I do not propose now to discuss any of the more unusual and abnormal forms of leaves: the pitchers of Nepenthes or Cephalotus, the pitfalls of Sarracenia or Darlingtonia, the spring-trap leaves of Dionæa, 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 spontaneous 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 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 "crystallize," as it were, into some particular form, quite irrespective of any advantage 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 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 fourteen square inches, and the section of the twig has a diameter of ·06 of an inch. In the beech the leaves are rather larger, six of them having an area of perhaps eighteen 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, cœteris 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.
