ON LEAVES. 345 



I do not propose now to discuss any of the more unusual and ab- 

 normal forms of leaves : the pitchers of Nepenthes or Gephalotus, the 

 pitfalls of Sarracenia or Darlingtonia, the spring -trap leaves of 

 Dionaza, the scarcely less effective though less striking contrivances 

 in our own Drosera or Pinguicula, nor the remarkable power of 

 movement which many leaves present, whether in response to an 

 external stimulus, as in certain mimosas, oxalises, etc., or as a spon- 

 taneous periodic movement, such as the "sleep" of many leaves, or 

 the nearly continuous rotation of the lateral leaflets of Desmodium. 

 I propose, rather, to ask you to consider with me 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 de- 

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

 there is some special form, size, and texture of leaf inherently char- 

 acteristic of each species ; that the cellular tissue tends to " crystallize," 

 as it were, into some particular form, quite irrespective of any advan- 

 tage to the plant itself. 



Neither of these 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 

 toward the end of the shoot, while in trees the leaves, though not iden- 

 tical, 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 fourteen square inches, 

 and the section of the twig has a diameter of *0G of an inch. In the 

 beech the leaves are rather larger, six of them having an area of per- 

 haps eighteen inches, and, corresponding with this greater leaf-surface, 

 we find that the twig is somewhat stouter, say "09 of an inch. Fol- 

 lowing this up we shall find that, cceteris paribus, the size of the leaf 

 has relation to the thickness of the stem. This is clearly shown in 

 the following table : 



Impression of Stalk below the Sixth Leaf. 



Hornbeam. Beech. Elm. Nut. Sycamore. Lime. Chestnut. 



9 G > 



Mountain-ash Elder. Ash. Walnut. Ailantus. Horse-Chestnut 



