Relation between Structure of Pine Leaves and their Position on the Tree

Abstract

THE quantitative anatomical method has recently acquired an increasing importance in determining the face, the form, or even the variety of various cultivated plants. It has also been applied in the study of the leaf of the pine (Pinus silvestris L.)^1,2,3. Unfortunately, however, scarcely any of the investigators have considered the position on the tree to which are attached the leaves taken for study. Whereas, so early as 1904, Zalensky⁴ discovered the law which has, in the Russian literature, acquired the name of Zalensky's law (Maximov⁵), but which, unfortunately, remained unknown in western Europe and was considerably later rediscovered by a number of investigators⁶. Zalensky, studying monocotyledonous and dicotyledonous plants on a very extensive and variegated material, showed that the higher up on the tree the leaf grows (or the nearer to the end of the branch) the greater the xeromorphic properties it acquires ; that is, the epidermis and mesophyll cells are smaller, the conducting strand is thicker, the cuticle and wax coating are thicker, the stomata are greater in number and smaller, the palisade tissue is more clearly defined. It was to be expected that a similar law would be found for Gyrnnospermse, except that, in connexion with the peculiar structure of the leaf in conifers, it would take a slightly different form.