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). Family 2. =Florideæ.= The thallus has one or more apical cells, grows principally by apical growth, and may be differentiated into root, stem, and leaf. The chromatophores vary in form, but have a red or brownish colour, due to chlorophyll and phycoerythrin. Asexual reproduction by motionless tetraspores, which generally arise by the division into four of the contents of the tetrasporangium. The carpogonium has a trichogyne, and the carpospores, which are formed indirectly from the fertilised oosphere, possess a cell-wall. [Illustration: FIG. 73.--_Callithamnion elegans_: _a_ a plant with tetraspores (× 20); _b_ apex of a branch with tetraspores(× 250).] [Illustration: FIG. 74.--_Polysiphonia variegata_: _a_ a portion of a male plant with antheridia; _b_ spermatia; _c_ transverse section of thallus.]