Structure and Reproduction in Chara-Fresh Water Green Algae

Chara grows submerged in fresh water and upon muddy or sandy bottoms of pools and ponds, or in limestone streams. Certain species have the capacity of precipitating calcium carbonate from the water and covering themselves with calcareous layers. Clear hard water is suitable for their growth. Mature plant is enriched with CaCO₃. Therefore, they are called as stone worts.

Structure of Chara

The plant body or structure of chara is as follow

Thallus

The plant body is thallus. The plant attains a height of 20-30cm. It has following demarcations.

Rhizoids

The erect, branched attached the substratum by means of rhizoids.

Branches

The thallus has long slender upright branches. The axis is differentiated into node and internodes. From each node arise the following four types of appendages,

Branchlets: Each node bears a whorl of branches, which has limited growth. These are called branchlets.

Long Branches: There are one or two branches of unlimited may also be present in each node. They arise singly at the older nodes of the main axis. They often called as axillary branches. The axillary branch continues the growth of thallus.

Stipuloids: These are unicellular out growth that arises from the basal node of each branchlet. They are usually two in number.

Cortex: In some species, the intermodal cells is covered by a sheath of vertically elongated narrow cells, the cortex Such species are called as corticated.

Cell Structure

Young cells are always uni-nucleated structures without conspicuous vacuole. In the mature cells, the vacuole may be developed and may become multinucleated due to amitosis. There are many small ellipsoidal chloroplasts arranged longitudinally in peripheral portion of cytoplasm. The Cell wall of the internodal cell is impregnated with silica and calcium carbonate. The reserve food material is starch and the cytoplasm show characteristic cyclic movement.

Growth of the Thallus

The growth of the thallus is by dome-shaped apical cell which cuts off derivatives at its posterior surface Each derivative undergoes a transverse division to produce two daughter cells. The upper cell acts as nodal initial and the lower as internodal initial. The internodal initial elongates many times its original length and matures to form internode of the axis. The nodal initial divides by vertical divisions intersecting each other to produce two central cells surrounded by 6-20 peripheral cells. Each peripheral cell cuts off apical cells of branches of limited growth.

Reproduction

The Chara reproduces by vegetative and sexual reproduction. Asexual reproduction by spore formation is absent.

Vegetative Reproduction

The vegetative reproduction in Chara involves various kinds of reproductive bodies which on detachment from the parent plant give rise to a new plant. The common means of vegetative reproduction are as followings:

Amylum Stars: These are star-shaped groups of cells developed from the lower nodes. The cells contain amylum starch. After being detached these develop into new plants.

Bulbils: These are small, rounded or star-shaped, tuberous bodies that develop either on rhizoids (Chara aspera) or on stem nodes (Chara baltica). These develop into new plant on detachment.

Secondary Protonema: In some cases, protonema-like outgrowths arise from the node of primary protonema or from the basal cell of the primary rhizoid. These are called secondary protonema and develop into new plants like primary protonema.

Sexual Reproduction

The sexual reproduction is an advanced type of oogamous sexual reproduction. The gametes are produced. In antheridia and oogonia that are enveloped in multicellular sheaths formed of cells derived from the cells present below the sex organs. These specialist complex structures containing the sex organs are called globule (male fructification) and nucule (female fructification). Some species are dioecious while the others are monoecious. The monoecious species are protandrous, i. e., the male sex organs develop first.

Globule

The male fructification is called as globule. The mature globule is bright yellow in color. It consists of following parts.

Pedicel cell

The globule is attached to the plant by a large cell called as pedicel cell. It extends within the cavity of the globule and join with the primary capitula

Shield cells

Each globule is in the form of ball like structure whose wall consists of large plate like eight cells called as shield cells. The outer wall of each shield cell has radial out growth therefore it appears as multicellular structure.

Manubrium

On the inner side of shield-cells in the center is attached an elongated cell called as manubrium.

