The plants in this class are multicellular, and brown, with apical growth, new cells being derived either from a flat apical cell, or from a border of apical cells. The thallus is flat, leaf- or strap-shaped, attached by haptera, which are either found only at the base, or on the whole of the lower expansion of the thallus. The cells are differentiated into the following systems of tissues: an external, small-celled layer of assimilating cells, generally one cell in thickness, and an internal, large-celled layer of one or only a few cells in thickness, forming the mechanical and conducting tissues. All the reproductive cells are motionless. Asexual reproduction by naked, motionless spores (tetraspores) which are formed 1–4 in each tetrasporangium, the latter being outgrowths from the surface cells of special, sexless individuals. Zoospores are wanting. The sexual organs are of two kinds, oogonia and antheridia, which are formed from the surface cells, either on the same or different individuals. The oogonia are spherical or oval, and are generally placed close together; each contains one oosphere, which on maturity is ejected into the surrounding water, and is then naked and motionless. The antheridia are formed of longitudinal cells, united in groups, whose contents by repeated divisions--transverse and longitudinal--are divided into a large number of small, colourless, motionless spermatia--round or elongated--which are set free by the dissolution of the wall of the antheridium. The process of fertilisation has not yet been observed. 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.