Susan H. Barnes

EMBRYOPHYTE SPORE WALLS: ORIGIN, DEVELOPMENT, AND HOMOLOGIES

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Pollen wall morphogenesis in Tragopogon porrifolius L. (Compositae: Lactuceae) and its taxonomic significance

Abstract Pollen development in Tragopogon porrifolius has been studied in the scanning electron microscope to establish the origin of taxonomically significant features. As in other members of the tribe recently investigated the echinolophate condition is shown to result from differential deposition of the tetrad callose wall and, subsequently, of primexine. Tragopogon differs from other Lactuceae in the absence of tectum and columellae in the lacunae, this also appears to be the result of differential primexine deposition. The development of onci and their extrusion at the apertures are described. The contribution of developmental studies of pollen grains to systematics is discussed.

Cosmos pollen ontogeny: A scanning electron microscope study

SummaryOntogenetic data concerning pollen not only clarifies the mode of deposition of the elaborate walls but has considerable functional and taxonomic relevance. Hitherto such studies have used optical or transmission electron microscopy but here a recently devised preparative technique has enabled pollen development inCosmos bipinnatus to be studied using the scanning electron microscope. The technique involves freeze-fracturing of osmium fixed, cryoprotected anthers, maceration in dilute osmium tetroxide, critical point drying, sputter coating and examination. The processes of pollen wall development can then be observed in three dimensions, an important aid to understanding the spatial relationships involved in the determination of ornamentation and apertures. Details of the pollen and tapetum are described at various stages between meiosis and anthesis. A close conformity is demonstrated between the results obtained and those of earlier transmission electron microscopic studies of the same and related species although very different interpretations are made.

Scanning electron microscopy of chloroplast ultrastructure

Abstract A range of fracturing and sectioning techniques are now available which permit intracellular structures to be observed in the scanning electron microscope. One such technique, based on the method of Tanaka (1981), has been used to study chloroplast ultrastructure in Japan laurel, Aucuba japonica. Small pieces of leaves were fixed, fractured whilst frozen and transferred to a dilute solution of osmium tetroxide in which cytoplasmic maceration took place. Specimens were dehydrated, critical point dried and examined was required to remove the stroma from fractured chloroplasts. Following this treatment details of the chloroplast envelope, frets, grana and plastoglobuli could be observed. The results were compared with conventionally prepared thin sections examined in the transmission electron microscope and with the three dimensional reconstructions described in the literature.

Preliminary observations on the formation of the male germ unit in Catananche caerulea L. (Compositae: Lactuceae)

SummaryWe have used the freeze fracture and cytoplasmic maceration technique to study microsporogenesis inCatananche caerulea L. and here we present preliminary observations of the male germ unit (MGU)in situ. The technique has the advantage of enabling the complex structure of the MGU to be seen in a single fracture plane whereas previously this was only possible by the reconstruction of transmission electron micrographs of serial thin sections. The arrangement of the two generative cells and the vegetative nucleus closely resembles that described from other species includingPlumbago zeylanica.

Harmomegathy in the Acanthaceae

Abstract Harmomegathy was investigated in Diflugossa colorata, Peristrophe speciosa, and Schaueria calycotricha (Acanthaceae) using a SEM with cryo-stage facility. Hydrated pollen from undehisced anthers was compared with relatively dehydrated pollen from naturally dehisced anthers and with acetolysed pollen examined by conventional techniques. The observations suggest that pseudocolpi do not always play the same role in harmomegathy and do not necessarily function in the same way as colpori. Diflugossa exhibits a previously undescribed harmomegathic mechanism. The taxonomic significance of the observations are discussed.

Freeze Fracture and Cytoplasmic Maceration of Pollen Grains

Abstract The application of the freeze fracture and cytoplasmic maceration technique to mature pollen grains is described. In this method anthers are freeze fractured and the cytoplasm of exposed cells is macerated by treatment with a dilute solution of osmium tetroxide. When examined in the scanning electron microscope the specimens reveal details of cell walls and membraneous organelles. It is possible to examine fractured pollen walls complete with intine, the inner surface of which may possess endosculpturing. The disposition of pollen grains within anthers and such features as pollen connecting viscin threads may be studied with the technique.

Low temperature scanning electron microscopy of pollen

Abstract Low temperature scanning electron microscopy using a cryo-stage permits the examination of sputter coated frozen hydrated material. The technique has been used to compare morphological differences between hydrated pollen from either undehisced anthers or stigmas and dehydrated pollen from dehisced anthers. This enables observations of harmomegathic mechanisms, for which optical microscopy has previously been used, to be made at higher magnifications in the scanning electron microscope.