Algae

The term algae has been derived from the Latin word alga, meaning seaweed.

The study of algae is known as phycology or algology.

According to the notes, there are about 1,500 genera with 17,535 species of algae (Smith, 1955).

General Characteristics of Algae

• They are autotrophic thallophytes.
• They are generally aquatic in habitat.
• Their thallus ranges from simple unicellular forms to highly complex multicellular forms.
• The cell wall is mainly made up of cellulose.
• Reserve food is generally stored in the form of starch.
• Photosynthetic pigments include chlorophylls, carotenoids, xanthophylls and phycobilins.
• Reproduction takes place by vegetative, asexual and sexual methods.
• Asexual reproduction occurs through motile and non-motile spores.

Types of Algae

The notes classify algae into three important classes:

Comparison of Chlorophyceae, Phaeophyceae and Rhodophyceae

Spirogyra

Habit and Habitat

Spirogyra is a green alga found in freshwater bodies such as ponds, rivers, lakes and streams.

It is commonly known as pond silk or water silk.

Initially, it remains attached to the substratum by means of a holdfast. Later, it may become detached and float on the surface of water.

It is slippery to touch due to the presence of a mucilaginous substance.

Structure of Spirogyra

Spirogyra is a filamentous alga. Its filament is long, hair-like or thread-like and consists of cells arranged one above another in a single row.

The plant body is a thallus and is not differentiated into roots, stem and leaves.

Each cell is approximately rectangular in shape and is surrounded by a cellulose cell wall.

A large central vacuole is present, due to which the cytoplasm is pushed towards the periphery.

The nucleus is suspended inside the cell by thin, thread-like cytoplasmic strands.

A characteristic ribbon-shaped, spirally coiled chloroplast is present.

Reserve food material is stored as starch associated with pyrenoids.

1. Vegetative Reproduction

Vegetative reproduction takes place by fragmentation.

It generally occurs during favourable conditions. The filament breaks into smaller fragments due to mechanical injury or decay of intercalary regions.

Each fragment then develops into a new filament.

2. Asexual Reproduction

According to the notes, asexual reproduction in Spirogyra occurs by formation of:

• Akinetes
• Aplanospores
• Parthenospores

3. Sexual Reproduction

Sexual reproduction in Spirogyra is described as isogamous because there is no obvious difference in the shape of the gametes.

Sexual reproduction takes place by conjugation.

Conjugation occurs mainly by:

1. Lateral conjugation
2. Scalariform conjugation

A. Lateral Conjugation

Lateral conjugation is described in the handwritten notes as the homothallic mode of sexual reproduction.

It occurs between adjacent cells of the same filament.

1. The entire contents of the cells round off to form male and female gametes.
2. The wall between the two adjacent cells develops small outgrowths called papillae.
3. The common wall becomes partially dissolved and a minute passage called the conjugation tube is formed.
4. The male gamete migrates through the conjugation tube.
5. It fuses with the female gamete and forms a zygote.
6. The zygote secretes a thick wall around itself and forms a spore-like structure called a zygospore.
7. The zygospore remains dormant during unfavourable conditions and later develops into a new filament when favourable conditions return.

B. Scalariform Conjugation

Scalariform conjugation is described in the notes as the heterothallic mode of sexual reproduction.

It occurs between two different filaments.

1. Two filaments of opposite strains come close to each other.
2. They are held together by mucilaginous substances secreted by both filaments.
3. The contents of the cells round off to form gametes.
4. Small protuberances arise from corresponding cells of the two filaments and meet in the centre.
5. The walls at the point of contact dissolve, resulting in formation of a minute passage known as a conjugation tube.
6. The male gamete migrates through the conjugation tube towards the female gamete of the opposite filament.
7. Fusion results in the formation of a zygote.
8. The zygote develops a thick wall and becomes a zygospore.
9. The zygospore undergoes a resting period during unfavourable conditions.
10. It develops into a new filament after the return of favourable conditions.

Germination of Zygospore

The zygospore is a thick-walled resting spore.

It is surrounded by two walls:

• Outer exine
• Inner intine

The zygospore remains dormant during unfavourable conditions and germinates when favourable conditions return.

1. Before germination, the nucleus undergoes meiosis.
2. Four haploid nuclei are formed.
3. Out of the four nuclei, three degenerate and one remains functional.
4. The zygospore absorbs water and swells.
5. The outer exine ruptures.
6. The inner intine comes out in the form of a germ tube.
7. Repeated mitotic divisions take place.
8. Finally, a new filament is formed.

Important Questions

1. What are algae?
2. What is phycology?
3. Write the general characteristics of algae.
4. Name the three major classes of algae described in the notes.
5. Differentiate between Chlorophyceae, Phaeophyceae and Rhodophyceae.
6. Write the systematic position of Spirogyra.
7. Describe the structure of Spirogyra.
8. Why is Spirogyra called pond silk or water silk?
9. Explain vegetative reproduction in Spirogyra.
10. Describe asexual reproduction in Spirogyra.
11. What are akinetes?
12. What are aplanospores?
13. What are parthenospores?
14. Explain lateral conjugation in Spirogyra.
15. Explain scalariform conjugation in Spirogyra.
16. Describe the germination of zygospore in Spirogyra.