Order 1. Goniotrichaceæ.—The thallus consists of a branched cell-filament without rhizoids. Tetraspores are formed directly from the entire contents of the mother-cell, without any preceding division. Fertilisation unknown. Asterocystis, Goniotrichum.
The Goniotrichaceæ, through the blue-green Asterocystis, are allied to the Myxophyceæ, and through Goniotrichum to the Porphyraceæ.
Order 2. Porphyraceæ.—The thallus is formed of an expansion consisting of a layer of 1–2 cells, which, at the base, are attached to the substratum by means of a special form of haptera (Porphyra, Diploderma); or of unbranched (very rarely slightly branched) filaments, attached at the base by haptera (Bangia): or it extends from a prostrate cell-disc (various species of Erythrotrichia). Tetraspores are formed after one or more divisions of the mother-cell, either from the whole or only a part of its contents; they possess amœboid movements, or have a jerky, sliding-forward motion. The antheridia have the same appearance as the vegetative cells, but divide several times, and several spermatia are formed, either simultaneously from the whole contents (Porphyra, Bangia), or the spermatia are successively formed from a part of the contents of the antheridium (Erythrotrichia). The carpogonium is without a trichogyne, but the oosphere has a colourless spot which may sometimes rise a little above the surface of the thallus, and may be considered as an early stage in the development of the trichogyne. The spermatia form a canal through the membrane of the carpogonium, and their contents coalesce with the oosphere at its colourless spot. The fertilised oosphere divides on germination into a number of carpospores, which are set free as naked, motionless masses of protoplasm, which grow and give rise to new individuals (alternation of generations).
The thallus has one or more apical cells, grows principally by apical growth, and may be differentiated into root, stem, and leaf. The chromatophores vary in form, but have a red or brownish colour, due to chlorophyll and phycoerythrin. Asexual reproduction by motionless tetraspores, which generally arise by the division into four of the contents of the tetrasporangium. The carpogonium has a trichogyne, and the carpospores, which are formed indirectly from the fertilised oosphere, possess a cell-wall.
Fig. 73.—Callithamnion elegans: a a plant with tetraspores (× 20); b apex of a branch with tetraspores(× 250).
Fig. 74.—Polysiphonia variegata: a a portion of a male plant with antheridia; b spermatia; c transverse section of thallus.
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.
Fig. 75.—Ceramium diaphanum (nat. size).
Fig. 76.—Delesseria sanguinea (about ⅓).
Tetraspores may be wanting (e.g. Lemanea) or may often arise on special, non-sexual individuals. In some (e.g. Batrachospermum) only one tetraspore is formed in each tetrasporangium, but the number is generally four, which may be formed tetrahedrally (Fig. 73) or by divisional walls perpendicular to each other, or even in a single row. The tetrasporangia in some species are free (Fig. 73), but in the majority they are embedded in the thallus.
Fig. 77.—A Lejolisia mediterranea: r haptera; s longitudinal section through a cystocarp; p the empty space left by the liberated spore (t). B-E Nemalion multifidum: a antheridia; b procarpium with trichogyne, to which two spermatia are adhering.
