Male and female gametophyte development in Achillea tenuifolia (Asteraceae)

The anther, pollen and ovule development in Achillea tenuifolia were studied with a bright field microscopy. Results showed that the anther is of tetrasporangiate type and the anther wall is composed by four layers: an epidermis, an endothecium, one middle layer and a tapetum layer. Tapetum is of secretory type and its cells showed polyploidy. Pollen tetrads were tetrahedral, microspores were very irregular and contained large amounts of starch at the time of dispersion. Pollen grains were generally tricolporate, and in some cases were tetracolporate or even pentacolporate with spines on surface. The size of the pollen grains varied and ranging from 18-42.5 μm at the polar axis and from 16.5-35.5 μm at the equatorial axis. Ovule is anatropous, unitegmic and tenuiucellate. The archesporium may consist of one or more archeosporial cells, but only one of them undergoes meiosis, forming a linear or T-shaped tetrad. A 7-celled embryo sac is formed corresponding to the Polygonum type. Embryo sac is very tiny at the beginning of its development, its size was increased considerably at late growth stages. The relationship between Asteraceae, Calyceraceae and Goodeniaceae are discussed but based on embryological evidence, Goodeniaceae appear to be the putative sister group of Asteraceae. To understand more exact relationships within the order Asterales, embryological studies are recommended.