Surface-viewed shoot apex ofAngiopteris lygodiifolia Ros. (Marattiaceae)

Marattian ferns are thought to be an exception to the rule that a single apical cell is always present in the shoot apex of ferns; the occurrence of plural apical initials has been generally accepted for these ferns. However, a contradicting conclusion was reached in this study which examined the apical organization of the shoot ofAngiopteris lygodiifolia Ros., using fresh materials which had not been fixed. Shoot apices were hand-sectioned transversely into thin sections, including the surface layer of the shoot apex, which were observed by differential interference contrast microscopy without staining. In contrast with the generally accepted view, the shoot apex ofA. lygodiifolia was found to usually possess a single apical cell with three cutting faces. The segments cut off from the apical cell are regularly arranged in a helical sequence. The apical cell seems to actually function as an initial cell of the whole shoot apex. The shoot apices, particularly those of plants cultivated in a greenhouse, sometimes show somewhat irregular organization. In extreme cases, no apical cell is recognizable. However, even in these exceptional cases of such apparently irregular shoot apices, plural apical initials are not found.