Charophytes | Classification of Charophytes | Cell structure and thallus morphology | Reproduction and life cycle of Chara

 Charophytes

• The charophytes are green algae, which occupy an isolated group between Chlorophyta and Bryophyta. 
• They are multicellular organisms. 
• They like to live in fresh and brackish water and are mostly confined to the bottom of clear lakes. 
• The body is attached to the substratum by using its rhizoids.
• The cells are long either uninucleate or multinucleate. 
• Thallus growth of charophytes is apical.
• It mostly takes place through a meristematic cell which divides transversely into 2 daughter cells.
• It forms the upper nodal cells and the lower internodal cells. 
• The internodal cell will elongate to form the internode. 
• The nodal cell divides further to form the nodal zone and meristematic cell. 
• From the nodal zone, the branches arise.
• The members of Charophytes are important experimental organisms.

 Classification:

• Class: Charophyceae
• Order: Charales 
• Family: Characeae
• Sub-family: Charae and Nitellae 

There are four genera and more than 300 species of Charophytes. Chara and Nitella are well-known genera.

 Cell structure:

• The cell wall of charophytes is composed of cellulose. 
• A large size vacuole is located in the center of the cell. 
• The cytoplasm is divided into an ectoplasm or ectoderm and an endoplasm or endoderm.
• The ectoplasm contains chloroplasts and some other organelles. 
• The endoplasm has numerous microfilaments, which are involved in cytoplasmic streaming and movement of materials.
• The cell also consists of Chloroplast and pigments for photosynthesis and stored materials.
• Starch is formed in chloroplasts as the end product of photosynthesis. 

The photosynthetic pigments are: 

1. Chlorophyll: chlorophyll a and chlorophyll b. 
2. Carotenoids: α carotenoids and β carotenes 
3. Xanthophylls: Antheraxanthin, neoxanthin, violaxanthin, and zeaxanthin

Thallus morphology and growth:

• The body of charophytes can be seen macroscopically. 
• They are erect in quiet waters and bend with the current of running water. 
• They may be 30 cm or more in length. 
• The main axis of the plant body is divided into nodes and internodes. 

1. Each node has a whorl of branches composed of several cells. These branches do not grow after attaining a certain length. 
2. The internodes consist of a single large cell with a considerably large vacuole. The branching is of two types i.e. branches of limited and unlimited growth. 

 Reproduction:

 Reproduction takes place through both the vegetative method and the sexual mode of reproduction.

Vegetative reproduction: 

• Any fragment or part of the body from a node can produce rhizoids and adventitious shoots and can establish a new individual body. 
• Moreover, the rhizoids can spread and develop bulbs and give rise to colonies of erect photosynthetic shoots.

Sexual reproduction: 

• Sexual reproduction is an advanced oogamous type. 
• The male sex organ is the globule or antheridium while the female sex organ is the nucule or oogonium. 
• In Chara, they are born on a single individual. 
• The antherozoids are formed inside the globule in the form of filaments and are liberated at maturity.
• The microtubular antherozoids are motile and composed of three regions.

1. The head region contains mitochondria
2. The middle region contains the nucleus 
3. The tail region consists of plastids

• Both male and female gametes fuse and form a zygote. 

Life cycle or Reproduction cycle of Chara.

• The nucleus in its center has oogonium, which contains an egg. 
• When the egg is mature, the nucule makes an opening in it for the entry of antherozoids. 
• The antherozoids swim to the egg to fuse with it and form the zygote. 
• The zygote develops a thick wall around it and forms the oospore, which is released to the outside environment.
• At the time of germination, the oospore undergoes a meiotic division to form 4 haploid nuclei, which are soon partitioned into two unequal cells. 
• The upper cell has one nucleus and the lower cell has 3 nuclei and many reserve materials. 
• Soon the nuclei in the lower cell degenerate and the partition membrane between both cells breaks away. 
• Then this cell divides longitudinally. 
• One of the two cells produces rhizoids and the other will grow out in opposite directions to form an erect thread. This structure is called protonema. 
• The mature individual arises as a lateral branch from protonema.

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