' r 



.p- 



470 



DR. A. AKBEIl ON THE LEAF-TirS 



(figs. 2 A & b), Dlsticlda clandestlna, Baclien. (figs. 6 a-d) and EUg'm ileusta 

 Kuiitli (figs. 5, A & b), I liiivc also found a. closed ring oC bundles. In 

 the loaf of the Bracccna cultivated under tlie name " Uncliess of York " 

 (figs. 3 a & b), the main lateral bundles {in.L) were observed to become, 

 towards tlie apex, more conspicuous tban tlie midrib [m.h.). In two leaves 

 of Dnmvna fragrans, Ker-Gawl. (figs. 4 A-d), \v1i1c1i I examined, this process 

 was carried so far that the main bundle disa[)pcared by fusion with one oE 

 the latonils, with the result that, towards the extremity of the leaf-tip, the 

 vascular system consisted of a pair of bundles (fig. 4c), which ultimately 

 fused (fig. 4 d). The subsidiary part played by the midrib in such cases 

 may possibly, as I have previously suggested in connection with some other 

 members of the Liliacea^ *, be an indication of phyllodic origin, but this is a 



point on which little stress can be laid. 



In the apex oE a species of Bromelia, again, iT have found a potiole-liko 



vascular scheme, with inver 



tod bundh'S towards the adaxial surface 



(figs. 8 A-c), but in Tillandsia Lescalllei, Wright (figs. A-c), and in Aldris 

 farinosa, L. (figs. 7 A-c), the anatomy of the tip departs less from that of tho 



limb. 



Lonay t has described a ring of bundles in the " acumen " terminating 



the loaf of Ornithogalum caudatum, Ait. 



If the explanation previously advanced | for the leaf-tip of Tnlipa bo 

 accepted, there seems no reason why it should not be applied to the further 

 cases which we have been considering in tho present section of this paper. 

 It is, liowever, possible to hold the view tliat we are dealing with vestigial 

 petioles in the case of those better-developed apices which reveal an almost 

 radial type of vascular symmetry (e.g., Donjanthes, .DldicJda^ and Elegki), 

 while reserving judgment in the case of the relatively inconspicuous leaf- 

 tips of Aletiis, Tillandsia, etc., \\ hich, instead of being reduced petioles, may 

 represent a secondary modification. 



Velonovsky §, wlio draws attention to a number of plants whose leaves 

 have solid apices, points out that tins form of tip is frequent in Mono- 

 cotyledons, but very rare in Dicotyledons, nnd is characteristically associated 

 with a simple typo of leaf — umliffei-entiated into sheath and liiid), and 

 without a llgnlo. Ho offers no explanation of these facts, whicl-, however, 

 become readily intelligible from the standpoint adopted in the present 

 paper ; if the leaves in question are merely highly-developed leaf-bases 



terminated by abortive petioles, it is obviously useless to look for tliem 



among the laminated leaves of Dicotyledons, while, in the Monocotyledonous 

 cases, the ligule, if appearing at all, would be located inunediately below tlie 

 solid leaf-apex. 



» Arbor, A. (1920'),, pp. 401-2. 

 t Loiiay, 11. (1902). 

 X AvhcT, A. (1920'). 

 § Velenovsky, J. (1907). 



