22 THE FORMS OF LEA VES. [May 



THE FOllMS OF LEAVES. 



AT an April meeting of tlie Linnean Society, .Sir John Lubbock 

 enlarged on some points he had dwelt on in a popular manner 

 in his lecture at the Eoyal Institution. He said he had long been 

 puzzled at the reason why some plants have cordate leaves, more or 

 less elongated, others palmate. Starting from a foliaceous expansion 

 of a stem, the former was evidently the simpler type. He had sug- 

 gested that the cordate shape was the earlier, from wliicli the palmate 

 was developed. The interesting consequence would follow, that if in 

 the same genus some species were cordate and others palmate, the latter 

 w^ere of later origin. This explanation presupposed that the palmate 

 form, at any rate under certain cu'cumstances, presented greater 

 advantages. Sir John suggested that this might have reference to 

 the power of self-support and of resisting wind. The weight of the 

 leaf and the force of the wind might be considered as if they were 

 concentrated on the central point of the leaf. Now in palmate 

 leaves the centre would be nearer the point of support, and the 

 resistance to any given force, therefore, would be more effectual. 

 This point was illustrated by experiment. Broad leaves, however, 

 were of two main types — cordate, witli veins following the curvature 

 of the edge ; and palmate or lobed leaves, with veins running direct 

 to the margin. jSTow, the fibres constituting the so-called veins of 

 leaves, contained elongated cells running parallel to the veins. Con- 

 seqiiently the sap passing up the leaf stalk and into the leaves 

 moved more readily along these libro-vascular bundles, having fewer 

 cell walls to traverse. It was almost like going along a road instead 

 of across country. The fibro-vascular bundles then acted as pipes 

 conveying the sap, and it is clearly more economical that they 

 should go straight to their destination, rather than wind in a curve. 

 As the sap passes more freely along the veins, the leaf also grows 

 there more rapidly, which seems to explain the common-lobed form 

 of leaf, with a vein running to the point of each lobe. 



Referring to some criticism on his views witli reference to the 

 relation between the size of leaves and the diameter of the stem, 

 Sir John examined especially those of the conifers. Thus the spruce 

 fir had shorter leaves than the Scotch fir, but they remained on 

 much longer — say seven or eight years instead of two or three, and 

 had a stem as thick or thicker. Amongst other interesting problems 

 presented by leaves, he discussed the probable reason for the larch 

 having deciduous leaves, and for the presence of two kinds of leaves 

 on certain cypresses and other trees. 



