Biology · Plant Kingdom · NEET
A prothallus is the gametophyte body of a fern (a pteridophyte). It is a small, green, flat, usually heart-shaped structure. NCERT calls it a 'free-living, mostly photosynthetic thalloid gametophyte called prothallus.' It is tiny (a few millimetres) and grows on damp soil. Its only job is to make sex organs and allow fertilisation. This matters for NEET because examiners love the exact word 'prothallus' as the name of the fern gametophyte.
The prothallus is HAPLOID (n). This is a very common NEET trap. It grows from a spore, and spores are made by meiosis, so they are haploid. Therefore every cell of the prothallus is haploid (n). The prothallus then makes gametes (also n) by mitosis. Remember: gametophyte = gamete-maker = haploid = prothallus.
The adult fern (the big leafy plant you see) is the diploid sporophyte. Its leaves bear sporangia, and inside them the spore mother cells do meiosis to make haploid spores. When a spore lands in a cool, damp, shady place, it germinates and grows into the prothallus (the gametophyte). So the order is: sporophyte to spore (by meiosis) to prothallus. NEET 2025 asked exactly this sequence.
The prothallus bears the male and female sex organs. The male organ is the antheridium (makes antherozoids = male gametes) and the female organ is the archegonium (makes the egg). Water is needed to carry the antherozoids to the mouth of the archegonium. After fertilisation, the zygote grows into a new diploid sporophyte on the prothallus. So the prothallus makes the SEX ORGANS, not spores.
The prothallus is the gametophyte: small, heart-shaped, haploid (n), short-lived, and makes gametes. The sporophyte is the main fern plant: large, leafy, has real roots-stem-leaves and vascular tissue, is diploid (2n), and makes spores. In ferns the SPOROPHYTE is dominant and the prothallus (gametophyte) is small and reduced. This is the opposite of bryophytes, where the gametophyte is dominant.
The prothallus is the GAMETOPHYTE. The easy way to remember: 'gamete-o-phyte' makes gametes, and the prothallus bears antheridia and archegonia which make gametes. It is haploid. The sporophyte is the leafy fern that makes spores. Do not mix them up — NEET often puts both words in one question to test you.
Given below are the stages in the life cycle of pteridophytes. Arrange them in the correct sequence: A. Prothallus stage; B. Meiosis in spore mother cells; C. Fertilisation; D. Formation of archegonia and antheridia in gametophyte; E. Transfer of antherozoids to the archegonia in presence of water.
Match the plant with the kind of life cycle it exhibits: (a) Spirogyra (b) Fern (c) Funaria (d) Cycas with (i) Dominant diploid sporophyte vascular plant, with highly reduced male or female gametophyte (ii) Dominant haploid free-living gametophyte (iii) Dominant diploid sporophyte alternating with reduced gametophyte called prothallus (iv) Dominant haploid leafy gametophyte alternating with partially dependent multicellular sporophyte.
Which of the following statements is true?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The fern prothallus is free-living and photosynthetic (it is green and makes its own food). This is different from gymnosperms, where the gametophyte is not free-living and stays inside the sporangium on the parent plant.
Yes. Water is required for fertilisation. The antherozoids (male gametes) released from the antheridia must swim through a film of water to reach the archegonium. This is why ferns grow in cool, damp, shady places.
After the egg is fertilised, the zygote (2n) grows into a new diploid sporophyte — the leafy fern plant. It develops on top of the small prothallus and later becomes independent.
The idea is the same (the prothallus is the gametophyte), but details differ. In homosporous ferns, one prothallus bears both antheridia and archegonia. In heterosporous plants like Selaginella, the gametophytes are separate and much reduced. Learn homosporous vs heterosporous next.
NEET repeatedly tests the exact name 'prothallus', its ploidy (haploid n), and its place in the fern life cycle sequence (spore to prothallus to sex organs to fertilisation). It also appears in match-the-column and life-cycle questions almost every year.
