The most ancient of these is the unicellular life-form, which occurs in each of the major algal lineages. The unicellular life-form has two variants - the uninucleate and the multinucleate (e.g., Chlamydomonas and Vaucheria, respectively). In the latter, karyokinesis occurs without coincident cytokinesis. With indeterminate growth in cell size, this condition results in the siphonous growth habit.
The multinucleate unicellular life-form occurs in only two algal lineages (the Chrysophyta and the Chlorophyta). Despite the absence of transverse cell walls in the vegetative phase of growth, some siphonous species are capable of achieving the size and morphological complexity of many terrestrial vascular plants (e.g., Caulerpa).
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