4 88 



THE POPULAR SCIENCE MONTHLY. 



Fia. 32. 



In looking at foliage I have often been much puzzled as to why 

 the leaves of some species are tongue-shaped, while others are lobed. 

 Take, for instance, the black bryony ( Tamus communis) and the com- 

 mon bryony {Bryonia dioica). Again, why are the veins in some 



leaves pinnate, like those of the beech and 

 elm, and others palmate, as in the maple 

 and sycamore ? 



My first idea was that this might have 

 reference to the arrangement of the woody 

 fibers in the leaf -stalk. If we make a sec- 

 tion of the stalk of a leaf, we shall find that 

 in some cases the woody fibers are collected 

 in the middle, while in others there are sev- 

 eral distinct bundles, separated by cellular 

 parenchyma. My first idea was that each 

 of the primary ribs of a leaf might repre- 

 sent a separate woody fiber in the leaf- 

 stalk, so that leaves with a single bundle 

 of woody fibers would be pinnate ; those 

 with several distinct bundles, palmate. 



The first species which I examined fa- 

 vored this view. The melon, geranium, mal- 

 low, cyclamen, and other species with palmate leaves, had, sure enough, 

 several woody fibers ; while, on the contrary, the laurel, rhododendron, 

 privet, beech, box, castanea, arbutus, phillyrea, and other leaves with 

 pinnate veins, had one central bundle. But I soon came across numer- 

 ous exceptions, and had to give up the idea. 



I then considered whether the difference could be accounted for 

 by the mode of growth of the leaf, and I am still disposed to think 

 that it has some bearing on the subject, though this requires further 

 study. 



The next suggestion which occurred to me was that it might be 

 connected with the " prefoliation " or arrangement of the leaves in the 

 bud. The first palmate leaves which I examined were what is called 

 " plicate," or folded up more or less like a fan ; while the leaves with 

 pinnate veins were generally " conduplicate," or had the one half 

 applied to the other. But, though this was true in many cases, it was 

 not a general rule, and I was obliged to give up this idea also. 



It then occurred to me to take climbing plants, and see whether I 

 could find any relation between palmate and tongue-shaped leaves on 

 the one hand and the mode of growth on the other whether, for in- 

 stance, the one turned generally up, the other down ; whether the one 

 were generally twining and the other clasping, or vice versa. All these 

 suggestions one by one broke down. 



Among monocotyledons, however, the tongue-shaped preponderates 

 greatly over the palmate form of leaf. With very few exceptions, the 



