Describe briefly the structure and life cycle of Chara.

Ans. Systematic Position:

Class : Chlorophyceae

order : Charales

Family : Characeae (According to Fritsch)

Genus : Chara

Occurrence : Chara is a submerged aquatic plant and grows attached to the muddy or sandy bottom of the pools, lakes and the slow flowing streams. Normally it prefers clear, fresh, hard and still water. A few species may be marine.

Structure : The plant body consists of an erect branched kis which may grow to the height of about 20-30 cm. The axis has distinct nodes and internodes. From each node arise a whorl of laterals (branches) of limited growth, the so called leaves. From the axis of some of the leaves branches of unlimited growth may arise (Thus the plant body becomes more or less like Equisetum). Each branch, whether limited or unlimited growth, is also differentiated into nodes and internodes. The plant is anchored to substratum by means of colourless, branched, multicellular rhizoids, which arise from the lower nodes of the axis.

The growth of the axis in length takes place by means of a single dome shaped apical cell.

Each node consists of a plate of cells, while the internode consists of a single elongated which is unsheathed by a cortex of elongated cells. Each cell has a cell wall made up of cellulose and a deposit of CaCO₂. They contain a single nucleus, dense cytoplasm with many discoid chloroplasts.

Reproduction: Reproduction in Chara may take place by the following two methods :

(1) Vegetative

(2) Sexual.

(1) Vegetative reproduction :

(a) By amylum stars or starch stars formation: In this method some of the cells of the lower nodes form a mass of special type of cells which are starshaped and are called amylum starch as they contain amylum starch in their cells.

(b) By bulbils: In this method some of the rhizoids or lower nodes may form bulbils which also give new plants when detached.

(c) By protonema formation: In this method sometimes on the nodes protonema like branches are developed and they also form new plants.

(2) Sexual reproduction: This is highly advanced oogamous type and takes place with the help of male and female reproductive organs known as globule (antheridium) and nucule (oogonium) respectively. Mostly the species are homothallic or monoecious, a few may be heterothallic or dioecious. The two structures are found just opposite at a node. Nucule being above the globule.

As we know the plant body of chara consists of a main axis which is differentiable into nodes and internodes. From the nodes come out branches of limited growth. This also possesses nodes and internodes. The sex organs are developed on the nodes of these branches.

Structure of the globule: The mature globule or antheridium is circular in outline and red or orange in colour. The wall of the globule consists of eight shield cell, the outer wall of each shield cell is marked with numerous infoldings giving the idea that the wall of antheridium consists of more than eight c but this is only apparent. From the centre of each shield cell there arises a rod like out growth, the manubrium which bears at its upper end a primary capitulum cell. The primary capitulum may form secondary and tertiary cells. The secondary capitulum cell usually bears the branched, uniseriate spermatigenous filament which are divided into small segments by transverse septa. Each segment functions as a single antheridium. The cytoplasmic contents of each segment give rise to a single spermatozoid or antherozoid. Each antherozoid is spirally coiled and biflagellate.

Development of the globule: Globule, the male reproductive organ, arises in the axis of the branches of limited growth, from the single superficial cell. This cell cuts off one or two discoid cell at its basal end then becomes spherical. The lower two cells form a pedicle while the upper cell enlarges in size and becomes hemispherical in shape. The upper spherical cell divides by two longitudinal and one transverse division to form octant (8 celled structure). This octant divides by two successive curved plates or shields and constitutes the wall of globule. As the shield cells mature, they develop red pigments and radial growth. The mature, shield cells expand laterally and thus a cavity is formed inside the globule. The wall at the base of the globule is completed by the upper discoid basal cell (pedicle cell) which elongates and protrudes up into the cavity of the globule. The middle eight cells elongate radially to form a rod shaped manubrium which projects inward from the centre of curved shield cells. Each of the inner eight cells becomes a primary capitulum borne at the tip of manubrium. The capitulum cells lie quite close together in the middle of the cavity of the globule. Each primary capitulum buds off about 4 to 6 secondary capitulum cells which may further give rise to tertiary and quaternary ones. Now from each capitulum cell there develop antheridial filament. Each antheridial filament consists of upto 200 discoid cells, the antheridium. Within each antheridium is produced a single elongated, spirally coiled and biflagellate antherozoid.

When the globule or antheridium is matured, the shield cells fall apart and the antherozoids are liberated by the gelatinisation of antheridial walls or through a pore formed in each antheridial cell.

Structure of nucule: The nucule or oogonium is a short stalked body attached to the body of a primary lateral or leaf or dwarf shoot or branch of limited growth just above the antheridium. A mature oogonium consists of a large oval or elliptical egg surrounded by a cover of five tubular cells which make two or more clock-wise spiral turns around it. From the upper end of each of the regular cells, a cell is cut off, thus constituting the crown or corona of oogonium.

Development of nucule: The oogonium or nucule, the female reproductive gan, develops in the axil of the branches of limited growth on the adaxial side. It develops from single superficial adaxial cell. This cell undergoes two successive divisions thus forming three cells. The lower most cell elongates and forms pedicel. The middle cell gives rise to five peripheral cells and uppermost act as oogonial mother cell. Each of the peripheral cell divides transversely forming an upper smaller cell corona cell and the lower larger cell tube cell. The five corona cells elongates little and mature into the corona. The five tube cells elongate many times and becomes spirally coiled around the oogonium. The terminal oogonial mother cell elongated vertically and divides transversely to form a short stalk cell and an elongated oval shaped oogonium containing single uninucleated egg.

The nucule when mature, the tube cells separate from one another just below corona to form five small slits for the entrance of the antherozoids.

Fertilization : At the maturity of oogonium, the tube cells separate, thus forming slits. The apex of the oogonium gelatinises. Several antherozoids enter through these slits but however, only one succeeds resulting in the formation of the zoospore or zygote.

Germination of zygote : The zygote, just after formation, secretes a coloured wall around it and undergoes a period of rest within the oogonium. It becomes red in colour, finally changing into black. It falls from the plant and sinks to the bottom of the pond or stream where it germinates after a few weeks or more.

At the time of germination, the zygote nucleus migrates to the apical pole and divides three into four daughter nuclei, each is haploid in nature The uppermost one nucleus gets separated from the rest of three nuclei by the formation of a transverse wall. The lower three nuclei degenerate. The one nucleus thus left is divided by a longitudinal division forming protonemal and rhizoidal initials. They form the protonema and rhizoid respectively. The protonema increases and develops into a young chara plant.

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