bio

Alternation of generations

The life cycle of every land plant: it alternates between two different multicellular bodies — a haploid gametophyte that makes eggs and sperm, and a diploid sporophyte that makes spores. What is remarkable is that dominance between the two has flipped over plant history. In a moss the gametophyte is the plant you see and the sporophyte is a small stalk living on it. In a fern it is reversed: the fern is the sporophyte and the gametophyte is a thumbnail-sized scrap on the soil. In a seed plant the gametophyte has shrunk to a handful of cells hidden inside pollen and ovule. That progressive takeover is arguably the largest reorganisation in the whole history of plant form.