v.] CA USES OF DIFFERENCES. 99 



of many leaves, or the nearly continuous rotation of 

 the lateral leaflets of Desmodium. I propose, rather, 

 to ask you to consider the structure, and especially 

 the forms, of the common every-day leaves of our 

 woods and fields. 



In talking the subject over with friends, I have 

 found a widely prevalent idea that the beauty and 

 variety of leaves are a beneficent arrangement made 

 specially with reference to the enjoyment and delight 

 of man. I have, again, frequently been met by the 

 opinion that there is some special form, size, and 

 texture of leaf inherently characteristic of each 

 species ; that the cellular tissue tends to " crystal- 

 lize," as it were, into some particular form, quite 

 irrespective of any advantage to the plant itself. 



Neither of these views will, I think, stand the test 

 of careful examination. 



In the first place, let us consider the size of the 

 leaf. On what does this depend ? In herbs we very 

 often see that the leaves decrease towards the end of 

 the shoot, while in trees the leaves, though not identical, 

 are much more uniform in size. 



Again, if we take a twig of Hornbeam, we shall find 

 that the six terminal leaves have together an area of 

 about 14 square inches, and the section of the twig 

 has a diameter of *o6 of an inch. In the beech the 

 leaves are rather larger, six of them having an area 

 of perhaps 18 inches, and, corresponding with this 

 greater leaf-surface, we find that the twig is somewhat 

 stouter, say '09 of an inch. Following this up, we 

 shall find that, cateris paribus, the size of the leaf has 



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