A contribution to our knowledge of seedlings / by the Right Hon. Sir John Lubbock.

  • John Lubbock, 1st Baron Avebury
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: A contribution to our knowledge of seedlings / by the Right Hon. Sir John Lubbock. Source: Wellcome Collection.

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    FORMS OF LEAVES the evidence seems to point strongly to the latter view.1 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 traclieid the water-current has to traverse seventy cell-walls instead of one.’ 2 In reply, then, to the question why some plants should have cordate leaves with curved veins, while others have palmate leaves with straight ones, I suggested that the first retains the old form of leaf, while the latter has assumed one which presents certain advantages. We can also see a relation between the shape and size of the leaf and the length of the internode. For instance, in the common Lime the leaf-stalks are arranged, at an angle of about 40° with the branch, and the upper surfaces of the leaves are in the same plane with it. The result is, they are admirably adapted to secure the maximum of light and air. ‘ In the Beech (fig. 3) the general plane of the leaves corresponds with that of the branch, but the leaves them- selves are ovate in form, and smaller, being only from two to three inches in length. On the other hand, the dis- tance between the nodes is also smaller, being, say, one and a quarter inch against something less than two inches. The diminution in length of the internode is 1 See, for instance, Darwin and Phillips, ‘ On the Transpiration Stream in Cut Branches,’ Proc. Cambridge Phil. Sob. vol. v. 2 Loc. cit. p. 364. b 2