Biology · Plant Kingdom · NEET
Algae reproduce by three methods. (1) Vegetative reproduction, done by fragmentation. (2) Asexual reproduction, done by spores (mainly zoospores). (3) Sexual reproduction, done by the fusion of two gametes. NCERT lists these three in exactly this order, so remember them as one set. This matters for NEET because match-the-column and 'which is correct' questions often mix up which method uses fragments, spores or gametes.
Fragmentation is a VEGETATIVE method, not asexual. The thallus (algal body) simply breaks into pieces, and each fragment grows into a new, complete thallus. No spores and no gametes are involved. Students often wrongly call fragmentation 'asexual' because no gametes are used. NCERT is clear: fragmentation = vegetative reproduction. Asexual means spores. So keep fragmentation and spores in separate boxes.
Zoospores are the most common asexual spores in algae. 'Zoo' means moving, so zoospores are flagellated and motile (they can swim using flagella). On germination each zoospore grows into a new plant, without any fusion of gametes. Because no two cells fuse, it is asexual, not sexual. Brown algae (Phaeophyceae) produce special biflagellate, pear-shaped zoospores with two unequal lateral flagella. This exact fact was tested in NEET 2018 and 2024.
All three are types of SEXUAL reproduction (gamete fusion), told apart by gamete size and motility. Isogamy = two gametes similar in size (can be flagellated like Ulothrix, or non-flagellated like Spirogyra). Anisogamy = two gametes different in size (both may be motile), e.g. Eudorina. Oogamy = a large, non-motile female gamete (egg) fuses with a small, motile male gamete, e.g. Volvox and Fucus. Memory: iso = same, an-iso = not same size, oo = one big egg (oogamy).
Yes. Isogamy only means the two gametes are SIMILAR IN SIZE. It does not require them to swim. In Ulothrix the isogametes are flagellated (motile), while in Spirogyra the isogametes are non-flagellated (non-motile) but still similar in size. Both are isogamous. This is a classic trap: students think non-motile gametes cannot be isogamous, but size is what defines isogamy, not movement.
No. Red algae reproduce asexually by NON-MOTILE spores (not motile zoospores) and sexually by NON-MOTILE gametes. They have no flagellated cells at any stage. This is why NEET 2018 marked 'Uniflagellate gametes - Polysiphonia' as the wrong match: Polysiphonia is a red alga with non-motile gametes. So link 'red algae = no flagella, non-motile spores and gametes' firmly in memory.
Which one is wrongly matched?
Read the following statements and choose the set of correct statements. In the members of Phaeophyceae: A. Asexual reproduction occurs usually by biflagellate zoospores. B. Sexual reproduction is in oogamous method only. C. Stored food is in the form of carbohydrates which is either mannitol or laminarin. D. The major pigments found are chlorophyll a, c and carotenoids and xanthophyll. E. Vegetative cells have a cellulosic wall, usually covered on the outside by a gelatinous coating of algin.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The zoospore. Zoospores are flagellated and motile, and on germination each one grows into a new plant. NCERT names the zoospore as the most common asexual spore of algae.
Isogamous: Ulothrix (motile) and Spirogyra (non-motile). Anisogamous: Eudorina. Oogamous: Volvox and Fucus. These NCERT examples are frequently tested, so memorise them as pairs.
Neither. Fragmentation is a VEGETATIVE method of reproduction in algae. The thallus breaks into pieces and each fragment forms a new thallus, without spores or gametes.
Red algae (Rhodophyceae) have no flagellated cells at any stage of their life. So they reproduce asexually by non-motile spores and sexually by non-motile gametes, never by motile zoospores.
Brown algae (Phaeophyceae) produce biflagellate, pear-shaped (pyriform) zoospores that bear two unequal, laterally attached flagella. This exact point was tested in NEET 2018 and 2024.
