Elvira P.R., Sekida S. and Okuda K. 2012. Rhizoid formation in Valonia (Siphonocladales, Chlorophyceae). Phycologia 51: 391–402. DOI: 10.2216/11-31.1Rhizoids were artificially induced by the contact or approach of substrata toward Valonia macrophysa, V. fastigiata and V. aegagropila cell surfaces. A single, spherical cell produced rhizoids locally at the portion that contacted with a glass coverslip, a sheet of cellophane or surface of another Valonia cell. Local induction of rhizoid formation did not always require direct contact with substrata: occurring when two living cells were placed apart but closer than 0.5 mm. Induction of rhizoid formation required continuous contactless exposure to a substratum for at least 48 h. In these cases, amorphous materials were secreted to external surfaces and accumulated in the space between two adjacent cells, but when washed out, the number of rhizoids induced decreased remarkably. The amorphous materials were stained with periodic-acid Schiff's and Alcian Blue and conjugated fluorescein isothiocyanate–labeled lectins that recognize β-d-glucose, α-d-mannose, β-d-galactose, N-acetyl-d-galactosamine and N-acetyl-glucosamine residues. Further analysis using thin-layer chromatography confirmed the presence of galactose, N-galactosamine and N-glucosamine in the amorphous materials; in addition, a high-Rf monosaccharide was also detected. When rhizoid formation was induced, a local aggregation of protoplasm began concomitant with cortical microtubules changing in arrangement from parallel to radial and the contraction of actin filaments. This was followed by disassembly of perinuclear and cortical microtubules in the protoplasmic aggregation. The protoplasmic aggregation was then split from the cell by a septum to become a small lenticular cell, which eventually elongated toward a substratum, differentiating into a rhizoid.