PRINCIPLES OF FOLIAR ORGANIZATION SHOWN BY SUN‐SHADE LEAVES FROM TEN SPECIES OF DECIDUOUS DICOTYLEDONOUS TREES

Abstract

THE MORE CONSPICUOUS differences between sun and shade leaves are well known and references in this field are numerous. Many papers have dealt more or less critically with the structural features shown by cross sections of the lamina; several of these consider environmental factors and a few take into account the vascularization of the blade. A recent paper by the writer (Wylie, 1949) summarizes briefly some related papers in this field in connection with a comparative study of leaf organization at four stations in the crown of a single tree (Acer platanoides). The general structural concept of the foliage leaf has been largely based upon the study of transections of the blade. Early students had only freehand sections and later workers, with modern histological methods available, have based most of their findings on transections. These show favorably the blade thickness and the depth of its several layers as well as the form of individual palisade cells but afford little information about the spongy mesophyll or the general relations of its cells. Paradermal sections, cut in the plane of the blade, supplement by showing the size and shape of epidermal cells, stomatal distribution and the inter-relations of palisade cells, together with critical information on the structure of the spongy mesophyll. These sections also favor study of the minor venation, which is closely associated with the spongy mesophyll, and permit measurement of the vascularization. The present paper summarizes a survey of the blade tissues for ten species of deciduous woody dicotyledons, using leaves from three widely spaced stations in the crown of each tree. The purpose of the study, in addition to obtaining information on the structure and relative volume of the several tissues at the different stations, was to review their tissuemodifications and shifts in vascularization in relation to problems of foliar organization. MATERIALS AND METHODS.-Of these ten species, listed in table 1, six are native to Iowa; two, Liquidamber styracifiua and Catalpa,speciosq, are of southern range, while Acer platanoides and Lonicera tatarica are introduced north temperate plants. This series is a heterogeneous assemblage, selected in part because of problems presented by the structure of their sun leaves. Catalpa, and several others in lesser degree, have a vertical trend among their spongy mesophyll cells. Others, including the oaks, Acer platanoides, and Catalpa, have a dominant palisade, and a majority of them have patterns of spongy mesophyll differing from that of typical mesophytic foliage. Ulmus americana was included for various reasons primarily to learn more about the puzzling leaf of this widely distributed tree. While Acer platanoides had been previously sur-