2 ON SEEDLINGS 



veins following the curvature of the leaf as, for instance, in 

 Tamus (fig. 1) the vascular bundles pursue necessarily a 

 curved course ; while in palmate leaves, as in Acer (fig. 2), the 

 veins are 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 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 trans- 



FIG. 1. Leaf of Tamus, to show the 

 curved course of the veins. 



FIG. 2. Leaf of Sycamore, to show 

 the straight course of the veins. 



versely the length of a single tracheid 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. 



In my book on Flowers, Fruits, and Leaves, I have also 

 given some account of the causes which have determined the 

 form and structure of seeds and fruits. 



These considerations naturally led me to the study of 

 cotyledons. 



1 See for instance Darwin and Phillips, ' On the Transpiration Stream in 

 Cut Branches,' Proc. Cambridge Phil. Soc. vol. v. - Loc. cit. p. 364. 



