The Stigma, Style, and Ovarian Transmitting Tract in the Oncidiinae (Orchidaceae): Morphology, Developmental Anatomy, and Histochemistry

Abstract

Development of stigma and the pollen tube transmitting tract in column and ovary is described in Lemboglossum maculatum and L uro-skinneri. The most immature stigma examined comprises a shallow depression lined by an epidermis and two to three layers of isodiametric, densely staining, nonvacuolate subepidermal cells. A thin, homogeneous cuticle overlies the epidermis. The stylar and ovarian transmitting tracts are less differentiated than the stigma. Development is marked by an increase in complexity of the stigmatic cuticle and amount of intercellular secretion and by a reduction in cell-cell attachment. A basipetal developmental gradient is apparent along the length of the pistil. The mature stigma is an oval, fluid, and cell-filled depression. The stigma tissue type is fully continuous with that in the tripartite stylar transmitting tract, which traverses the column and branches at the entrance to the ovary to produce six ovarian transmitting tracts. A pair of tracts is associated with each of three placental ridges which, in the unpollinated pistil, bear undifferentiated ovular tissue. In the mature stigma, four layers have been identified. The first layer is a highly modified cuticle, comprising cutin and lipid, enclosing islands of polysaccharide-rich secretory products. Beneath the cuticular layer, an unbroken epidermis overlies a subepidermal "alveolar secretory zone" in which cells are bathed in polysaccharide-rich intercellular secretion. The innermost "transition zone" of small tightly packed cells borders the column cortex. At anthesis the epidermal layer becomes disrupted, and adaxial linkages in the subepidermal layer become weaker. The stylar and ovarian transmitting tissues are anatomically and histochemically similar to the stigma. A cuticular layer is present in only the top half of the style and is progressively less complex than its stigmatic counterpart. Stigma, stylar, and transmitting tract secretions stain positively for protein, carbohydrate, acidic polyanions, and pectic substances. The aspects of the transmitting tract described appear to be common throughout the genera Lemboglossum, Odontoglossum, and Oncidium.