ANATOMY OF THE EMBRYONIC LEAF

Abstract

Ash. In Fraxinus peunsylvanica Marshall the essential structure of the embryonic leaf is revealed by a study of an unswollen leaf bu-id of late March. Each leaflet (fig. I, 2) consists of midrib and conduplicately folded wings. The wings are composed of a limiting layer of small, brick-shaped cells and a mesophyll region of cells of somewhat similar shape and rather indefinitely layered into 5-6 rows. Provascular areas are embedded at intervals in the middle rows. In figure 2 two provascular areas are recognizable. The area which is near the tip of the leaflet shows a cell of the middle mesophyll layer divided horizontally into 2 daughter cells; the other area shows a stage in which spiral protoxylem elements are present. The midrib of the leaflet is composed of a central stelar region containing primary xylem and phloem parenchyma areas, a cortical region, and a limiting epidermal layer. Basswood. In Tilia glabra Ventenat the young leaf throughout its period of development from May until the following February (fig. 3-II) is composed of 5 rows of brick-shaped, densely protoplasmic parenchymatous cells of unifornm size so closely packed together that no intercellular spaces are present. This regular stratified arrangement of cells is broken at intervals by provascular areas in varying degrees of development. Each of these provascular areas arises through the horizontal division of one or more contiguous cells of the middle layer of the mesophyll, thus cutting each mother cell into an upper and lower cell. Such a stage is seen between the first and second major lateral veins in figure io. This first division is followed by other divisions in the mother cells and in adjacent mesophyll cells until very definite areas of irregularly arranged cells are gradually built up. These progressively more complex stages in the development of veins are well illustrated by the provascular areas sketched in figures 6, 4, II (near tip), 7, and 1 Contribution from tlhe Osborn Botanical Laboratory, Yale University, Seessel Fellow. Awarded secondl place, Walker Prize Contest, Boston Society of Natural History