Histogenese und morphologische Deutung der Doppelnadelnvon Sciadopitys

Abstract

Summary The vegetation cone of the long shoots of Sciadopitys shows the zonation pattern characteristic for higher Gymnosperms with a tunicalike outer layer dividing periclinally in the apicalzone only occasionally (F oster’s apical initial group). Beneath this region larger cells are located with less dense cytoplasm (central mother cells) from which all cell rows of the vegetation cone originate. Below the central mother cells develops a cup-shaped zone of active cell divisions (cambi-forme meristem) which corresponds to the cambial-like zone of P opham and C han . Pith meristem and peripheral meristem arise from the central-mother-cell zone. The leaf formation is initiated by divisions of subepidermal cells of the vegetation cone. The leaf primordia develop apical initials which divide peri- and anticlinally according to the 2-cell-rhythm. Owing to a stronger development of the dorsal segments the leaf primordium hyponastically overarches the vegetation cone. Later on a subepidermal dorsal meristem originates on the dorsal side of the scale leaves. The double needles are formed as axillary shoots in the axils of the scale leaves predo-minantely by periclinal divisions of the subepidermal cells of the vegetation cone. In this way anticlinal cell rows are brought about. In more advanced stages the double needles show a similar zonation at their tips as the vegetation cones of the long shoots. The apex of the double needle is covered with a single layered tunica which does not divide periclinally. The formation of this tunica layer can be explained by the fact that the dermatogen of the long shoot apices is already developed as an individualized layer at the level of the axillary buds. Below the tunica there is a group of central mother cells in which all cell rows converge. A distinct initial group at the tip of the short shoots is missing; this lack may perhaps be interpreted by the early cessation of the apical growth of the double needles. The absence of the cambiforme meristem in the apex of the double needles can probably be deduced from the fact that practically no organ formation takes place. In the last stage of the apical growth of the double needles the cell division activity in the middle of the tip is strongly reduced while 2 protuberances are formed on both flanks by continous periclinal divisons of the first subepidermal layer. Thus, they owe their origin to a cambium-like meristem (arranged in cell rows) and are perhaps to be interpreted as rudimentary leaves. The apical growth of the short shoots is thereby stopped and replaced by a basal intercalary meristem. Also growth in width and thickness originates from a subepidermal meristem (arranged in cell rows) which consequently surrounds the whole primordium of the double needle. Marginal cells which are characteristically for the margin formation of leaves are not developed. Since the main part of the double needles is formed by elongation of the basal zone below the vegetation cone, a comparison of the double needles of Sciadopitys with2needles of a dwarf shoot of Pinus congenitally grown together seems to be questionable. Die Arbeit wurde von der Deutschen Forschungsgemeinschaft gefordert, wofur ich hiermit meinen Dank aussprechen mochte. Herrn Professor Dr. E. M. Gifford und Frau Professor Dr. K. Esau, University of California, Department of Botany, Davis/California, sowie Herrn Professor Dr. W. Rauh, Botanisches Institut der Universitat Heidelberg, bin ich fur wertvolle Ratschlage und Hinweise, die sie mir zuteil werden liesen, zu Dank verpflichtet. Herrn Dr R. Schindler, Botanisches Institut der Universitat Heidelberg, danke ich fur die Ausfuhrung der Zeichnungen in Tusche.