Did Multicellular Plants Invade the Land?

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Abstract
Evidence and arguments are presented to support the hypothesis that plants entered terrestrial habitats as unicellular, relatively undifferentiated forms. The multicellular forms placed by Stewart and Mattox in their Phylum Charophyceae evolved principally in moist terrestrial habitats, and modern orders of this phylum (Zygnematales, Coleochaetales and Charales) probably are secondarily aquatic. The main terrestrial line of this phylum is extinct except for a few simple filamentous forms. Between the time when the Charales diverged and the appearance of the grade represented by modern mosses and liverworts, as well as the upper Silurian vascular plants, isomorphic alternation of generations evolved. Its adaptive value lay partly in the adaptiveness of polyploidy itself, which is demonstrated by the presence of polyploid species of algae, and partly in the adaptiveness of a haploid-diploid cycle in a seasonally fluctuating habitat or in a place where 2 somewhat dissimilar habitats exist side by side. The differentiation of vascular from nonvascular archegoniates was associated with adaptation of vascular forms to favorable stable or climax habitats, and of nonvascular forms to temporary pioneer habitats.

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