No abstract

Anther development, microsporogenesis and microgametogenesis of six species of the genera Corchorus, Heliocarpus, Luehea and Triumfetta were analysed. The genera were found to share the following characters: ontogeny of anther wall of basic type; simultaneous microsporogenesis, resulting mainly in tetrahedral tetrads; secretory tapetum and pollen grains shed at the bicellular stage. Moreover, the characters that differentiate them are: presence of uninucleate tapetal cells in Heliocarpus and Triumfetta; binucleate cells in Corchorus and multinucleate cells in Luehea; differentiation of the thickenings of the endothecium at free microspores stage in Corchorus, Heliocarpus and Triumfetta, whereas in Luehea differentiation occurs at the mature pollen grains stage; late disintegration of sporogenous tissue cell walls in Luehea; and the presence of orbicules, absent only in Corchorus. This is the first embryological report of the Grewioideae subfamily, contributing to the characterization of the genera studied. The results are discussed in relation to the known data for the family.

The pollen of Cliococca selaginoides (Lam.) C. M. Rogers & Mildner and eight Linum L. species from Argentina were examined using light and scanning electron microscopy. Both genera share pollen grains which are 3zonocolpate, isopolar, radiosymmetric, spheroidal and medium to large in size. The sculptural elements of the exine are gemmae or clavae, which allow the genera to be distinguished and the species to be characterised. In Cliococca Bad., the pollen is exclusively gemmate whereas in Linum the pollen is gemmate and clavate (except in L. catharticum L. whose pollen has only gemmae which are apically microechinate). The statistical analyses of quantitative and qualitative morphological characteristics were performed. A key to identify and distinguish the pollen types and subtypes is also provided. A generalized procrustes analysis (GPA) of joint characterisation based on both qualitative and quantitative characters identified four distinct groups.

The style morphology and anatomy vary among different species. Three basic types are: open, closed, and semi-closed. Cells involved in the pollen tube pathway in the different types of styles present abundant endoplasmic reticulum, dictyosomes, mitochondria, and ribosomes. These secretory characteristics are related to the secretion where pollen tube grows. This secretion can be represented by the substances either in the canal or in the intercellular matrix or in the cell wall. Most studies suggest that pollen tubes only grow through the secretion of the canal in open styles. However, some species present pollen tubes that penetrate the epithelial cells of the canal, or grow through the middle lamella between these cells and subepithelial cells. In species with a closed style, a pathway is provided by the presence of an extracellular matrix, or by the thickened cell walls of the stylar transmitting tissue. There are reports in some species where pollen tubes can also penetrate the transmitting tissue cells and continue their growth through the cell lumen. In this review, we define subtypes of styles according to the path of the pollen tube. Style types were mapped on an angiosperm phylogenetic tree following the maximum parsimony principle. In line with this, it could be hypothesized that: the open style appeared in the early divergent angiosperms; the closed type of style originated in Asparagales, Poales, and Eudicots; and the semi-closed style appeared in Rosids, Ericales, and Gentianales. The open style seems to have been lost in core Eudicots, with reversions in some Rosids and Asterids.