22. _Plocamium coccineum_, with capsule (coccidium). 23. _Plocamium coccineum_, portion of frond. 24. _Plocamium coccineum_, tetraspore from sporophyll. 25. _Polysiphonia fastigiata_, portion of. 26. _Polysiphonia fastigiata_, filament with capsules (ceramidia). 27. _Polysiphonia fastigiata_, portion of filament. 28. _Corallina officinalis._ 29. _Corallina officinalis_, portion of filament. 30. _Corallina officinalis_, capsule (ceramidium). 31. _Enteromorpha compressa._ 32. _Enteromorpha compressa_, cells of frond. 33. _Hypnea purpurascens_, capsule (coccidium). 34. _Hypnea purpurascens_, spores. 35. _Hypnea purpurascens_, filament. [Illustration: Plate IV. W Bagg sculp _London: John Van Voorst._] _Fúcus vesiculósus_, with its parallel-sided or linear olive-brown fronds, is known to every one as the seaweed which is hung up to act as a weather-glass. The fronds have a central stout vein, or midrib, and scattered air-bladders, mostly in pairs. The fructification consists of yellowish oval enlargements of the ends of the fronds, called the _receptacles_ (fig. 16); but these are somewhat variable in form, being often angular or truncate. On holding one of the receptacles to the light, it will appear to contain a number of little grains imbedded in its substance, slightly projecting above the surface, and in the centre of each is a minute dot or pore. These grains are the capsules, or _conceptacles_, and contain the spores. The substance of the receptacles is composed of a beautiful network of colourless, jointed, cellular fibres (figs. 17 _a_ and 18 _a_), the meshes of which are filled with a transparent gelatinous substance; but immediately around the conceptacles the cells are shorter and more closely packed. The spores (fig. 19) are arranged in the conceptacles in a radiate manner; they are brown, and surrounded by a colourless sac, called the _perispore_ (περἰ, around, σπορἀ, seed); and between them are numerous slender, colourless, jointed filaments, the _paraph´yses_. The spores are not, however, truly single spores, for they ultimately divide into eight segments or sporules, each of which is capable of producing a new plant. In the conceptacles of some fronds of _Fucus_ no spores will be found, the conceptacles (fig. 18) being filled with elegantly branched colourless filaments (fig. 20), the ends of many of them being distended into little yellowish sacs; these are the _antherid´ia_. The antheridia contain large numbers of exceedingly minute spermatozoa, furnished with two cilia, and very similar to those existing in the antheridia of the mosses; these, escaping through the pore of the conceptacle, fertilize the spores.