The Dictyotales, in having tetraspores and spermatia, deviate considerably from the Phæophyceæ, but may be classed near to the Tilopteridæ, in which there are asexual spores with 4 cell-nuclei, which may be considered as an indication of the formation of tetraspores.
Order 1. Dictyotaceæ. Dictyota dichotoma which has a thin, regularly dichotomously divided thallus, occurs on the coasts of the British Isles. Padina is found on the south coast.
Class 10. Rhodophyceæ (Red Seaweeds).
The plants comprised in this class are multicellular; they are simple or branched filaments, or expansions consisting of 1 to several layers of cells; the thallus may be differentiated (as in many Florideæ), to resemble stem, root, and leaf. The cells contain a distinctly differentiated nucleus (sometimes several), and distinct chromatophores, coloured by rhodophyll. The chlorophyll of the chromatophores is generally masked by a red colouring matter (phycoerythrin), which may be extracted in cold, fresh water; or rarely by phycocyan. Pyrenoids occur in some. Starch is never formed in the chromatophores themselves, but a modification—Florideæ starch—may be found in the colourless protoplasm. Asexual reproduction by motile or motionless spores (tetraspores) which are devoid of cilia and of cell-wall. Swarmspores are never found.
Sexual reproduction is wanting, or takes place by the coalescence of a spermatium and a more or less developed female cell. The spermatia are naked masses of protoplasm, devoid of cilia and chromatophores. The female cell (carpogonium) is enclosed by a cell-wall, and after fertilisation forms a number of spores, either with or without cell-walls (carpospores), which grow into new individuals.
The Rhodophyceæ may be divided into two families:
1. Bangioideæ.
2. Florideæ.
Family 1. Bangioideæ.
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.