The thallus consists of a branched or unbranched cell-filament, formed
of a single row or of many rows of cells, or of an expansion, one or
two layers of cells in thickness, but without conspicuous pores for the
intercommunication of the cells. The growth of the thallus is chiefly
intercalary. The star-like chromatophores contain chlorophyll and are
coloured blue-green with phycocyan, or reddish with phycoerythrin;
all these colouring matters are occasionally found in the same cell
(_Bangia_-species). Asexual reproduction by tetraspores, without cilia,
but capable of amœboid movements.
Sexual reproduction is wanting, or takes place by the coalescence of a
spermatium with a carpogonium, which is only slightly differentiated
from the vegetative cells, and is devoid of a trichogyne. The
carpospores are destitute of cell-wall and arise directly by the
division of the fertilised oosphere. The Bangioideæ occur chiefly in
salt water.
Order 1. =Goniotrichaceæ.=--The thallus consists of a
branched cell-filament without rhizoids. Tetraspores are
formed directly from the entire contents of the mother-cell,
without any preceding division. Fertilisation unknown.
_Asterocystis_, _Goniotrichum_.
The _Goniotrichaceæ_, through the blue-green _Asterocystis_,
are allied to the Myxophyceæ, and through _Goniotrichum_ to the
_Porphyraceæ_.
Order 2. =Porphyraceæ.=--The thallus is formed of an expansion
consisting of a layer of 1–2 cells, which, at the base, are
attached to the substratum by means of a special form of haptera
(_Porphyra_, _Diploderma_); or of unbranched (very rarely
slightly branched) filaments, attached at the base by haptera
(_Bangia_): or it extends from a prostrate cell-disc (various
species of _Erythrotrichia_). Tetraspores are formed after one
or more divisions of the mother-cell, either from the whole or
only a part of its contents; they possess amœboid movements,
or have a jerky, sliding-forward motion. The antheridia have
the same appearance as the vegetative cells, but divide several
times, and several spermatia are formed, either simultaneously
from the whole contents (_Porphyra_, _Bangia_), or the spermatia
are successively formed from a part of the contents of the
antheridium (_Erythrotrichia_). The carpogonium is without a
trichogyne, but the oosphere has a colourless spot which may
sometimes rise a little above the surface of the thallus, and
may be considered as an early stage in the development of the
trichogyne. The spermatia form a canal through the membrane of
the carpogonium, and their contents coalesce with the oosphere
at its colourless spot. The fertilised oosphere divides on
germination into a number of carpospores, which are set free as
naked, motionless masses of protoplasm, which grow and give rise
to new individuals (alternation of generations).