RELATIONS BETWEEN TISSUE ORGANIZATION AND VEIN DISTRIBUTION IN DICOTYLEDON LEAVES

Abstract

THIS PAPER deals with certain relations between leaf structure and the distribution of minor veins in the blade. In connection with work on the wound responses of foliage leaves the writer built up several series of slides which were equally favorable for the study of normal leaf tissues. It seemed from reviewing these preparations that vein spacing was often related to mesophyll organization. A careful analysis of leaves of three series of plants seemed to confirm this generalization and revealed a fairly close correlation between the photosynthetic and conductive tissue of the blade in these species. Several investigations have dealt quantitatively with the development of the minor venation in the leaf blade. Zalenski (1902), using leaves cleared in chloral hydrate and mounted in glycerin, called attention to variations in total length of vein per unit area in plants of given species even in similar habitats. This led to his later paper (1904), which is discussed at length by Maximov. He demonstrated a wide range in the structure of leaves distributed vertically along a single stem. On shrubby or herbaceous plants of moderate size he found leaves of higher insertion successively more xeromorphic with progressively greater vein length per unit area than those lower on the same stem. While this feature was associated with several other structural differences in the leaves investigated, he considered these vein specializations as most closely correlated with cell size in the different leaves. Schuster (1908) surveyed a considerable number of plants, giving attention to differences between sun and shade leaves. With few exceptions, shade leaves were larger than sun leaves of the same species, but the shaded leaves had reduced venation, in terms of total vein length per unit area of blade, compared with those in the sun. Benedict (1915) made an intensive vein study of a few plants but concerned himself particularly with ritis vulpina. He held that vein spacing (which he termed the vein-islet) for leaves of this species was correlated closely with the plant's age, diminishing regularly through successive years after the launching of a new plant from an embryo started by fertilization. His records were made photographically by transmitted light, using untreated living leaves, which method probably failed to record some of the lesser veinlets. While he noted the earlier work of Zalenski (1902), he does not include the very important paper by that writer published in 1904. Strain (1933) recently made a study of the types of vascular elements in the vein endings of a large number of species, by clearing whole leaves, cotyledons, and petals. He finds type patterns of tracheal organization rather than variations due to environ-