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.