In this article we will discuss about the classification of ulotrichales.
Plants are filamentous:
Filaments are with or without an attaching base; typically unbranched, composed of cylindrical uniseriate cells with thin to gelatinous walls. Cells possess lateral plate-like or band-like chromatophores with or without pyrenoids.
Asexual reproduction is by bi- and quadriflagellate zoospores, although a few genera (e.g., Stichococcus, Radiofilum, and Geminella) fail to do so. Sexual reproduction is isogamous (Ulothrix), anisogamous, or oogamous (Cylindrocafisa).
This is a fresh-water alga of the filamentous or confervoid kind (in the days of Linnaeus such forms were included in the old genus Conferva, being simple filamentous), growing preferably in cold-flowing water. The un-branched slender filament consisting of a single row of cells, differentiated into base and apex, anchors to its substrate at one end by a specialized basal cell of the filament to form a holdfast (Fig. 41 A).
The filaments can, however, live equally well detached, floating in tangled masses on the- surface of water. The filament is composed of numerous cylindrical cells placed end to end, each of which contains a single nucleus, and a band-shaped or plate-like parietal chloroplast resembling a slightly open ring curved to fit the rounded contour of the cell (Fig. 41B-G).
Usually one or more pyrenoids are found in the chloroplast. Within the cell wall is the cytoplasmic layer, or primordial utricle, in which the nucleus is embedded (Fig. 41C). Walls surrounding the cells may be thick or thin, and homogeneous or stratified. The filament grows throughout its length, excepting the basal cell, and has no special growing point.
Apart from vegetative multiplication by accidental fragmentation of the filament, Ulothrix reproduces by means of swarmers. All the cells but the holdfast are capable of producing swarmers. Three types of swarmers may be produced by the common species of Ulothrix (U. zonata).
Depending on their shape and behaviour they may be named as:
(1) Quadriflagellate macrozoospores, formed in small numbers from the mother cell,
(2) Quadri- or biflagellate microzoospores, formed in large numbers, and
(3) Biflagellate gametes, formed in large numbers.
The macro- and micro-zoospores are formed in separate sporangia. The sporangia are nothing but the vegetative cells
of the filament in which the zoospores are formed by the repeated division of the protoplast (Fig. 41D to F). The zoospores are of the type characteristic of the motile vegetative cells of the Volvocales, only difference being in size and number of flagella. The zoospores escape by the breaking down of the sporangial wall (Fig. 41G).
After emerging, each zoospore swims rapidly for a short period, then comes to rest, loses its flagella, becomes attached to some convenient substrate, and grows into a new plant (Fig. 41H to K).
In some species, the protoplast divides to form non-motile, round daughter protoplasts. These protoplasts instead of bearing flagella secrete a wall and become aplanospores. Under condition of drought the entire protoplast may round up to form a single, thick-walled aplanospore or the hypnospore (Fig. 41L).
Occasionally wall of the sporangium producing the aplanospores gelatinizes followed by the gelatinization of the wall of the aplanospores as a result of which a number of green bodies become embedded in a mass of mucilage, this is known as palmella-stage, because the plant when in this state somewhat resembles the genus Palmella.
The palmella-stage of Ulothrix is chiefly found when the filament is left by the receding water on the damp parts of the pool or stream in which it grows. Ultimately these cells reproduce the normal filaments, either by direct germination or by forming zoospores.
Sexual reproduction is isogamous and involves the formation of small biflagellate isogametes. Formation of gametes is like the zoospores, each gametangium producing to the extent of 64 gametes (Fig. 41M). After emerging from the gametangium, the gametes swarm around in water. The fusing gametes come from different filaments indicating heterothallic condition (Fig. 41N to Q and Fig. 42).
The zygote remains motile for a short time after its formation but soon becomes a resting cell, and later germinates, during which meiosis of the zygotic nucleus takes place. Four or more zoospores or aplanospores are formed, each of which produces a new filament (Fig. 41R).
Some Indian species:
Ulothrix acqualis Kutz.; U. oscillarina Kutz.; U. tenerrima (Kutz.) Kutz.; U. tenuissima Kutz.; U. variabilis (Kutz). Kutz.; U. zonata (Web.) Kutz.
1. Filamentous un-branched plant body differentiated into base and apex.
2. Band-shaped chloroplast.
3. Asexual reproduction by bi- and quadriflagellate zoospores.
4. Sexual reproduction isogamous.