1. By fission: Unicellular algae grow in size by taking food and divide into two new algae by fission process. Like- Chlorella, Euglena, Navicula, Diatom, Protococcus, Gloeocapsa, Synechococcus etc.
2. Segment: When filaments and colonies of multicellular algae are divided into two or more segments by injury or physical reasons, a new algae is formed from each segment. E.g. Ulothrix, Oedogonium, Spirogyra, Nostoc, Oscillatoria etc.
3. Budding: In favorable environment, the algae grows in size by taking food and outgrowths form from the side of the body. A bud or bud is then produced from the subgrowth and the bud separates from the mother body to give rise to new algae. Eg- Protosiphon.
4. Hormogonia: Trichrome of blue-green algae breaks down to form hormogonia. The hormogonium is covered by a thick wall to form the hormocyst. New algae are produced from hormogonia in favorable conditions. Eg Nostoc, Oscillatoria etc.
5. Hormocysts: In unfavorable environment hormogonium of algae is covered by thick wall to form hormocyst. Under favorable conditions, hormocysts germinate and give rise to new algae. Eg Nostoc, Oscillatoria etc.
6. Tuber/Bulbil: Tuber or bulbil is produced by the accumulation of food above and below the rhizoid in some algae. These separate from the mother body and form new algae. Like- Chara, Cladophora, Botrydium etc.
7. Akinity: In unfavorable environment, one or more cells of some protozoan algae swell to store large amounts of food and develop into resting spores covered by a thick wall. These cells are called akinites. Each actinomycete germinates into a new alga in a favorable environment. Such as Nostoc, Oscillatoria, Pithophora, Ulothrix, Gloeotrichea, Revularia etc.
8. Star stach: White matter accumulates in the body of some algae and forms a star-shaped organ. They are called starch stars or amylum stars. Later new algae are born from the starch star. Like Chara, Botrydium etc.
9. Protonema: Protonema is formed from the rhizoids of some algae. New algae grow from these protonema. Like- Chara, Nitella etc.