Developmental Studies in Ptychosperma (Palmae). I. The Inflorescence and the Flower Cluster

Abstract

This paper describes inflorescence structure, including organogenesis of the panicle and flower clusters and vasculature of flowering branches, for two species of Ptychosperma, a genus of arecoid palms. The inflorescence is an infrafoliar panicle with up to four orders of branches in a spirodistichous arrangement conforming to an irregular one‐half phyllotaxy. The primordium of the inflorescence is crescentic and the apex has two tunica layers, a group of central cells, and a rib meristem. The distal flower‐bearing parts or rachillae of all branches develop acropetally early in ontogeny and are vertically oriented in the bud. Although these rachillae terminate branches of different sizes and orders, they are similar in size and in number of flower clusters produced. Internodes and lower parts of branches develop later. Bracts of four types are produced: a prophyll and empty peduncular bract, bracts which subtend lateral branches, bracts subtending triads, and floral bracteoles. The prophyll and peduncular bracts are tubular and completely closed around all branches until about three months before the flowers reach anthesis. Bracts subtending lateral branches and those that subtend triads enlarge by small amounts of apical, adaxial, and marginal growth to cover subtended apices during early ontogeny, but are small to absent at maturity. Flower clusters are triads of two lateral staminate and a central pistillate flower. Organogenesis indicates that the triad is a sympodial unit. Flowers develop successively, each floral apex bearing a bracteole that subtends the next flower. The vasculature of the inflorescence may be divided into two systems. Bundles of the main axis extend acropetally into the vertically oriented branches as they are initiated and form a central cylinder of larger bundles in each branch. Flower clusters are supplied by a peripheral system of smaller bundles that develop later in relation to the developing floral organs. Bundles of the peripheral system branch frequently, but branching levels are irregular. The irregular branching of peripheral bundles appears related to the phyllotaxy of the flower clusters and the random right or left position of the first flower of the triad. The level of branching of a bundle may depend on the position of a floral primordium with respect to an existing procambial strand. Three (‐4) bundles supply each staminate flower and six (‐10) the pistillate flower. The histologically specialized inflorescence has stomata and contains abundant starch. Tannins and raphides, spherical silica bodies, and various forms of sclerenchyma appear in sequence and apparently provide support and protection during the long exposure of the branches.

The palm family (Arecaceae): a microcosm of sexual system evolution

“…The triad is a highly condensed sympodial flower cluster consisting of a female flower flanked by two male flowers (Fig. D, E) (Uhl, , ). Male and female functions may be further separated by the production of fully developed triads at the base of the rachillae and only dyads of male flowers or single male flowers (interpreted as degenerate triads in which the female flower is lacking) present in the distal part.…”