The process of sexual reproduction in which sperm and egg produced in the same thallus combine to form a zygote is called homothelic sexual reproduction. E.g. Spirogyra, Chlamydomonas etc.

According to the nature of the thallus, the sexual reproduction process of algae can be divided into two types.
1. Homothelic: The process of sexual reproduction in which sperm and egg produced in the same thallus combine to form a zygote is called homothelic sexual reproduction. E.g. Spirogyra, Chlamydomonas etc.
2. Heterothelic: The process of sexual reproduction in which sperm and egg produced in different thallus combine to form zygote is called heterothelic sexual reproduction. eg Ulothrix.
According to the nature of the reproductive cells or gametes, the process of sexual reproduction of algae (heterothallic) can be divided into three types.
1. Isogamy: The union of two gametes of the same nature in terms of size, shape and structure is called isogamy. Gametes that participate in sexual reproduction are called isogametes. It can be flagellated and motile or non-flagellated and immobile. Such as Ulothrix, Chlamydomonas, Spirogyra etc.
2. Anisogamy: The union of two gametes of different nature in terms of size, shape and structure is called anisogamy. Gametes that participate in sexual reproduction are called anisogametes. It can be flagellated and motile or non-flagellated and immobile. One is large in size and the other is small in size. Larger gametes are called macrogametes or female gametes and smaller gametes are called microgametes or pungametes. Such as Pandorina, Chlamydomonas, Caulerpa, Ectocarpus, Ulothrix etc.
3. Oogamy: The process of sexual reproduction in which the male gamete is small and motile and the female gamete is large and stationary is called oogamy. A single ovule is produced in the female genitalia. A zygote unites with an egg to form a zygote. Such as Fucus, Oedogonium, Chlamydomonas, Vaucheria, Volvox, Chara, Polysiphonia, Sargassum etc.

Algae of Bacillariophyceae complete asexual reproduction by producing a special type of spore called axospores. Eg – Chaemaesiphon, Navicula etc.

Class Bacillariophyceae and some blue-green algae reproduce asexually by endospore formation. For example, Chaemaesiphon.

Aplanospore stores a lot of food in harsh environment and is surrounded by a thick wall. It is called Hypnospore. Such as Ulothrix, Botrydium, Chlamydomonas, Vaucheria, Pediastrum, Sphaerella etc.

In extremely dry conditions, the protoplast of the algal body cell divides to form numerous daughter cells. These non-flagellated (aplanospore) germ cells are covered by a gelatinous membrane. This condition is called Pamela Dasha. Under favorable conditions the gelatinous coating dissolves in water flow and each cell transforms into a zoospore. Each zoospore gives rise to new algae. Such as Fucus, Sargassum, Chlamydomonas etc.

Very small aplanospores are formed in the algal body similar to the mother cell. These spores are called autospores. Autospores germinate to form new plants. For example – Chlorococcus, Chlorella, Scenedesmus etc.

Aplanosporangium is formed in the algal body in unfavorable environment. Non-flagellated cells are produced inside the aplanosporangium. These molecules are enveloped by the wall and become aplanospores. Spores germinate in favorable environment to form new filaments. Such as Vaucheria, Ulothrix, Microspora, Chlorella etc.

One or more cells of algae transform to form zoosporangium. The protoplast of zoosporangium divides to produce two or more zoospores. These zoospores have two or four flagella and are motile. However, a single zoospore with many flagella is produced inside the zoosporangium of algae of class Oedogoniales and family Vaucheriaceae. It is called synzoospore. These zoospores germinate and give rise to new algae. Such as Ulothrix, Chara, Vaucheria, Ectocarpus, Oedogonium etc.

1. Zoospore: One or more cells of algae transform to form zoosporangium. The protoplast of zoosporangium divides to produce two or more zoospores. These zoospores have two or four flagella and are motile. However, a single zoospore with many flagella is produced inside the zoosporangium of algae of class Oedogoniales and family Vaucheriaceae. It is called synzoospore. These zoospores germinate and give rise to new algae. Such as Ulothrix, Chara, Vaucheria, Ectocarpus, Oedogonium etc.
2. Aplanospore: Aplanosporangium is formed in the algal body in unfavorable environment. Non-flagellated cells are produced inside the aplanosporangium. These molecules are enveloped by the wall and become aplanospores. Spores germinate in favorable environment to form new filaments. Such as Vaucheria, Ulothrix, Microspora, Chlorella etc.
3. Autospore: Very small aplanospores are formed in the algal body similar to the mother cell. These spores are called autospores. Autospores germinate to form new plants. For example – Chlorococcus, Chlorella, Scenedesmus etc.
4. Pameloid: In extremely dry conditions, the protoplast of the algal body cell divides to form numerous daughter cells. These non-flagellated (aplanospore) germ cells are covered by a gelatinous membrane. This condition is called Pamela Dasha. Under favorable conditions the gelatinous coating dissolves in water flow and each cell transforms into a zoospore. Each zoospore gives rise to new algae. Such as Fucus, Sargassum, Chlamydomonas etc.
5. Hypnospore: Aplanospore stores a lot of food in harsh environment and is surrounded by a thick wall. It is called Hypnospore. Such as Ulothrix, Botrydium, Chlamydomonas, Vaucheria, Pediastrum, Sphaerella etc.
6. Endospore: Class Bacillariophyceae and some blue-green algae reproduce asexually by endospore formation. For example, Chaemaesiphon.
7. Axospore: Algae of Bacillariophyceae complete asexual reproduction by producing a special type of spore called axospores. Eg – Chaemaesiphon, Navicula etc.
8. 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, Oedogonium, Gloeotrichia, Oscillatoria, Pithophora, Ulothrix, Revularia, Cladophora etc.