1. Sub-Family. Zoogonicæ.
Reproduction by means of gametes and zoospores.
Order 1. Ectocarpaceæ. The thallus consists of single or branched filaments with intercalary growth, extending vertically from a horizontal, branched filament or a disc, but sometimes it is reduced to this basal portion only. Zoosporangia and gametangia (for fertilisation see Fig. [66]) are either outgrowths or arise by the transformation of one or several of the ordinary cells. The most common genera are: Ectocarpus and Pylaiella.
Order 2. Choristocarpaceæ. Choristocarpus, Discosporangium.
Order 3. Sphacelariaceæ. The thallus consists of small, parenchymatous, more or less ramified shoots, presenting a feather-like appearance. In the shoots, which grow by means of an apical cell (Fig. [68] S), a cortical layer, surrounding a row of central cells, is present. Sporangia and gametangia are outgrowths from the main stem or its branches. Sphacelaria, Chætopteris are common forms.
Fig. 68.—Apex of the thallus of Chætopteris plumosa. S Apical cell.
Order 4. Encoeliaceæ. Punctaria, Asperococcus, Phyllitis fascia.
Order 5. Striariaceæ. Striaria, Phlœospora.
Order 6. Dictyosiphonaceæ. Dictyosiphon.