UNIDIRECTIONAL ORGAN INITIATION IN LEGUMINOUS FLOWERS

The order of initiation of floral organs is compared in several legumes. In Bauhiniafassoglensis, a caesalpinioid, the sepals are initiated helically, with the first one forming abaxially. In Genista tinctoria and Lupinus aff nis (both papilionoids) the sepals are initiated unidirectionally, with the first forming on the abaxial side of the floral apex and subsequent sepals initiating laterally and then adaxially. All three taxa show unidirectional order of initiation for petals, first-whorl stamens, and second-whorl stamens. In each whorl, the first member or members form on the abaxial side, next to the subtending bract, then the lateral ones, and lastly the member(s) on the adaxial side, next to the axis. In Lupinus and Genista there are overlaps in time of initiation between organs in different whorls; for instance, the first stamens begin initiating before the last petals appear. Size differences among members of a whorl are evident in early stages, but may disappear after organogeny ceases, when the members become equal in size in each whorl. This precocious onset of dorsiventrality in floral development is viewed as a specialized feature. THE ORDER of floral appendage initiation is usually described as either helical, or simultaneous and whorled. A third possibility, unidirectional, from one side to the other of the floral apex, has long been known and reported for certain legumes (Rohrbach, 1870; Frank 1876; Eichler, 1878; Hartog, 1888), but has been overlooked in recent work on floral apices. In this paper, the order of floral organ initiation in representative caesalpinioid and papilionoid legumes is described, and a survey of unidirectionality reported among legumes is presented. Unidirectional organ initiation in an apical meristem is combined in many legumes with overlapping initiation of two kinds of organs simultaneously; the implications of this phenomenon do not fit well with interpretations of the apical meristem and its stepwise competencies. The current paper is a progress report of work to date on a broad, comprehensive study of comparative floral development in legumes, the scope of which was established in a previous paper (Tucker, 1984). METHODS AND MATERIALS-Developmental stages of inflorescences and flowers of about 150 species of legumes, including Lupinus af' Reeeived for publieation 25 November 1983; aeeepted 19 February 1984. This work was funded by National Seienee Foundation Grant DEB-82-04 132. The author is indebted to Kittie S. Derstine for her skilled assistanee with SEM and photography. The generous help in allowing colleetions of plant material at Fairehild Tropieal Gardens, Miami, Fla., and at the Royal Botanie Gardens, Kew, Richmond, Surrey England, is aeknowledged with appreeiation. finis Agardh and Genista tinctoria L., were collected at the Royal Botanic Gardens, Kew, England, in the spring of 1982 and fixed in formalin-acetic acid-alcohol (FAA) or in "Kew Mixture," in which glycerine is substituted for the acetic acid. Woody tropical plants, including Bauhinia fassoglensis Kotschy, were collected at Fairchild Tropical Gardens, Miami, Fla. in November 1982 and March 1983, and fixed in FAA. Later, some of the inflorescences and flowers were dissected in 95% alcohol under a dissecting microscope to reveal early floral stages as well as later stages of critical structures. These were dehydrated and critical point dried in a Denton DCP1 apparatus, mounted on stubs with Tubekote, and coated with goldpalladium in a Hummer II sputter coating apparatus. The preparations were viewed with a Hitachi S-500 scanning electron microscope (SEM) at 25 kv, and SEM micrographs were taken. Terminology-Terms diXer for referring to a particular side of the flower. To facilitate comparing our descriptions with conflicting terminologies in the literature, they are here explai-ned. The side of the flowest closest to the subtending bract is termed the abaxial or anterior side; the side closest to the main axis is termed the adaxial or posterior side. The vexillum or standard petal is on the adaxial posterior side, as are the vexillary stamen and the carpellary cleft. RESULTS-Investigations are complete on the order of floral organ initiation in three legume species, and preliminary or partial results are