The sexual reproduction (discovered by Thuret and Bornet, 1867) differs in the essential points from that of all other plants, and approaches most nearly to the sexual reproduction of the Bangioideæ. The sexual cells are developed from the terminal cells (never nodal cells) of the branched cell-filaments, which constitute the thallus. The mother-cells of the spermatia (spermatangia) are generally arranged in a group, in the so-called antheridia (Figs. 74, 77 A, a). On becoming ripe the membrane of the spermatangium ruptures and the spermatia emerge as spherical or ovoid, naked (a little later they may possess a cell-wall) masses of protoplasm which are not endowed with the power of motion, and hence are carried passively by the current of the water in which they may happen to be, to the female cell. This latter is analogous with the oogonium of the Green Algæ. The female reproductive organ is termed the procarpium, and consists of two parts, a lower swollen portion—the carpogonium (Fig. 77 b in A and B)—which contains the cell-nucleus, and an upper filamentous prolongation—the trichogyne (Fig. 77 B)—which is homologous with the colourless receptive spot of the oosphere of the Green Algæ, and the Porphyraceæ. In the sexual reproduction of the majority of the Florideæ, a very important part is played by certain special cells, rich in cell-contents—the auxiliary cells. These are either dispersed in the interior of the thallus, or are arranged together in pairs with the cell-filament which bears the carpogonium, and are generally united with this to form an independent multicellular procarpium. The spermatia attach themselves firmly to the trichogyne and surround themselves with a cell-wall. The dividing wall at the point of contact is perforated, and the nucleus of the spermatium probably travels through the trichogyne to the swollen part of the procarpium—the carpogonium—and fuses with its nucleus. After fertilisation the trichogyne withers (Fig. 77 C), but the lower portion of the procarpium, constituting the fertilised oosphere, grows out and forms in various ways, first a tuft of spore-forming filaments known as gonimoblasts, and finally the carpospores. These latter form a new asexual generation (compare the germination of the oospore of Œdogonium and Coleochæte).
The gonimoblasts may arise in three ways:—
1. In the Nemalionales, branched filaments grow out from the oosphere and form an upright, compressed or expanded tuft of spore-forming filaments.
2. In the Cryptonemiales, several branched or unbranched filaments (ooblastema-filaments) grow out from the oosphere, and conjugate in various ways with the auxiliary cells. The gonimoblasts are then formed from the single cells produced by the conjugation.
3. In the Gigartinales and Rhodymeniales the oosphere conjugates with an auxiliary cell by means of a short ooblastema-filament, and from this auxiliary cell a gonimoblast is produced.
The motionless carpospores, which sometimes in the early stages are naked, and afterwards invested with a cell-wall, are developed from the terminal cells (and perhaps also from some of the other cells) of the branches of the gonimoblast. The gonimoblasts constitute sharply defined parts of the plant in which the carpospores arise. These parts are called cystocarps and are either naked (Fig. 77 E), or surrounded by a covering (pericarp or involucre, Fig. 77 A) formed in different ways. On this account the Florideæ were formerly divided into Gymnosporeæ (Batrachospermum, Nemalion, Ceramium, etc.) and Angiosporeæ (Farcellaria, Lejolisia, Delesseria, Melobesia, etc.).
The Florideæ are divided into four sub-families:—
Sub-Family 1. Nemalionales. The fertilised oosphere produces directly the gonimoblast.
Order 1. Lemaneaceæ. Algæ of brownish colour and living in fresh water. They lack tetraspores, and the very sparingly branched fertile filaments, composed of many rows of cells, grow out from a proembryo, which consists of a single row of cells bearing branches. Lemanea fluviatilis, often found on rocks and stones in quickly flowing streams.
Order 2. Helminthocladiaceæ. Tetraspores are generally wanting (e.g. in Nemalion) or arise one in each tetrasporangium (e.g. Batrachospermum) and it is only in Liagora that four cruciate tetraspores are formed. Chantransia corymbifera consists of simple, branched cell-rows, and is an independent species. Several other Chantransia-forms, living in fresh water, are “proembryos” of species of the genus Batrachospermum. The germinating carpospore grows out into filaments and forms a so-called proembryo which, if not shaded, attains only a small size, but when growing in shady situations presents a much greater development. These highly developed proembryos have been described as species of Chantransia. The proembryo can reproduce by division, or by tetraspores which are developed singly in the sporangia; in B. vagum and B. sporulans which do not possess fully developed female reproductive organs, the proembryos serve almost entirely to reproduce the species. The young Batrachospermum-plant arises from the end of an upright filament of the proembryo. The proembryo is generally persistent, and continually produces new Batrachospermums. These latter bear the sexual reproductive organs and also whorls of branches: the central row of cells is enclosed by cells growing from the base of the whorls of branches, and from these cortical cells secondary proembryos are developed. In this alternation of shoots there is really no alternation of generations, since the proembryo and the shoots with the sexual reproductive organs are parts of the same thallus.