Primary capitula

The inner ends of manubrial cells are united to form eight isodimetric cells called as primary capitula

Secondary capitula

Each primary capitula has one or two smaller cells towards the cavity of globule called as secondary capitula.

Antheridial Filament

Attached to the primary or secondary capitula are several branched uniseriate filament called as antheridial filament. Its contents are metamorphosed into a single antherozoid.

Antherozoid

Each antherozoid is an elongated somewhat coiled structure. It bears two flagella.

Liberation of Antheridia

When the antherozoids are mature, the shield cells are separated from each other, exposing the antheridial filament. Antherozoids are than escape through a pore. The liberation of antherozoids may takes place in the morning and they may swim until the evening.

Development of Globule

The development of the globule starts from the adaxial peripheral cell of the lower node of a branch of limited growth. It divides by a periclinical division into an outer globule initial and an inner cell that undergoes another periclinal division. The lowermost cell act as internodal cell and the middle one forms the basal node of the antheridium. The basal nodal cell divides to produce five peripheral cells, the uppermost cell acting as nucule initial.

The globule Initial divides transversely into a basal pedicel cell and a terminal antheridial mother cell. The pedicel cell does not divide further rather elongates and protrudes into the antheridial cavity. The antheridial mother cell becomes spherical and divides by two vertical divisions at right angles to each other to form a quadrat (only two are visible in a section). All these cells divide transversely to produce an octant of eight cells. Each of these eight cells divide periclinally into an outer and an inner cell. The cells of outer layer divide periclinally again and as a result three layers of cells are formed lying one above the other in the same radius. At this stage, the antheridium consists of twenty-four cells arranged in eight diagonal series of three cells each.

Nucule

The female fructification is called as nucule. It consists of following parts.

Pedicel cell

It is present at the base of the nucule on which are present central and stalk cell.

Oogonium

Upper to the stalk cell is a very much-enlarged structure called as oogonium. Its contents are transformed into a single large uninucteate egg.

Tube cells

The oogoniurn is covered by five elongated spirally twisted cells, called as tube cells.

Corona

At the top of the oogonium is a crown of small five cells called the corona.

Development of Nucule

The nucule initial divides twice transversely to produce a filament of three cells. The lowermost cell elongates and function as pedicel of the nucule. The middle cell divides by vertical divisions to form five sheath initials surrounding a central cell. The terminal cell functions as oogonium mother cell. It elongates and divide transversely into a lower small stalk cell and an upper oogonium. The oogonium enlarges and its contents metamorphose into a single egg.

The sheath initials elongate, grow upward, and divide transversely to form two tiers of five cells each. The cells of the upper tier function as corona cells and these forms the corona of the nucule. The cells of the lower tier act as tube cells. The tube cells elongate and twist spirally in clockwise direction around the oogonium. The egg becomes filled with starch and oil, its nucleus migrates towards the lower side and a receptive spot develops at the top of it.

Fertilization

When the nucule is Mature, the spirally twisted tube cells separated from each other just below the corona to form five small slits. Antherozoids swim through these slits and enter into the sheath of nucule. One of the antherozoid enters the egg and fertilization is completed.

The zygote secretes a thick wall and become oospore. The zygospore falls to the bottom of the pond and germinates after a period of rest of few weeks or more.

Structure and Germination of Oospore

The oospore germinates after a period of rest. Its nucleus divides by meiosis to produce four haploid nuclei. The oospore divided into two unequal cells by a wall. The upper cell is small and uninucleate and the lower cell is larger and contains three nuclei which disintegrate later on. The oospore wall burst open to expose the upper cell. It divides by an oblique longitudinal wall into a protonemal initial and a rhizoidal initial. The rhizoids develop from the rhizoidal initial and the protonemal initial develops into an erect primary protonema which is differentiated into nodes and internodes later on. A new adult plant develops from the primary protonema.

Classification of Chara

The classification of Chara is as follow

Division: Charophyta

Class: Charophyceae

Order: Charales

Family: Characeae

Genus: Chara