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.
[Illustration: FIG. 68.--Apex of the thallus of _Chætopteris
plumosa_. _S_ Apical cell.]
Order 4. =Encoeliaceæ.= _Punctaria_, _Asperococcus_, _Phyllitis fascia_.
Order 5. =Striariaceæ.= _Striaria_, _Phlœospora_.