Chara: Salient Features, Occurrence, Thallus Structure, Reproduction

Chara is a multicellular submerged freshwater green alga of the class Charophyceae. It is usually found in freshwater ponds, lakes, tanks, etc.

On the surfaces of the Chara, cyanobacteria have been found to grow as epiphytes. Chara is commonly known as “stonewort” due to its being covered with calcium carbonate.

Salient Features of Chara

The salient features of Chara are as follows:

  • The plant body shows an elaborate complexity in its structure. It is green, branched, and filamentous.
  • Branches are dimorphic, i.e., two types of branches can be seen in Chara; branches of limited growth and branches of unlimited growth.
  • The plant body remains attached to the substratum by colourless branched rhizoids.
  • The main axis and branches are differentiated into nodes and internodes.
  • The branches of limited growth bear sex organs at each node.
  • Vegetative reproduction takes place by the formation of amylase stars, bulbils, amorphous bulbils, and secondary protonema.
  • Asexual reproduction in Chara is completely absent due to the lack of motile and non-motile spores.
  • Sexual reproduction is of the advanced oogamous type.
  • Sex organs are complicated and highly specialised in structures. The male sex organ is the globule and the female sex organ is the nucule. The nucule is situated above the globule.
  • The globule is round and produces many antherozoids or sperm.
  • The nucule is oval-shaped and surrounded by a protective sheath. It contains a single large, uninucleate egg.
  • The zygote shows very elaborate and long-cycled post-fertilization changes.
  • Typically, alternation with haploid and diploid generations is absent.
Diagram of thallus of Chara
Figure: Thallus or plant body of Chara

Occurrence of Chara

Chara is a genus of about 180 species (Wood and Mahori; 1959). It is fresh water in habit and is found submerged in shallow water ponds, lakes, tanks and slow-running water. The thallus remains attached to the sandy or muddy bottom with the help of rhizoids.

Most of the species prefer to grow in hard freshwater, deficient in oxygen or rich in organic matter and calcium. Some species grow in hot springs, e.g., C. fragilis and C. tragilis. Chara baltica occurs in stagnant brackish water. Species like C. hatei grow trailing on the soil, while C. grovesii and C. nuda are found on mountains.

The plant body of Chara is covered with calcium carbonate and magnesium carbonate, particularly on the plants growing in heavy water. Due to the presence of sulphur compounds, it often emits a disagreeable onion-like odour.

Thallus Structure of Chara

The plant body or thallus is multicellular, filamentous, and green in colour. The filaments are profusely branched. The plant is generally 20–40 cm in height but may often be up to 1 m. Some species are small, such as C. hatei (2 to 3 cm long).

The plant body of Chara in appearance resembles the Pteridophyte Equisetum. Hence, Chara is often called aquatic horsetail.

The thallus is differentiated into rhizoids and the main axis.

Rhizoids

The rhizoids are thread-like, multicellular, colourless, uniseriate, and branched structures. They arise either from the lower part of the thallus or from peripheral cells of the lower nodes of the main axis.

The rhizoids are obliquely septate and also show apical growth. They possess minute solid particles at the tips that function as statoliths.

Rhizoids attach the main axis to the substratum (mud or sand) and help in the absorption of water and minerals. They also help in vegetative reproduction by forming bulbils and secondary protonema.

Main axis of Chara
Figure: Apex of the main axis showing its different parts

Main Axis

The main axis of Chara is erect, long, branched, and differentiated into nodes and internodes.

Internodes

The internode is cylindrical and consists of a single, much elongated or oblong inter nodal cell or axial cell.

The axial cells in some species may be surrounded by a one-celled thick layer called the cortex, and such species are called corticate species, e.g., C. zeylanica, C. hatei, C. fragilis, etc. The species in which the cortical layer is absent are known as ecorticate species, e.g., C. wallichii, C. corallina, C. braunii, C. succinata, etc.

The cortex is composed of a sheath of vertically elongated, narrow cortical cells. Each cortical cell is much smaller in diameter than the axial cell.

The cortical cells derive from the node. After originating, 50% of the cortical cells grow upward as the ascending filaments and the remaining 50% grow downward as the descending filaments.

The ascending filaments surround the lower half, and the descending filaments cover the upper half of the axial cell.

Nodes

The node is complex in structure and consists of a pair of central small cells surrounded by 6–20 peripheral cells. Three types of appendages are developed from each node of the main axis. These appendages are:

  • Branches of limited growth or branchlets
  • Branches of unlimited growth or long branches
  • Stipulodes
Branches of Limited Growth

The branches of limited growth, or primary laterals, are short branches arranged in whorls on the nodes of the main axis or on branches of unlimited growth. These branches are also known as branchlets, branches of the first order, or leaves.

