ONTOGENY OF THE INFLORESCENCE OF SAURURUS CERNUUS (SAURURACEAE)

The spicate inflorescence of Saururus cernuus L. (Saururaceae) results from the activity of an inflorescence apical meristem which produces 200-300 primordia in acropetal succession. The inflorescence apex arises by conversion of the terminal vegetative apex. During transition the apical meristem increases greatly in height and width and changes its cellular configuration from one of tunica-corpus to one of mantle (with two tunica layers) and core. Primordia are initiated by periclinal divisions in the subsurface layer. These are common" primordia, each of which subsequently divides to produce a floral apex above and a bract primordium below. The bract later elongates so that the flower appears borne on the bract. All common primordia are formed by the time the inflorescence is about 4.4 mm long; the apical meristem ceases activity at this stage. As cessation approaches, cell divisions become rare in the apical meristem, and height and width of the meristem above the primordia diminish, as primordia continue to be initiated on the flanks. Cell differentiation proceeds acropetally into the apical meristem and reaches the summital tunica layers last of all. Solitary bracts are initiated just before apical cessation, but no imperfect or ebracteate flowers are produced in Saururus. The final event of meristem activity is hair formation by individual cells of the tunica at the summit, a feature not previously reported for apical meristems. INFLORESCENCE MERISTEMS provide some of the most common examples of determinate apical meristems. As part of a research program on apical meristem cessation, the inflorescence of Saururus cernuus L. (Saururaceae) is being investigated developmentally. Although the flower has been thoroughly studied (Murty, 1959; Raju, 1961; Tucker, 1975; 1976a), the inflorescence has received little attention. The aims of the present paper are 1) to examine changes in size, shape, and configuration of the inflorescence meristem during its active phase and during onset of senescence, and 2) to follow the sequence of organ initiation by the inflorescence meristem and to compare it with that of other prolific apical meristems. A third aim of the study is to provide detailed information about the last few primordia around the inflorescence summit, as a basis for comparison with other saururaceous genera having terminal flowers or peloria. MATERIALS AND METHODS-A series of early stages of inflorescences and terminal mixed buds of Saururus cernuus were collected from wild populations at Burden Research Plantation in Baton Rouge, Louisiana, and near Alexandria, I Received for publication 9 January 1978; revision accepted 14 November 1978. The capable technical assistance of Michael T. Postek is acknowledged with thanks. The work was funded on NSF grants GB40083 and DER7610108. Louisiana, over a two-year period. Buds were dissected immediately, and inflorescences were fixed for scanning electron microscopy, for paraffin embedding, and for plastic embedding. Methods used for paraffin work (Tucker, 1975) and for plastic embedding (Tucker, 1976b; Postek and Tucker, 1976) have been described previously. Light micrographs were prepared with a Leitz Ortholux microscope and Orthomat automatic camera. Methods for SEM preparation for the early work (Tucker, 1975, 1976a) have been somewhat modified for more recent work. The fixative was 10% paraformaldehyde buffered in s-collidine at pH 7.4 (Winborn and Seelig, 1970) for 2 hr; the material was postfixed in osmium and thiocarbohydrazide in a ligand-osmium binding technique (Postek and Tucker, 1977). The material was dehydrated chemically with 2,2-dimethoxypropane (DMP) (Postek and Tucker, 1976) for rapid and effective removal of all water. The preparations were critical-point-dried with a Denton DCP-1. The ligand-osmium-impregnated material did not require coating; some of the inflorescences prepared earlier and not treated in this way were sputter coated with gold-palladium with a Technics sputter coater. Most SEM micrographs were prepared with a Hitachi S-500 at 25kV; some of the early work was done with a