Pteridophytes

Ferns illustrate the alternation of generations explained in NCERT Class 11, Chapter 3 (Plant Kingdom), where the dominant phase in pteridophytes is the diploid sporophyte. The familiar large, leafy fern plant is this sporophyte; it bears leaves called sporophylls whose undersurfaces carry sporangia in clusters known as sori. Within the sporangia, meiosis produces haploid spores. Therefore the spore-bearing leafy plant is the diploid sporophyte generation. The haploid gametophyte option is incorrect because in ferns the gametophyte is a small, short-lived, inconspicuous heart-shaped structure called the prothallus, not the large leafy plant. A diploid gametophyte is a contradiction, since gametophytes are by definition haploid. A haploid sporophyte is equally invalid, as sporophytes are diploid by definition. The tiny green prothallus that develops from a germinating spore bears the antheridia and archegonia and depends on a film of moisture for fertilisation, after which a new diploid sporophyte grows directly from the resulting zygote. A consistency check confirms the reasoning: the structure producing spores by meiosis must be diploid, because meiosis reduces a diploid cell to haploid spores, so the spore-bearing fern plant is necessarily the diploid sporophyte.