Ulothrix: Salient Features, Occurrence, Thallus Structure, Reproduction

Ulothrix is an unbranched filamentous green alga of the class Chlorophyceae. It is commonly found in freshwater ponds, pools, lakes, etc.

The word Ulothrix means shaggy or curly hair (in Greek, oulos = shaggy or curly and thrix = hair).

Salient Features of Ulothrix

The salient features of Ulothrix are as follows:

  • The plant body is a multicellular, green, unbranched filament.
  • The filament is differentiated at the apex and base.
  • A gridle-shaped, collar-shaped, or ring-shaped chloroplast is present in each cell, which contains one or more pyrenoids.
  • Asexual reproduction occurs through biflagellate or quadriflagellate zoospores.
  • Sexual reproduction is of the isogamous type. It takes place by the fusion of two biflagellate isogametes.
  • On germination, the zygote produces 4–16 zoospores.
Ulothrix

Occurrence of Ulothrix

Ulothrix is a genus of about 30 species. It occurs in a variety of habitats around the world.

Most of the species are found in stagnant or slow-running freshwater ponds, pools, and ditches (e.g., U. zonata, U. aequalis). Some species, such as U. psudoflacea and U. flacca, are marine. Ulothrix flacca often occur in the intertidal zone.

U. zonata is a common freshwater species that prefers to grow in cold water during the rainy season as well as in the spring.

The filaments are attached to rocks, stones, or some other substratum. They may form bright green tufts over the substratum.

A few species are lithophytes (e.g., U. implexa).

  • Some Ulothrix species are: U. flacca, U. variabilis, U. zonata, U. implexa, U. aequalis, etc.

Thallus Structure of Ulothrix

The plant body of Ulothrix is multicellular, long, filamentous, and bright green in colour.

Diagram of Ulothrix
Figure: Plant body of Ulothrix

The filaments are unbranched and uniseriate, i.e., the cells are arranged in a single row or attached end to end.

Each filament is differentiated into three types of cells.

  • Basal Cell or Holdfast
  • Middle or Intercalary Cells
  • Apical Cell

Basal Cell or Holdfast

The lowermost cell of the filament is the basal cell. It is modified into a holdfast.

The holdfast is long, gradually narrowing downwardly, and towards the basal part it expands to form a disc-shaped outgrowth.

Basal or rhizoidal cell
Figure: Basal cell of Ulothrix filament

The basal cell is brown or colourless and lacks chloroplasts. It attaches the filament to the substratum.

Middle or Intercalary Cells

All cells of the filament between the basal and apical cells are similar in structure. These cells are known as intercalary cells.

Intercalary cells are rectangular in shape, usually much broader than their length. In U. moniliformis, the cells may be quadrangular, while in U. subconstricta, they may be much longer (2–6 times) than their breadth.

Vegetative intercalary cells in Ulothrix
Figure: A portion of filament showing vegetative cells

Apical Cell

The apical cell is the topmost cell of the filament. It is dome-shaped with a hemispherical outline.

Due to the presence of chloroplasts, the apical cell is green in colour. It can take part in photosynthesis.

Cell Structure of Ulothrix

The vegetative cells of Ulothrix are surrounded by a rigid cell wall, which often becomes stratified (e.g., U. moniliformis). The cell wall is composed of two concentric layers: an outer pectin layer and an inner layer of cellulose.

A thin plasma membrane encloses the protoplast. The protoplast contains the cytoplasm, a central vacuole, a chloroplast, and a single nucleus.

The cytoplasm forms the primordial utricle (a lining layer), adjacent to the plasma membrane.

In the centre of the cell, a large vacuole is present, which contains cell sap. The single nucleus lies inside the cytoplasm.

Structure of cell in Ulothrix
Figure: Portion of filament showing cell structure

A griddle-shaped, collar-shaped, or ring-shaped chloroplast remains embedded in the cytoplasm near the cell membrane. It contains one (e.g., U. rorida) or more (e.g., U. zonata) pyrenoids.

The chloroplast extends from one end of the cell to the other and encircles the protoplast completely (e.g., U. zonata) or partially.

Reproduction in Ulothrix

Reproduction in Ulothrix takes place by means of vegetative, asexual, and sexual methods.

Vegetative Reproduction

Ulothrix reproduces vegetatively through fragmentation and the formation of akinete.

In fragmentation, the filament can break into small fragments due to accidental breakage. Each fragment is capable of developing into a new filament.

In some species (e.g., U. aequalis), akinetes are formed during unfavourable conditions.

Some vegetative cells of the filament become enlarged, accumulate much food material in their protoplast, and secrete a highly resistant, thick wall around them. Such modified vegetative cells are called akinetes, or resting spores.

With the onset of favourable environmental conditions, akinete may germinate into a plant.

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