Cell-wall formation in Pelvetia embryos. A freeze-fracture study.

The cell-wall formation in the egg of Pelvetia fastigiata (J.G. Agardh) DeToni (Fucaceae) was studied with freeze-fracture. 1. The wall is lamellated with microfibrils approximately parallel in each lamella. The average orientation of microfibrils turns about 35° in each subsequent lamella. This slow turn gives rise to bow-shaped arcs when the wall is obliquely cross fractured. 2. The organization of the fibrils in the innermost lamellae is visualized by their imprints on the plasma membrane. These imprints are the result of both turgor pressure and adhesion of fibrils to the membrane. 3. Strings of membrane particles appear on the plasma membrane shortly after fertilization. They seem to be formed by a fertilization-induced aggregation of isolated membrane particles. Later each string comes to lie under a fibril and along its imprint. Peculiar lateral rips indicate that some strings are tightly bound to a fibril and may be involved in its orientation. 4. Wall formation in Pelvetia is marked by pronounced secretory activities. Following fertilization, the fusion of cortical vesicles and other vesicles make numerous loci in the plasma membrane. In older embryos, fibril-free patches in the plasma membrane mark the position of microfibril elongation centers in the wall matrix. Prior to germination, these elongation centers and their corresponding membrane patches reach a high density at the presumptive rhizoid end.