VASCULARIZATION OF THE VEGETATIVE SHOOTS OF HELIANTHUS AND SAMBUCUS

ALONG WITH Linum perenne, considered in a previous article (Esau, 1943a), certain other plants were examined to determine the course of development of the first vascular elements in vegetative shoots. The present paper describes the differentiation of the first phloem and xylem in the shoots of Helianthus annuus L. and Sambucus glauca Nutt. These two plants were chosen because, having large leaves and trilacunar or multilacunar nodes, they strikingly contrast with Linum perenne which has small leaves and a single trace2 per leaf. MATERIAL AND METHODS.-The sunflower plants were raised in a greenhouse and the elderberry-was growing wild in the vicinity of Davis, California. Five shoots of Helianthus and one of Sambucus were sectioned transversely, several others longitudinally. Ordinary paraffin and staining methods were used in preparing the slides. Some of the prepared slides were kindly supplied by Dr. R. H. Wetmore of Harvard University. In the examination of the sections the same procedure was followed as that outlined in connection with similar studies on flax (Esau, 1942, 1943a). In all illustrations the leaves were numbered beginning with the youngest at the apex of the shoot. If the leaves were paired at the nodes, the two members of the pair received different numbers, though the two leaves were of the same age. Such numbering was convenient for the marking of the traces within the stem. To simplify the descriptions of the shoots, all leaves including the youngest, are spoken of as "leaves" and the terms "embryonic leaves" or "leaf primordia" are rarely employed. Heliaqnthus.-Table 1 characterizes the five sunflower plants used for detailed study. All were young plants and showed no evidence of a change from the vegetative to the reproductive stage of development. Column 2 in table 1 gives the numbers of foliage leaves in the five plants. Besides these leaves each I Received for publication September I2, 1944. 2 The term "trace" is applied throughout to each bundle which extends to a leaf. plant had two cotyledons. As Priestley and Scott (1936) have reported, Helianthus 'does not have a stable type of leaf arrangement. The first few leaves are more or less decussate' (fig. 2), later the phyllotaxis changes to some spiral system (fig. 9). Concomitant with this variability in leaf arrangement, the relation of the leaf traces to each other within the axial vascular skeleton shows no stable pattern. Each leaf usually has three traces, one larger, median and two smaller, lateral ones; occasionally there are more. (Leaf 15 of shoot 5 had four traces.) The apex of a Helianthus shoot in vegetative stage does not form a cone; instead, it has a flat surface and the newly forming leaves arise as mounds on the margins of this surface (fig. 1). The internodes associated with the youngest leaves do not elongate for a number of plastochrones so that several leaves at the apex are attached at close levels. The axis expands laterally just below the apical meristem and, in longitudinal sections, appears to form broad shoulders on either side of it. The uppermost leaves rest upon these shoulders (fig. 1). In transverse sections the stem apex occurs, even at its highest level, not as a separate entity but is continuous with a mass of tissue formed by the united bases of several leaves (fig. 3). The procambium was not studied in as much detail in the sunflower as in the flax (Esau, 1942) and the matter of the longitudinal course of procambial formation will not be commented upon in this paper. Procambial differentiation was usually evident in connection with leaves that were recognizable as small elevations above the apex; but no procambial strands were found that could have been interpreted as traces to future leaves whose primordia had not yet arisen. As in the flax (Esau, 1942), the procambium of the youngest organs is initiated through a series of longitudinal divisions. Figures 4 to 7 show the beginning of procambial differentiation in leaf 1 of the shoot depicted in figures 2 and 3; and figure 8 illustrates a portion of the procambial strand of