Tapetum Types and Forms in Angiosperms

Based on the analysis of literature and our own data, we have suggested a new version of the typification of types and forms of the tapetum. It is proposed to distinguish two types of tapetum: parietal and periplasmodial. Parietal tapetum lines the locule of microsporangium and this position is maintained throughout the development. A periplasmodial tapetum is characterised by the formation of a coenocyte as a result of the fusion of protoplasts, while the cytoplasm and nucleus are located not only between the developing microspores and pollen grains, but also come into contact with the inner layers of the cavity. The differences between tapetum types relate to the peculiarities of structural and temporal reorganisation in anther development. The protoplasts that form after the disappearance of the cell walls (usually at the stage of microspore tetrad, or after their disaggregation), gradually break down (form 1 – typical parietal tapetum), or they form protrusions inside the microsporangium cavity (form 2 – amoeboid tapetum). The protoplasts in the periplasmodial tapetum are formed before or during meiosis. They fuse resulting in a symplast. It is possible to distinguish two forms of periplasmodial tapetum: combining and invasing of protoplasts into the locule of microsporangium (form 1 – typical periplasmodial tapetum), and almost or partly fusion of protoplasts, which do not organise the invasions and maintain the initial position (form 2 – bordering symplast). Data on the diversity and structure of the tapetum, like any other characters, are used to clarify the position of taxa on the phylogenetic tree.

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.

An acetolysis-resistant membrane investing tapetum and sporogenous tissue in the anthers of certain Compositae

After the breakup of the meiotic tetrads, it is usual in the Compositae for the cells of the previously parietal tapetum to invade the anther loculus and form a Plasmodium ramifying between the young spores. In species of three tribes, Helenieae, Heliantheae, and Ambrosieae, an extratapetal membrane is formed at this time, which totally encloses the tapetum and the spore mass. The extratapetal membrane is resistant to acetolysis, and is considered to be chemically similar to the sporopollenin of the pollen grain exine.