Except for Biflagellate Gametes, there are NO Unicellular Forms. They have the Greatest Degree of Morphological Complexity among the Algae. Life Modes - The Phaeophyta are autotrophs. The Unicellular Filaments of some Phaeophytes have Intercalary Growth. This means that cells other than Apical Cells divide and contribute to the overall elongation of the Filament.
Growth can occur in any part of the filament.
Cell division is thus, diffuse. In the simplest case the divisions occur in only the Transverse Plane and thus, contribute to the length of the filament. However, longitudinal divisions can occur periodically and produces branches. Longitudinal divisions by cells of the Primary Axis can produce a partial sheet-like morphology. This is called a Corticated Thallus.