Figure 3: Diagram of the components believed to be localized at the fixed site of polar growth in spherical Fucus zygotes. Localization of actin, DHP receptors and free calcium ions are shown at the site of polar growth just prior to emergence of the rhizoid. Since the contents of the F granules (e.g. F2 and VnF) have been incorporated into the cell wall, the site is fixed and apparently signals to the underlying cytoplasm (arrows), providing positional information to initiate polar growth, target secretory vesicles, and orient the plane of the first cell division.