Several species of Batrachospermum have a bluish green or verdigris colour. Nemalion multifidum has a brown-red thallus, slightly branched, which is attached to rocks near the water’s edge.
Order 3. Chætangiaceæ. Galaxaura has a thallus thickly incrusted with lime.
Order 4. Gelidiaceæ. Naccaria, Gelidium.
Sub-Family 2. Gigartinales. The fertilised auxiliary cell grows towards the thallus, to produce the gonimoblasts. Procarpia generally present.
Order 5. Acrotylaceæ. Acrotylus.
Order 6. Gigartinaceæ. Gigartina, Phyllophora, Ahnfeltia; Chondrus crispus, with dark red, dichotomously branched thallus, is common on the coasts of Scandinavia and Great Britain.
Order 7. Rhodophyllidaceæ. Rhodophyllis, Euthora; Cystoclonium purpurascens is common, and sometimes the ends of its branches may be modified into tendril-like haptera.
Sub-Family 3. Rhodymeniales. The fertilised auxiliary cell forms the gonimoblast on the side away from the thallus. Procarpia are abundantly produced.
Order 8. Sphærococcaceæ. Gracilaria.
Order 9. Rhodymeniaceæ. Rhodymenia palmata is a common species. Lomentaria, Chylocladia, Plocamium.
Order 10. Delesseriaceæ. Delesseria sanguinea; D. alata and D. sinuosa are handsome forms which are not uncommon.
Order 11. Bonnemaisoniaceæ. Bonnemaisonia.
Order 12. Rhodomelaceæ. Rhodomela, Odonthalia; Polysiphonia, of which many species are to be found on the coasts of Great Britain, has a filamentous, richly branched thallus consisting of a central row of cells surrounded by a varying number of cortical cells of similar size—the so-called “siphons.”
Order 13. Ceramiaceæ. Pretty Algæ, often branched dichotomously, or unilaterally pinnate. Spermothamnion, Griffithsia, Callithamnion, Ceramium, Ptilota.
Sub-Family 4. Cryptonemiales. The cells formed by the coalescence of the auxiliary cells and the ooblastema-filaments, produce the gonimoblasts. The carpogonium-filaments and the auxiliary cells are scattered singly in the thallus.
Order 14. Gloiosiphoniaceæ. Gloiopeltis.
Order 15. Grateloupiaceæ. Halymenia, Cryptonemia.
Order 16. Dumontiaceæ. Dumontia, Dudresnaya.
Order 17. Nemastomaceæ. Furcellaria, which has dichotomously branched, round shoots, is common on the coasts of Great Britain.
Order 18. Rhizophyllidaceæ. Polyides, Rhizophyllis.
Order 19. Squamariaceæ. The Algæ belonging to this order form crust-like coverings on stones, mussel-shells, and on other Algæ, but are not themselves incrustated: Petrocelis, Cruoria, Peyssonellia.
Order 20. Corallinaceæ. Partly crustaceous, partly erect, branched Algæ, thickly incrusted with lime, so that a few species (Lithothamnia, also called Nullipora) occur in fossilized condition from Jurassic to Tertiary periods. Melobesia, Lithophyllum, Lithothamnion, Corallina.
Uses. “Carragen” is the thallus of Chondrus crispus (Irish Moss) and Gigartina mamillosa. It is a common article of food on the coasts of Ireland, and swells to a jelly when cooked. It is officinal. Rhodymenia palmata is generally eaten as food in Ireland and in some places on the west coast of Norway; it is also used as food for sheep and hence is termed “Sheep-seaweed.” Agar-Agar is the jelly obtained from species of Gelidium and Gigartina growing in China and Japan.