In a whorl, there are about 6 to 16 branchlets developed. Each branchlet is also divided into 5–15 nodes and internodes.

The nodes develop some unicellular, hair-like secondary laterals. Sex organs and stipulodes (unicellular outgrowths) are formed on the lower nodes of the branchlets.

Fertile branch of Chara
Figure: Portion of a fertile branch in Chara
Branches of Unlimited Growth

The branches of unlimited growth develop from the older nodes of the main axis (the axils of the branchlets). These branches are also called axillary branches or long laterals.

The axillary branches are differentiated into nodes and internodes like the main axis. Each node bears primary laterals.

Stipulodes

The stipulodes are short unicellular outgrowths developed from the lower nodes of the branches of limited growth (branchlets).

In most of the species of Chara, the number of stipulodes at each node is twice the number of branchlets. Such species are called bistipulate, e.g., C. tomentosa, C. baltica, C. burmanica, C. contraria, etc. Some species are unistipulate, e.g., C. braunii, C. coralline, and C. nuda, as the number of stipulodes at each node is equal to the number of branchlets at that node.

When the stipulodes are present in one whorl at each node, the species are called haplostephanous (e.g., C. braunii), and those with two whorls at each node are called diplostephanous (e.g., C. delicatula).

Cell Structure of Chara

The main axis of Chara is composed of mainly two types of cells: nodal cells and internodal cells.

Nodal Cells

The nodal cells are small, uninucleate, with dense granular cytoplasm. Each cell contains many discoid chloroplasts. The chloroplasts are without pyrenoids.

Small vacuoles may be present in the cytoplasm. The cell is surrounded by the cell wall, which is made up of cellulose.

Internodal Cells

The internodal cells are multinucleate (the single nucleus in the internodal cell is divided by amitosis division to form several nuclei) and much elongated. Each cell possesses a large central vacuole in the cytoplasm.

The cytoplasm is differentiated into the outer stationary ectoplasm and the inner rotatory endoplasm. Many discoid chloroplasts lie embedded in the outer exoplasm. Pyrenoids are absent in chloroplasts. The endoplasm develops a thin sieve around the large vacuole.

The endoplasm shows streaming movements. In a constant state of rotation, the endoplasm flows on one side of the vacuole and flows down on the other. The streaming of the cytoplasm is due to the alternating contraction and expansion of the protein fibrils, which remain fixed to the cell wall.

The cell wall of the internodal cell is also composed of cellulose. However, it is richly encrusted with calcium carbonate and silica, producing hardness and brittleness.

Growth of the Thallus

Growth of the thallus in Chara takes place by a single large dome-shaped apical cell, situated at the tip of the axis. The apical cell cuts off segments at its posterior surface.

Each segment divides transversely into two cells: an upper biconcave, the nodal initial, and a lower biconvex, the internodal initial.

The internodal initial does not divide further and elongates considerably to form the long internode of the axis.

The nodal initial divides by several vertical divisions and finally forms two central cells surrounded by 6–20 peripheral cells. Each peripheral cell divides into an apical initial, which later gives rise to a primary lateral and a basal nodal cell.

The basal nodal cell further divides into upper and lower cortical cells. In Chara, thus, the internode is surrounded by a cortical layer of vertically elongated rows of cortical cells.

The primary laterals grow in the same way, but the internodes are shorter than those of the main axis.

Reproduction in Chara

Chara reproduces by vegetative and sexual methods only. Due to the lack of non-motile and motile spores, asexual reproduction is absent in Chara.

Vegetative Reproduction

Vegetative reproduction in Chara occurs by various kinds of reproductive bodies which, on detachment from the parent plant, give rise to new plants.

The common methods of vegetative reproduction in Chara are as follows:

Amylum Stars

Amylum stars are star-shaped aggregates of cells  develop on the lower nodes of the main axis. The cells are densely filled with amylum starch and hence are called amylum stars.

After being detached from the parent plant, the Amylum stars directly develop into new Chara plants, e.g., C. stelligera.

Bulbils

The bulbils are small oval or spherical tube-like structures that develop either on rhizoids (e.g., C. aspera) or on lower nodes of the main axis (e.g., C. baltica).

Under favourable conditions, on detachment from the plant, each bulbil germinates into a new thallus.

Amorphous Bulbils

In some species of Chara, such as (e.g., C. delicatula, C. fragifera, and C. baltica), many small cells aggregate at the lower node of the main axis or at rhizoids and form many lateral outgrowths called amorphous bulbils. These clumps of cells are laden with food materials.

The amorphous bulbils get separated and grow into new plants.

Secondary Protonema

The secondary protonema are tubular or filamentous structures developed from the primary protonema or from the basal cell of the rhizoid.

Like the primary protonema, the secondary protonema develops into Chara plants.

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