In the Thallophytes the cytoplasm may be segmented by constriction, due to the in-growth of a new cell wall from the old one, as in Spirogyra and Cladophora, or by the formation of cleavage furrows in which the new cell-wall is secreted, as occurs in the formation of the spores in many Algae and Fungi.
In Ulva and Mesocarpus the chromatophore is a single plate, which in the latter genus places its edge towards the incident light; in Spirogyra they are spiral bands embedded in the primordial utricle; in Zygnema they are a pair of stellate masses, the rays of which branch peripherally; in Oedogonium they are longitudinally-disposed anastomosing bands; in Desmids plates with irregular margins; in Cladophora polyhedral plates; in Vaucheria minute elliptical bodies occurring in immense numbers.
Usually two cilia are present; in Botrydium and Hydrodictyon only one is present; in certain species of Cladophora four; in Dasycladus a chaplet, and in Oedogonium a ring of many cilia.
The escape of zoospores is effected by the degeneration of the sporangial wall (Chaetophora), or by a pore (Cladophora), a slit (Pediastrum), or a circular fracture (Oedogonium).
Cladophora and Bryopsis among monosiphonous forms, Chara, Polysiphonia, Ceramium and Cystoseira among larger algae, are illustrations of this.
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