Fig. 52.—Ulothrix zonata: a portion of a filament with zoospores, which are formed two in each cell (zoosporangium); the dark spots are the red “eye-spots”; 1, 2, 3, 4, denote successive stages in the development of the zoospores; b a single zoospore, v the pulsating vacuole; c portion of a filament with gametes, sixteen are produced in each gametangium; d free gametes, solitary or in the act of conjugation; e the conjugation is completed, and the formed zygote has assumed the resting-stage.
Order 2. Ulothricaceæ. The thallus consists normally of a simple unbranched filament (sometimes a small expansion consisting of one layer of cells is formed, as in Schizomeris and Prasiola which were formerly described as separate genera). Asexual reproduction takes place by means of zoospores (with 1, 2, or 4 cilia), akinetes or aplanospores; the last named may germinate immediately, or only after a period of rest. Sexual reproduction takes place by the conjugation of gametes of about the same size, each having two cilia (Fig. [52] d). The zygote of Ulothrix, on germination, produces a brood of zoospores which swarm for a time and then elongate to become Ulothrix-filaments (alternation of generations). The gametes may also germinate without conjugation in the same manner as the zoospores. The principal genera are: Ulothrix, Hormidium, Conferva, Microspora.—Ulothrix zonata is very common in running fresh water. Nearly all the species of Hormidium occur on damp soil, tree-stems and stones.
Order 3. Chætophoraceæ. The thallus consists of a single, branched, erect or creeping filament of cells, often surrounded by mucilage. The cells have only one nucleus. Asexual reproduction by zoospores with 2 or 4 cilia, by akinetes, or aplanospores. In many, conjugation between gametes with 2 cilia may be found. They approach on one side, Ulothricaceæ, and on the other, Mycoideaceæ. The principal genera are: Stigeoclonium, Draparnaldia, Chætophora, Entoderma, Aphanochæte, Herposteiron, Phæothamnion, Chlorotylium, Trichophilus, Gongrosira, Trentepohlia. Most of the species of Trentepohlia are coloured red by the presence of a red colouring material, which occurs in addition to the chlorophyll. They are aerial Algæ which live on stones (T. jolithus, “violet stone,” so named on account of its violet-like odour in rainy weather), on bark and old wood (T. umbrina), or on damp rocks (T. aurea). Trichophilus welckeri lives in the hair of Bradypus.
Order 4. Mycoideaceæ. The thallus is discoid, consisting of one or more cell-layers, and is always attached. Asexual reproduction by zoospores with 2 or 4 cilia. Sexual reproduction in some species by the conjugation of gametes with 2 cilia. This order forms the connecting link between Chætophoraceæ and Coleochætaceæ. The species occur in fresh water (Chætopeltis) as well as in salt (Pringsheimia), on the carapace of tortoises (Dermatophyton = Epiclemmydia), or endophytic between the cuticle and the epidermal cells of the leaves of tropical plants, destroying the leaf-tissue (Mycoidea).
Order 5. Cylindrocapsaceæ. The thallus consists of a simple (rarely, in parts, formed of many rows) unbranched filament, attached in the young condition, which has short cells with a single nucleus, and is enveloped in a thick envelope with a laminated structure. Asexual reproduction by zoospores with 2 cilia, which are formed 1, 2, or 4 in each vegetative cell. The antheridia are produced by a single cell, or a group of cells, in a filament, dividing several times without increasing in size. Two egg-shaped spermatozoids, each with 2 cilia (Fig. [53] D), are formed in each antheridium, and escape through an aperture in the side; in the first stages they are enclosed in a bladder-like membrane (Fig. [53] B, C). Other cells of the filament swell out and form oogonia (Fig. [53] A), which resemble those of Œdogonium. After fertilisation, the oospore surrounds itself with a thick wall, and assumes a reddish colour. The germination is unknown. The unfertilised oospheres remain green, divide often into 2–4 daughter-cells, and grow into new filaments.
Fig. 53.—Cylindrocopsa involuta. A Oogonium with oosphere (o) surrounded by spermatozoids (s). B Two antheridia, each with two spermatozoids. C Spermatozoids surrounded by their bladder-like membrane. D Free spermatozoid.
This order, which only includes one genus, Cylindrocapsa, forms the connecting link between Ulothricaceæ and Œdogoniaceæ. The few species (4) occur only in fresh water.
Order 6. Œdogoniaceæ. The thallus consists of branched (Bulbochæte) or unbranched (Œdogonium) filaments, attached in the early stages. The cells may be longer or shorter, and have one nucleus. Asexual reproduction by zoospores, which have a chaplet of cilia round the base of the colourless end (Fig. [6] a). Sexual reproduction takes place by oogamous fertilisation. On the germination of the oospore, 4 zoospores are formed (Fig. [54] F). They occur only in fresh or slightly brackish water. The division of the cells takes place in quite a peculiar and unusual manner. At the upper end of the cell which is about to divide, a ring-shaped thickening of soft cellulose is formed transversely round the wall; the cell-nucleus of the mother-cell and the protoplasm then divide by a transverse wall into two portions of similar size, and the cell-wall bursts transversely along the central line of the thickened ring. The cell-wall thus divides into two parts—the upper one short, the “cap,” and the lower one much longer, the “sheath.” The portions of the original cell-wall now separate from each other, the cellulose ring extending, and supplying an additional length of cell-wall between them. The cap and sheath will project a little in front of the piece thus inserted. The dividing wall between the two new cells is formed near to the uppermost edge of the sheath, and gradually becomes thicker and firmer. The inserted piece of wall forms the larger part of the wall of the upper cell: the remainder is formed by the cap. This mode of division is repeated exactly in the same way, and new caps are formed close below the first one, one for every division.