Growth and Development of the Shoot of Gnetum gnemon L. I. The Shoot Apex and Pith

Abstract

shoot apex of gyimnosperms has been inteiisively studied since Hofmeister (1857) first assumed that the primary tissues of both vascular cryptogams and phanerogams were derived from an apical cell. This assumption was soon refuted by many investigators and Koch (1891) was able to demonstrate conclusively that the shoot apex of the conifers consisted of (a) an outer zone from which epidermis, cortex, procambium and leaf primordia were derived and of (b) an inner zone which produced the pith. A second and quite different type of zonation was discovered by Foster (1938) in Gtnkgo. Here the major portion of the apex was readily traceable to a unique group of central mother cells. Comprehensive summaries of the literature up to 1939 are to be found in the papers of Koch (1891), Schiiepp (1926), Korody (1937), and Foster (1938). Subsequently Foster (1939, 1940, 1941, 1943), Cross (1939, 1941, 1942, 1943a, 1934b), Kemp (1943), Johnson (1939, 1944), Sterling (1945, 1946) and Gifford (1943) have shown that the gymnospermous apex possesses variations in architecture ranging from the massive, zoned apices of the eyeads to the smaller apices of the higher gymnosperms in which the trend is toward the elimination of periclinal divisions in suirface layer and flanking layers. Cross (1943b), in comparing the apices of the Taxodiaceae with those of the angiosperms states: The shoot apices of the Taxodiaceae differ from those of the Angiospermns with respect to only one essential feature, viz., the frequency (greater) with which perielinal divisions occur in the surface layer of the summit. closest approach to complete elimination of perielinal divisions in the superficial layer has been reported from the Gnetales by Gifford (1943) for Ephedra altissima Desf.; here one such division was found in a critical study of seventy stem tips. Little has appeared in the literature on the shoot apex of Gnetum. Bower (1882), in a paper largely devoted to the germination and embryogeny of Gnetutm gnemnon, observed that while the apex of dormant buds was clothed with a discrete superficial layer that of seedlings was frequently occupied by a single large cell and that the superficial cells on the flanks of the apical cone frequently divided perielinally. Haining (1920), however, in a study on the development of the embryo found that the cotyledons, hypocotyl, and