Every plant lives two lives, and in mosses the hidden generation is the one you see
Plants alternate between two separate multicellular bodies: one carrying a single set of chromosomes, one carrying two. In trees and flowers the double-set generation dominates and the other shrinks almost to nothing. In mosses it is reversed. The green carpet you walk on is the gametophyte, with its partner living attached to it.
The gametophyte grows from a spore that has one set of chromosomes. It is the sexual phase of the plant, forming organs that make eggs and sperm. When they fuse, the resulting zygote has two sets and grows into the sporophyte, which in turn produces spores by meiosis, and the cycle begins again. In some green, red and brown seaweeds, including Ulva lactuca, the two generations look identical, and Ulva's gametes are all alike in size and shape. In land plants, eggs and sperm always differ.
Mosses, liverworts and hornworts put the gametophyte centre stage. It lives longer, feeds itself, and carries the sporophyte, which depends on it. A germinating moss spore first grows a thread of cells called a protonema before developing leafy shoots bearing sex organs: archegonia for eggs and antheridia for sperm. Some liverworts raise their gametes on special stalks called gametophores.
Vascular plants tip the balance the other way, and as seed reproduction evolved, female gametophytes became ever smaller and more dependent. In most ferns the gametophyte is still a free-living, photosynthesising sheet called a prothallus, which nurtures the young sporophyte. Some fern groups keep theirs underground, fed by partnerships with fungi. Plants making one kind of spore grow free-living gametophytes that may be bisexual or single-sexed; plants making two kinds develop them inside the spore wall, each producing only sperm or only eggs. In Isoetes and Selaginella, the large spore cracks open along a seam so sperm can reach the eggs inside.
Fossils show the gametophyte was once grander. Specimens from the Rhynie chert, including the early Devonian Aglaophyton, had conducting strands, a cortex and a cuticle with stomata, pores no living gametophyte possesses, though they were much smaller than their sporophytes.
Source: Gametophyte
