Part 18

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.