The red seaweeds differ much in the complexity of the plant body, but all agree in the presence of the red pigment, and, at least in the main, in their reproduction. The simpler ones consist of rows of cells, usually branching like _Cladophora_; others form cell plates comparable to _Ulva_ (Fig. 30, _C_, _D_); while others, among which is the well-known Irish moss (_Chondrus_), form plants of considerable size, with pretty well differentiated tissues. In such forms the outer cells are smaller and firmer, constituting a sort of rind; while the inner portions are made up of larger and looser cells, and may be called the pith. Between these extremes are all intermediate forms. They usually grow attached to rocks, shells, wood, or other plants, such as the kelps and even the larger red seaweeds. They are most abundant in the warmer seas, but still a considerable number may be found in all parts of the ocean, even extending into the Arctic regions. [Illustration: FIG. 29.--_A_, a red seaweed (_Callithamnion_), of the natural size. _B_, a piece of the same, × 50. _t_, tetraspores. _C_ i-v, successive stages in the development of the tetraspores, × 150. _D_ I, II young procarps. _tr._ trichogyne. iii, young; iv, ripe spore fruit. I, III, × 150. iv, × 50. _E_, an antheridium, × 150. _F_, spore fruit of _Polysiphonia_. The spores are here surrounded by a case, × 50.] The methods of reproduction may be best illustrated by a specific example, and preferably one of the simpler ones, as these are most readily studied microscopically. The form here illustrated (_Callithamnion_) grows attached to wharves, etc., below low-water mark, and is extremely delicate, collapsing completely when removed from the water. The color is a bright rosy red, and with its graceful form and extreme delicacy it makes one of the most beautiful of the group. If alcoholic material is used, it may be mounted for examination either in water or very dilute glycerine. The plant is composed of much-branched, slender filaments, closely resembling _Cladophora_ in structure, but with smaller cells (Fig. 29, _B_). The non-sexual reproduction is by means of special spores, which from being formed in groups of four, are known as tetraspores. In the species under consideration the mother cell of the tetraspores arises as a small bud near the upper end of one of the ordinary cells (Fig. 29, _C_ i). This bud rapidly increases in size, assuming an oval form, and becoming cut off from the cell of the stem (Fig. 29, _C_ ii). The contents now divide into four equal parts, arranged like the quadrants of a sphere. When ripe, the wall of the mother cell gives way, and the four spores escape into the water and give rise to new plants. These spores, it will be noticed, differ in one important particular from corresponding spores in most algæ, in being unprovided with cilia, and incapable of spontaneous movement.