STR J. LUBBOCK l^IIYTOBIOLOaiCAL OBSEUYATIOXS. 341 



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Phytobiological Observations; On the Perms of Seedlings add 

 tlie Causes to wincli they are due. By Sir Joiix Lubbock:, 

 Bart., Pres. Linnean Society, F.E.S., M.P., D.C.L., LL.D. 



[Read as a Presidential Address, 24tli May, 18S0.] 



O FOBMS OF LeATES AND OF COTYLEDOXS. 



Introduction: Forms of Leaves. 



m 



w 



I nATE elsewhere * called attention to tlie forms of Leaves, and 

 discussed the causes to which wc might ascribe the eudless differ- 

 ences which they assume. Vertical leaves, for instance, are gene- 

 rally long and narrow, horizontal ones have a tendency towards 

 width, which brings the centre of gravity nearer to the point of 

 support. Wide leaves, again, are sometimes heart-shaped, some- 

 times lobed. The former shape is obviously that which would 

 arise if a leaf is gradually widened at the base ; and I have pointed 

 out that in many species with lobed leaves— for instance Passi- 

 flora, Cephalandra, Hibiscus, &c.— the first, or few first, leaves are 

 cntirc'and more or less cordate. The cordate form, then, appears 

 to be the early, the palmate a later form. But what advantage 



docs the palmate form present ? 



Now in cordate leaves with veins following the curvature of 

 the leaf, as, for instance, in Tavms (fig. 1, p. 342), the vascular 

 bundles pursue necessarily a curved course ; while in palmate 

 leaves, as in Acer (fig. 2), the veins arc straight ; and it is 

 clearly an advantage that the main channels which convey the 

 nutritive fluid should hold a direct course. In such cases the 

 leaf naturally assumes the lobed form with a vein running to 

 the point of each lobe. There has indeed been some question 

 whether the path of the sap lies mainly in the cell-walls or in 

 the cell-cavities ; but the evidence seems to point strongly to the 

 latter viewf. The tracheids of, say, the Yew "are at least 

 seventy or eighty times as long as they are broad, so that in 

 travelling transversely the length of a single tracheid the water- 

 current has to traverse seventy cell-walls instead of one" J. 



In reply, then, to the question why some plants should have 

 cordate leaves with curved veins, while others have palmate 



Journal Eoyal Institution, 1885 ; ' Contemporary EctIcw,' 188.'^ ; and 



« 



' Nature Series— Flowers, Fruits, and Leaves.' 



■t See for instance, Darwin and Thillips, " On the Transpiration Stream m 

 Cut Branches,^' Proc. Cambridge Phil. Soc. vol. T. J Loc. cit. p. 304. 



LI>2f. JOUEN. 



BOTA>'Y, VOL XXII. 2 P 



