The multicellular life-form is distinguished from the other two major algal life-forms by virtue of the presence of cytoplasmic continuity among some or most adjoining cells. This feature permits the construction of true tissues and tissue systems, and of algal body sizes that rival those of some of largest tree species (e.g., specimens of Macrocystis can grow up to 50 m in length). Three variants of the multicellular life-form exist - the filamentous, the heterotrichous, and the parenchymatous life-form. The filamentous growth habit results when the plane of cell division is confined to one or only two directions with respect to the longitudinal axes of filaments. Unbranched filaments result when the plant cell division is confined to one orientation with respect to the longitudinal axis ofthe plant body; branched filaments result when cells are free to divide in two planes (e.g., Ulothrix and Stigeoclonoum, respectively). Some filamentous life-forms are capable of achieving complex morphologies by virtue of a pseudo-parenchymatous construction, which results from the weaving together of filaments (e.g., Desmarestia).
The heterotrichous life-form consists of a prostrate creeping system of filaments that bears erect filaments projecting into the aquatic medium (e.g., Giraudyopsis). Arguably, this life-form may be considered a variant of branched filamentous body plan provided that creeping and erect portions of the plant thallus are entirely filamentous. However, this lifeform can be structurally intermediate between the filamentous and parenchymatous life-forms when the creeping system of the plant body consists of branched filaments and aggregates of parenchymatous tissue (e.g., Fritschiella).
variants of the colonial life-form exist - noncoenobic colonies have the ability to increase in cell number indefinitely (e.g., Mycochrysis) and coenobic forms have a fixed number of cells once they achieve their mature size (e.g., Pediastrum).
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