The development of the exine of young acetolysed pollen grains ofEchinops banaticushas been studied by measuring the thickness of the mesocolpia and apocolpia. Within the pollen mother cell wall the exine develops more or less uniformly except at places where apertures will be formed. After the release of the pollen grains from the pollen mother cell wall the thickness of the apocolpia decreases gradually. At the same time the young pollen grain is stretched in the direction of the polar axis. It is suggested, that the volume enlargement of the cell content is correlated with material transport from the tapetum through the apocolpia.