The thallus may assume very different forms. In the simplest species it is filamentous and formed of single, branched rows of cells (_Callithamnion_, etc., Fig. 73). _Ceramium_ has a filamentous thallus, generally dichotomously forked (Fig. 75), or sometimes pinnately branched, which, at the nodes, or throughout its entire length, is covered by a layer of small cortical cells. _Polysiphonia_ (Fig. 74) has a filamentous, much branched thallus, made up of a central cylindrical cell, surrounded by a layer of other cells, cortical cells, which in length and position correspond to the central ones. In many of the Red Algæ the vegetative organs are differentiated into stems and leaves, the former having, as in _Chara_, unlimited growth in length, whilst the latter soon attain their full development. _Chondrus_ has a fleshy, gelatinous thallus, without nodes; it is repeatedly forked into flat branches of varying thickness. _Furcellaria_ has a forked thallus with thick branches and without nodes. The thallus of _Delesseria_ (Fig. 76) consists of branches, often bearing leaf-like structures, with a midrib and lateral ribs springing from it. These ribs persist through the winter, and at the commencement of the succeeding period of vegetation the lateral ribs become the starting points for new leaves. In _Corallina_ the thallus is pinnately branched, and divided into nodes and internodes. The name has been given to this genus from the fact that the thallus is incrusted with carbonate of lime to such a degree that it becomes very hard, and the whole plant adopts a coral-like appearance. Other genera which are similarly incrusted, and have a leaf-like or even crustaceous thallus (such as _Melobesia_, _Lithothamnion_), are included in this family. In some instances the cells of the thallus may be found _differentiated_ into more or less well defined tissues, so that it is possible to find special assimilating, mechanical, and conducting tissues, the last named in some cases having the double function of conducting and of serving as a reservoir in which starch is found as a reserve material. The cells of the Florideæ, which are formed by the division of a mother-cell into two daughter-cells of unequal size, have always larger or smaller pits in the cell-walls, and the thin cell-wall separating two pits from each other is perforated by a number of small holes. These pits are particularly developed in the conducting tissues, but sieve-tubes are very rarely to be found. [Illustration: FIG. 75.--_Ceramium diaphanum_ (nat. size).] [Illustration: FIG. 76.--_Delesseria sanguinea_ (about ⅓).]