Anther Ontogeny and Microsporogenesis in Helianthus annuus L. (Compositae)

In this study, anther ontogeny and microsporogenesis were analysed in Helianthus annuus L. The undifferentiated anther is ovoid-shaped and the differentiation starts with the appearance of archesporial cells. Mature anthers are tetrasporangiate. The anther wall is composed of epidermis, endothecium, middle layer and plasmodial tapetum. Endothecial cells show no fibrous thickening. Tapetum is amoeboid type with binucleate cells. Epidermal layer remains intact until anther dehiscence; however, middle layer, endothecium and tapetum disappear during development. At the end of regular meiotic division tetrahedral microspore tetrads are formed. Pollen grains are triporate, suboblate and angulaperturate.

Structure and systematics of Hanguana, a monocotyledon of uncertain affinity

Floral anatomy and some aspects of leaf and root anatomy of Hanguana are described for the first time and reviewed with respect to its systematic position in monocotyledons. Several characters support its placement in the commelinoid clade, including cell wall ferulates, silica bodies and tetracytic stomata with non-oblique cell divisions. Analysis of morphological data indicates that Hanguana is the sister taxon to Zingiberales. Characters supporting this association include spinulate inaperturate pollen, plasmodial tapetum type, mucilage-secreting intra-ovarian trichomes and modified septal nectaries. Gynoecial nectaries were present in male flowers (probably modified septal nectaries, as in some Zingiberales) and staminodial nectaries in female flowers examined.

Development of the sporangia and associated structures in Schizaea pectinata (Schizaeaceae : Pteridophyta)

A light microscope study of the initiation of the frond and sporangial development in Schizaea pectinata revealed that sporangia arose from single-celled initials in marginal positions on narrow, strap-shaped pinnae. The sporangia were displaced to a superficial position by marginal development of a false indusial (pseudoindusial) structure. Divisions of a single, central initial with four cutting faces produced the archesporial tissue and a two-layered tapetum that differentiated into a combination tapetum consisting of an outer, cellular parietal tapetum and an inner, plasmodial tapetum that was intimately associated with the archesporial tissue and later with the developing spores. Mature sporangia had an apical annulus consisting mainly of a single tier of cells that differentiated from the layer of cells forming the sporangium wall. Thirty-two spore mother cells were produced and if no abortion occurred, approximately 128 spores developed within each sporangium. Key words: Schizaea pectinata, sporangial ontogeny, parietal tapetum, plasmodial tapetum, combination tapetum.

Morphological and ultrastructural variations in Scliizjaea pectinata (Schizaeaceae: Pteridophyta)

Schizaea pectinata (L.) Sw. was collected from the extreme ends of its geographical range in South Africa for a study of sporangial development, sporogenesis and tapetal organisation. Differences were noted in the gross morphology , in sporangium size, spore size and in the patterning of the outer exospore from the two sites. Coiled structures were associated with the development of the inner perispore in spores collected from the Transvaal, whereas dense, heterogeneous bodies were associated with the formation of this layer in spores from the Cape. Differences were also noted in the organisation of the tapetum. A cellular, parietal tapetum and a plasmodial tapetum were present in the Cape material when the spores had developed the sculptured outer exospore. In sporangia from the Transvaal, however, only a plasmodial tapetum was present at the same stage of sporoderm development. A detailed study of S. pectinata throughout its distribution is required to determine the taxonomic importance of these findings.

Maturation of the megaspore in Marsilea vestita

The course of megasporogenesis was followed from the simultaneous cleavage of the mother cell at the termination of meiosis to the formation of the compound wall around the single surviving megaspore in the maturing sporangium. The favoured tetrad that yielded the surviving megaspore was always that closest to the stalk of the sporangium. Although no asymmetry could be detected in the tetrad, three of the spores regularly degenerated. The beginning of degeneration was sometimes evident before the release of the spores, but only after the spores had acquired distinct walls. Cytochemical tests indicated that separation of the spores was facilitated by the mother cell wall becoming fluid. Growth of the maturing megaspore was accompanied by conspicuous nucleocytoplasmic interaction. Tubular extensions of the nucleus could be followed in the cytoplasm for more than 1 μm, but no connections could be detected with any organelles. The central space into which the megaspore grew was probably generated partly by the expansion of the sporangium as a whole, and partly by the digestion of the plasmodial tapetum. The wall of the mature spore was clearly compound, the unstructured exine of the growing spore eventually meeting and fusing with a chambered component formed at the periphery of the tapetum.