See his elaborate 

 and most suggestive essay in Mem. Amer. Acad. Arts and Sciences, ser. 2, 

 ix. 370, mainly reprinted in Philosophical Discussions (posthumous), 296-328, 

 in which the whole subject of phyllotaxy is acutely discussed, especially 

 in its relation to questions of origin and developed utilities. His conception 



FIG. 247. Piece of a branch of the Larch, with two fascicles of leaves, i. e. two very 

 short and stout branchlets, bearing scars of former leaves or bud-scales below, and a 

 dense cluster of leaves of the season at summit. The main axis bears scars from which 

 the alternate leaves of the developed axis of the preceding year separated. 



FIG. 248. Piece of a branch of Pitch Pine, with three leaves in a fascicle or bundle 

 in the axil of a thin scale (a) which answers to a leaf of the main axis. The bundle is 

 surrounded at the base by a short shealh, formed of the delicate scales of the axillary 

 bud, of which the throe leaves are the developed foliage. 



