ULTRASTRUCTURE OF MITOSIS AND CYTOKINESIS INKLEBSORMIDIUM MUCOSUMNOV. COMB., FORMERLYULOTHRIX VERRUCOSA(CHLOROPHYTA)1

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Abstract
A transmission electron microscopy study of dividing cells ofUlothrix verrucosaLokhorst has provided clear evidence that this species differs in many respects from otherUlothrixKützing species. These differences include the presence of a microtubular sheath around the prophase nucleus, the complete disintegration of the nuclear envelope coinciding with the proliferation of extranuclear microtubules into the prometaphase nucleus and the intrusion of vacuoles into the interzonal spindle region in between the widely separated telophase nuclei. This necessitates the transfer ofUlothrix verrucosato the charophycean genusKlebsormidiumSilva, Mattox and Blackwell. The new combinationKlebsormidium mucosumis proposed. On account of its mitotic pattern, this species can be placed in the (charophycean) evolutionary line towards the higher plants. However, because of its cytokinesis (annular centripetal ingrowth of the plasmalemma) this species probably should be considered as a blind offshoot of this line. It is emphasized that furrowing green algae with a persistent interzonal spindle at telophase (including the presently studied alga) often show an ill‐defined cytokinetic microtubular system.