APERTURE EVOLUTION IN THE GENUS PTYCHOPETALUM BENTH. (OLACACEAE)

The genus Ptychopetalum (Olacaceae), disjunct between South America (2 spp.) and Africa (2 spp. West Africa; 1 sp. Rhodesia), exhibits predominantly diploporate pollen among the West African species and predominantly triporate pollen among the South American taxa. An analysis of aperture evolution suggests two possible schemes of aperture evolution for the genus. Scheme I avers the indep'endent origin of both triporate and diploporate apertures from a basic brevicolpate condition while scheme II proposes a basic diploporate condition in which the diploporate apertures have migrated to the equator, fused to form brevicolpi and have ultimately become reduced to pores. Pollen evidence in support of both schemes is discussed. The distribution of aperture types within the genus also suggests West Africa as the original site of aperture evolution and South America as the second area of aperture development with types traceable to West African stock. THE GENUS Ptychopetalum Benth. (Olacaceae), disjunct between South America (2 sp.) and Africa (2 sp. West Africa; 1 sp. Rhodesia), exhibits varying proportions of triporate, diploporate and brevicolpate grains among its populations. The following analysis of aperture variation suggests two possible schemes of aperture evolution for the genus. MATERIALS AND METHODS-Pollen was collected from dried herbarium sheets taken from the following herbaria: Arnold Arboretum (A), Field Museum of Natural History (F), and the Gray Herbarium (GH). Material was first boiled in 10% potassium hydroxide (KOH) for ten minutes, washed twice in glacial acetic acid followed by a ten minute boiling in acetolysis fluid (Erdtman, 1960). Pollen residues were then washed twice more in glacial acetic acid and once in distilled water. Acetolyzed pollen was processed for light, scanning and transmission electron microscopy. Material for light microscopy was mounted in glycerine jelly. Determinations of size and shape were made with a Zeiss screwpiece micrometer. Measurements taken include: polar diameter (pole. diam) and equatorial diameter (eq. diam). 1 Received for publication 30 December 1977; revision accepted 19 May 1978. 2 Present address, Department of Biological Sciences, University of Lethbridge, Lethbridge, Alberta, CANADA TIK 3M4. I wish to thank the curators and staff of the herbaria (A, F, GH) who allowed the removal of pollen material for this study. I am particularly grateful to Dr. Job Kuijt and Mr. Michael Forster for their critical comments and to Ronald Wibel for SEM assistance. This study was supported both by NSF grant #DEB7520279 and the Department of Botany, University of Massachusetts/Amherst. The ratio between these two (P/e) provides an expression of overall shape. Material for the scanning electron microscope was suspended in a drop of water, freeze dried and sputter-coated with gold. Specimens were observed with a Cambridge Mark IIA scanning electron microscope. Samples for transmission work were first placed in .5% aqueous osmium tetroxide for six hours at room temperature, washed in distilled water and embedded in 2% agar (Skvarla, 1966). The pollen/agar moiety was then minced into small pieces and subsequently dehydrated in acetone and embedded in Araldite 6005 (PolySciences). Thin sections were made with an LKB diamond knife, collected on uncoated grids, and stained in aqueous uranyl acetate (ten minutes, room temperature) and Reynolds' (1963) lead citrate (thirty minutes, 500 C). Sections were viewed and photographed on a Zeiss 9S2 transmission electron microscope. RESULTS-The pollen of the South American and West African species of Ptychopetalum is similar in most respects. The pollen is radiosymmetric, isopolar and ranges from suboblate to oblate in equatorial view. The reticulate to tectate-perforate sculpturing, characteristic of all species, is composed of wide, simplibaculate muri and small lumina (Fig. 3, 4). Ultrastructurally, the pollen wall is divided into a slightly undulate perforate tectum supported by thin columellae which rest on a thick smoothly surfaced foot layer (Fig. 1, 5). In contrast, the aperture types of the South American and West African species exhibit significant differences. Diploporate apertures characterize the West African taxa, while triporate