Fig. 54.—A Œdogonium ciliatum. A Female plant with three oogonia (og) and dwarf-males (m). B An oogonium with spermatozoid (z) seen entering the oosphere (o) having passed through an aperture near the summit of the oogonium; m dwarf-male. C Ripe oospore. D Œdogonium gemelliparum. F Portion of a male filament from which spermatozoids (z) are emerging. E Portion of filament of Bulbochæte; the upper oogonium still encloses the oospore, in the central one the oospore is escaping while the lower one is empty. F Four zoospores developed from an oospore. G Zoospore germinating.
Fertilisation takes place in the following way. The oogonium is a large ellipsoidal, swollen cell (og, in Fig. [54] A), whose contents are rounded off into an oosphere with a colourless receptive-spot (see B); an aperture is formed in the wall of the oogonium, through which the spermatozoids are enabled to enter (B). The spermatozoids are produced either directly, as in D (in pairs), in basal cells of the filament, or indirectly. In the latter case a swarmspore (androspore) is formed which comes to rest, attaches itself to an oogonium, germinates, and gives rise to a filament of a very few cells—dwarf-male (A, B, m). The spermatozoids are formed in the upper cell of the dwarf-male (m), and are set free by the summit of the antheridium lifting off like a lid. On the germination of the oospore (C), which takes place in the following spring, 4 zoospores are produced (F) (i.e. the sexual generation); these swarm about for a time, and ultimately grow into new filaments.
Fig. 55.—Coleochæte pulvinata. A A portion of a thallus with organs of reproduction; a oogonium before, b after fertilisation; c an antheridium, closed; d open, with emerging spermatozoid. B Ripe oogonium, with envelope. C Germination of the oospore. D Zoospore. E Spermatozoid.
Order 7. Coleochætaceæ. The thallus is always attached, and of a disc- or cushion-shape, formed by the dichotomous branching of filaments of cells united in a pseudo-parenchymatous manner. Each cell has only one nucleus. Asexual reproduction by zoospores with 2 cilia (Fig. [55] D), which may arise in all the cells. Sexual reproduction by oogamous fertilisation. The spermatozoids resemble the swarmspores, but are smaller (E), and originate singly (in the species figured) in small conical cells (c, d in A). The oogonia are developed at the extremities of certain branches: they are bottle-shaped cells with very long and thin necks (trichogyne), open at the end (a in A); at the base of each oogonium is a spherical oosphere. The spermatozoids reach the oosphere through the trichogyne, or through an aperture in the wall when the trichogyne is absent, and fertilisation having taken place, the oogonium becomes surrounded by a cell-layer (envelope), which grows out from the cells near its base (b in A), and in this way a kind of fruit is formed (B) (spermocarp, cystocarp).
The oospore, next spring, divides and forms a parenchymatous tissue (homologous with the Moss-sporophyte); this bursts open the envelope (C), and a zoospore (homologous with the spores of the Moss-capsule) arises in each of the cells, and produces a new Coleochæte. We have then, in this case, a still more distinct alternation of generations than in Œdogonium. Only one genus, Coleochæte, is known, but it contains several species, all living in fresh water.
Order 8. Cladophoraceæ. This order is probably derived from the Ulothricaceæ. The thallus consists of a single, unbranched or branched filament, generally with an apical cell. The cells have each 2 or more nuclei. Asexual reproduction by zoospores with 2 or 4 cilia, and by akinetes. Conjugation of gametes with 2 cilia is found in some genera. They occur in salt as well as in fresh water. The principal genera are: Urospora, Chætomorpha, Rhizoclonium, Cladophora; of the last named genus the species C. lanosa and C. rupestris are common in salt water; C. fracta and C. glomerata in fresh water.
Order 9. Gomontiaceæ. Gomontia polyrrhiza, the only species hitherto known, is found on old calcareous shells of certain salt water Molluscs.
Order 10. Sphæropleaceæ. The thallus consists of free, unbranched filaments, with very elongated multinuclear cells. The vegetative cells form no zoospores. Sexual reproduction by oogamous fertilisation (see page [13], Fig. [10] B). The oospore has a thick wall (Fig. [10] D) studded with warts, and assumes a colour resembling red lead. It germinates only in the following spring, and produces 1–8 zoospores, each with 2 cilia (Fig. [10] E), which grow into new filaments. Only one species, Sphæroplea annulina, is known.