1885.] Sir John Luhhoch on the Forms of Leaves. 197 



WEEKLY EVENING MEETING, 

 Friday, February 13, 1885. 



Sib Frederick Pollock, Bart. M.A. Manager an 1 Vice-President, 

 in the Chair. 



Sir John Lubbock, Bart. M.P. D.C.L. F.R.S. M.BJ. 



The Forms of Leaves. 



Greatly as we all appreciate the exquisite loveliness of flowers, it 

 must be admitted that the beauty of our woods and fields is quite as 

 much due to the marvellous grace and infinite variety of foliage. How 

 is this inexhaustible richness of forms to be accounted for ? Does it 

 result from an innate tendency of the leaves in each species to assume 

 some particular shape ? Has it been intentionally designed to delight 

 the eyes of man? Or has it reference to the structure and organisation 

 — the wants and requirements of the plant itself ? 



Size. 



Now, if we consider firstly the size of the leaf we shall find 

 that it stands in close relation to the thickness of the stem, and 

 that when strict proportion is departed from the difference can 

 generally be accounted for. This was shown, for instance, by a table 

 giving the leaf area and the diameter of stem of the Hornbeam, 

 Beech, Elm, Lime, Spanish Chestnut, Ash, Walnut, and Horse 

 Chestnut. 



The size, once determined, exercises much influence on the form. 

 For instance, in the Beech the leaf has an area of about 3 square 

 inches. The distance between the buds is about IJ inch, and the 

 leaves lie in the general plane of the branch, which bends slightly at 

 each internode. The basal half of the leaf fits the swell of the twig, 

 while the upper half follows the edge of the leaf above; and the 

 form of the inner edge, being thus determined, decides that of the 

 outer one also. In the Lime the internodes are longer, and the leaf 

 consequently broader. In the Spanish Chestnut the stem is nearly 

 three times as stout as that of the Beech, and consequently can carry 

 a larger leaf-surface. But the distances between the buds are often 

 little greater than those in the Beech. This determines, then, the 

 width, and, by compelling the leaf to lengthen itself, leads to the 

 peculiar form which it assumes. 



