Promotion of Fern Rhizoid Elongation by Metal Ions and the Function of the Spore Coat as an Ion Reservoir

The perine, or outer coat, of spores of the fern Onoclea sensibilis L. may be chemically removed by a brief treatment with dilute NaClO. Treated spores germinate normally on glass-redistilled H(2)O, but elongation of the rhizoid which is differentiated during germination is severely limited. Rhizoid elongation in perine-free spores, however, is normal when the spores are germinated on Knop's mineral medium or on single-salt solutions of Ca(2+), Mn(2+), or Mg(2+). In intact spores which retain their perine, rhizoid elongation is normal on distilled H(2)O, and the perine serves as a source of ions which are available to the spores and can sustain rhizoid elongation, even when the external medium is deficient. Electron micrographs show that there are structural differences in the rhizoid wall between perine-free spores germinated on distilled H(2)O or on nutrient solutions, and also a difference in the number of vesicles in the apical cytoplasm. Localization of Mg(2+) and Ca(2+) in the elongating rhizoid can be visualized with chlorotetracycline fluorescence. No concentration of these ions can be detected by this technique in the small rhizoid initial cell before cell elongation begins.