PHYLLOTAXIS AND VASCULAR ORGANIZATION OF THE CARPELS IN MICHELIA FUSCATA

TUCKEII, SiIIRLEY C. (U. Minnesota, Minneapolis.) Phyllotaxis and vascular organization of the carpels in Michelia fuscata. Amer. Jour. Bot. 48(1): 60-71. Illus. 1961.-Phyllotaxis pattern and vascular organization are closely related in the floral receptacle of Michelia fuscata (Magnoliaceae). The carpels arise in a spiral or helix. They are initiated alternately along each of 7, 8 or 10 helical parastichies according to a complex repetitive seqluence. The pattern of the dorsal carpellary trace fusions is orderly for each of the 10 flowers investigated. The dorsal carpellary traces in each parastichy diverge from the same vascular sympodium. Among flowers one finds differing numbers of parastichies, differing angles of divergence, and varying sequences of parastichies which reflect the order of carpel initiation. The angle of divergence, although consistent for any l parastichy in a flower, can vary greatly between parastichies. The natuie and importance of the organizers which determine appendage position at the apical meristem are considered. Changes in apical size, configuration, and activity are shown to be related to phyllotaxis. TiuE ONTOGENY of Michelia juscata Blume, a phylogenetically primitive flower in the Magnoliaceae, is being investigated with emphasis upon a study of factors which affect the position of the appendages at the time of their initiation upon the floral and vegetative meristems. Preliminary observations have been concentrated on 2 phases: the plastochronic changes in the floral apex during the initiation of successive floral appendages, and the relationship between phyllotaxis of floral parts and vascular organization. An earlier paper (Tucker, 1960) explored the cellular configuration, shape, and development of the floral meristem. The subjects of phyllotaxis and vascularization of the flower will be subdivided for convenience. The present paper describes the phyllotaxis and vascular pattern of the carpels which are located on the uppermost portion of the attenuate receptacle. Such information should facilitate the later study of phyllotaxis in the lower, "telescoped" portions of the receptacle which bear the perianth parts and the stamens. Floral phyllotaxis, like that of vegetative shoots, reflects the precise timing and pattern of initiation and differentiation of appendages on the apical meristem. But floral phyllotaxis, particularly of the stamens and the carpels in magnoliaceous flowers. is far more complex than that of vegetative shoots. The 80-90 or more appendages of a Michelia flower include 3 stipular bracts, 6 perianth parts, 32--40 stamens, and 36-50 carpels; these are initiated in acropetal alternate succession on a long, narrow receptacle. The large number of stamens and carpels in Michelia is useful in a study of phyllotaxis. In 1 Received for publication April 18, 1960. This research was supported in part by the National Science Foundation. I am indebted to the Junior F. Hayden fund, Department of Botany, University of Minnesota, for a grant to defray part of the cost of publication. flowers, the arrangement of parts is related to the number and size of appendages, and to the productive surface area on the flanks of the floral apex. The productive area may be extended in the dimension of time (as in Michelia, Tucker, 1960) as the result of prolonged apical activity. During this period many appendages are initiated, axial increments are added, and the apex undergoes plastochronic changes. The sequence of carpel initiation and the pattern of the vascular system provide objective bases for describing phyllotaxis in the flower. METIIODS AND MATERIALS.-The standard histological techniques were used for preparing serial microtome sections of Michelia fuscata flowers, as described in a previous paper (Tucker, 1960). Diagrams of phyllotaxis were constructed for each of 10 flowers on the basis of serial microtome transections. Camera lucida tracings of transections 7-35,u apart were made throughout each serially cut flower. The sections were selected to show the level of each departing dorsal trace and the level of exsertion of the carpel base from the receptacle. The carpels were numbered chronologically from the base to the tip of the receptacle. The angles of divergence were measured on the camera lucida drawings between median bisects of successive carpels at comparable levels. Phyllotaxis, determined after the ontogenetic order of carpels was known, was verified by measuring angles of divergence and studying the pattern of fusion in the vascular system. The course of the dorsal traces was followed basipetally from the summit; the traces were numbered on the drawings, and their fusions and changes in relative position were observed. Measurements were taken from the floral apex downward. A large number of flowers (with perianth and stamens removed) were cleared in sodium hypochlorite (5%)) for 6-24 hr. Basic fuchsin or safranin was used to stain the vascular system.