A SEASONAL STUDY OF THE VEGETATIVE SHOOT APEX AND THE PATTERN OF PITH DEVELOPMENT IN HIBISCUS SYRIACUS

TOLBERT, ROBERT J. (West Virginia U., Morgantown.) A seasonal study of the vegetative shoot apex and the pattern of pith development in Hibiscus syriacus. Amer. Jour. Bot. 48(3): 249-255. Illus. 1961.-The shoot apex of Hibiscus syriacus L. is described as having a cytohistological zonation superimposed on a tunica-corpus configuration. The apex is flat-topped or may have a saddle-back or concave appearance as seen in median longitudinal section. The metrameristem, consisting of the tunica and corpus initials, is comprised of large, light-staining, vacuolate cells that have thick cell walls and exhibit much dark-staining intercellular substance. Surrounding the metrameristem is the flanking meristem, which is responsible for the outer layers of the shoot, and from which the leaf primordia arise. The pith rib meristem lies below the metrameristem and consists of files of cells that are responsible for the pith. There are no major seasonal changes in the structure of the apex during the yearly cycle. The pith displays a long-shoot type of development with the cells remaining in distinct files during the first flush of growth in the spring. As growth slows and internode elongation is gradually reduced, the pith displays the characteristic short-shoot type of development, consisting of a spongy tissue of rounded cells with many intercellular spaces and no distinct files of cells. A crown is differentiated across the top of the pith at the end of the growth period. This consists of a band of cells with thick, darkstaining cell walls, which separates by the apex from the last year's' growth. In contrast to many gymnosperms, this crown is dispersed by renewed cell activity the following spring. THE PRESENT investigation deals with the structure of the vegetative shoot apex and pith development, as affected by the seasonal cycle, in Hibiscus syriacus L., a member of the Malvaceae. Seasonal studies have been made in Liriodendron (Millington and Gunckel, 1950), in Drimys (Gifford, 1950), and in several members of the Rosacea (Rouffa and Gunckel, 1951a,D) ; but as Gifford (1954) has remarked, " . . . the possibility of actual seasonal variation in the structure of the shoot apex of perennials remains relatively unexplored." The study of pith development in Hibiscus syriacus was prompted by the discovery made by Rouffa and Gunckel (1951b) that growth habit and seasonal change were associated with pith development rather than with changes in the structure of the shoot apex itself. MATERIALS AND METHODS.-All the material was collected during 1956 and 1957 from a single specimen of Hibiscus syriacus L., the 'Rose of Sharon,' growing on the campus of Rutgers University, New Brunswick, New Jersey. Collections were made monthly during the winter dormant period and at weekly intervals from early spring to fall. Formalin-acetic acid-alcohol was the killing and fixing fluid used. Immediately after collection, the samples were evacuated to insure removal of air and entry of the preservative. 1-Received for publication August 28, 1960. This paper is a portion of a dissertation submitted tothe Botany Department and the Graduate School of Rutgers, the State University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The author wishes to express his gratitude to Dr. Marion A. Johnson of Rutgers for his guidance and suiggestions throughouit the investigation. The material was routinely softened in 24% hydrofluoric acid (in 50% alcohol) for a period varying from a few days to several weeks. Collections from the winter period especially were very hard to cut, a problem somewhat alleviated by the prolonged softening period. The tips were dehydrated in a graded series of ethyl and normal butyl alcohols and embedded in 60-63?C. Tissuemat. Prior to sectioning, the embedded material with some tissue exposed by shaving with a razor blade was softened in glycerinealcohol (9 parts 60% alcohol, 1 part glycerine) at temperatures up to 45-50?C. The blocks were then iced and serial sections were cut at 6,u. Iron-mordanted safranin and celestine blue B (Gray and Pickle, 1956) was found to be especially effective in staining cell walls for photography. This is a "one-jar stain" and is highly recommended for ease of application and superior results. Cytological features were checked in preparations stained with Heidenhein's hematoxylin and safranin. Terminology.-Metrameristem.-Hibiscus syriacUs has a zoned apex consisting of a central core of tissue extending to the surface, a surrounding flanking meristem, and an underlying pith rib meristem. I have called this central core of tissue the metrameristem in keeping with the concept recently elaborated by Johnson and Tolbert (1960) and consider it to consist of the tunica initials and the corpus initials (fig. 1). I THE VEGETATIVE SHOOT APEX. -The terminal shoot apex of Hibiscus syriacus is always vegetative, because the floral primordia arise only in the axils of the leaves. This fact makes it possible to follow the activities of the vegetative shoot apex