Walker (1971a; Fig. 1) argued that the ancestral pollen type in Annonaceae was monosulcate, with a single elongate germination furrow, or sulcus, as generally assumed for angio- sperms as a whole, and that pollen grains of this type were modified into inaperturates (by loss of the sulcus), disulculates (with two furrows parallel to the equator and perpendicular to the polar axis, which is defined by the centre of the meiotic tetrad and the centre of the microspore), permanent tetrads and polyads