Figure 1. Schematic diagram of brown algal development. (A) After fertilization, the zygote is initially spherical and apolar. During the next 14 hours, the zygote will develop a cytological asymmetry that leads to polarity visible at the gross morphological level. The orientation of that polarity is determined by an external gradient, such as unilateral light (arrow). (B) The initial sign of polarity visible at the whole-cell level is rhizoid tip elongation, which occurs at 10 to 14 hours after fertilization. Secretory vesicles are associated with the localized tip growth that occurs on the shaded side of the zygote. (C) At approximately 24 hours post-fertilization, the initial cell division occurs, dividing the zygote asymmetrically and producing two morphologically distinct cells. The larger, nearly spherical thallus cell contains most of the chloroplasts, whereas the smaller, elongated rhizoid cell contains more secretory vesicles and mitochondria, and continues to exhibit tip growth. (D) Further divisions give rise to an embryo which resembles that of higher plants at the globular stage. Descendants of the thallus cell will eventually give rise to the reproductive and photosynthetic structures of the plant (the fronds). Cells derived from the rhizoid continue to elongate via tip growth, and will form the mature plant's holdfast, which attaches to the underlying rocks. (E) A mature germling with multiple rhizoids that form the holdfast and apical hairs.