FEATURES OF DICHOTOMIZING APICES IN FLAGELLARIA INDICA (MONOCOTYLEDONES)

During vegetative growth aerial shoots of Flagellaria indica bifurcate regularly at intervals even though axillary meristems are absent. Dichotomy involves no change in the cytohistological organization of the apical meristem; the shoot apex simply enlarges in the plane of distichy, a median furrow appears, and two new equal centers of meristematic activity are constituted in a process of continuous growth. Dichotomy can occur at any stage in the plastochron, and the morphology of the leaf immediately below the fork may be modified under the influence of the two new meristematic centers. The first leaf on each of the new apices is always on the same side, so there is no mirror image symmetry. The suggestion that dichotomy is the result of precocious axillary branching is discussed but rejected. RECENT collections of vegetative aerial apices of Flagellaria indica L. (Flagellariaceae) allow successive stages in vegetative dichotomy to be documented, and the anatomy of apices during this process of bifurcation is now reported in detail. In an earlier account (Tomlinson, 1970) essential morphological features of this process of dichotomy had been established, with the prediction that a similar method of branching might occur in other monocotyledons. This prediction has ' Received for publication 11 October 1976; revision accepted 20 January 1977. 2 Research on woody monocotyledons supported in part by NSF Grant GB-3 1844-X. The senior author is grateful for research support from the National Geographic Society, which made the visit to Papua New Guinea possible. The field assistance of Camilla Huxley, Department of Biological Sciences, University of Papua New Guinea is gratefully acknowledged. Anne Faulkner was responsible for all the paraffin sectioning. We thank J. G. Torrey and M. H. Zimmermann for critical reading of the manuscript. been substantiated by a series of papers which document in greater or lesser degree this distinctive kind of vegetative branching. As summarized by Fisher (1976), examples are now known in the Palmae, e.g., Chamaedorea (Fisher, 1974), Hyphaene (Halle and Oldeman, 1970), Nypa (Tomlinson, 1971), and in the Strelitziaceae, e.g., Strelitzia reginae (Fisher, 1976). Precocious branching, difficult to distinguish from dichotomy, has been recorded by Wilder (1974a, b) in certain members of the Alismatidae where it is characteristic of highly organized, usually reproductive, branch systems which are perhaps not comparable to the simple bifurcation of the vegetative shoot in woody monocotyledons. Furthermore, apical dichotomy has been recorded recently in the dicotyledons in the description of two species of Mammillaria (Cactaceae) by Boke (1976). From this it is clear that the angiosperm vegetative shoot apex has a capacity, rarely expressed, for equal dichotomy. KEY TO LABELING: P1, youngest leaf primordium visible on the shoot apex; P2, P3, P4, etc., the next oldest successive leaf primordia; P', P", leaf primordia formed on apices resulting from dichotomous branching; r, removed, e.g., rP, removed leaf primordium; RM, rib meristem; V, vegetative apical meristem. Note that magnification of all photomicrographs of whole buds is the same, as is the magnification of all corresponding sectioned material. Fig. 1-6. Side views of vegetative apices just prior to dichotomous branching. 1. Undivided apex; one leaf (rP3) removed to allow unobstructed view. 2. Median longitudinal section through same apex as that of Fig. 1. Note the clearly defined rib meristem (RM). 3. Broad apex, probably representing initial stages of branching. 4. Median longitudinal section of apex in Fig. 3. 5, 6. Two similar stages showing flattened top with enlarged cells (arrows), usually indicative of first stages in dichotomous branching. Fig. 7-12. Side views (Fig. 12 oblique top view) of critical stages in dichotomous branching and the establishment of two new growth centers. 7. Externally this flat-topped apex still appears undivided. 8. Median longitudinal section through same apex as in Fig. 7. 9. Two new centers of meristematic activity (V) just becoming visible externally. 10. Median longitudinal section through same apex as in Fig. 9. Note the periclinal divisions below tunica (arrows). 11, 12. Two views of the same apex at a critical, early stage in branching. Note enlarged epidermal cells (arrow) between growth centers. See above for Key to Labeling.