Development and expansion of the pollen wall in Barleria obtusa Nees (Acanthaceae)

Anthers containing pollen grains of Barleria obtusa at various stages of development were examined to investigate pollen wall ontogeny. Whole anthers containing pollen grains were viewed with a light microscope, firstly to identify stages of pollen grain development and secondly to assess the state of the tapetum. Developing pollen grains were sectioned with a Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) in order to determine the suitability of this method for an investigation of exine ontogeny. The pollen wall structure and in particular the development of the external elaborate, reticulate sculpturing were examined. The exine was deposited on the immature pollen grains during the tetrad phase of development with the sculpturing, although somewhat condensed, already apparent at this early stage. The subsequent stages of development show the sculpturing advancing towards its final expanded structure as the grain increases in size. The tapetum was unusual in that it not only lined the anther locule but also surrounded each tetrad group without invaginating the tetrad arrangement, thus defining it as a secretory type.