First all extant plants( all known fossil plants) have a dibiontic lifecycle: each species has a multicellular gametophyte and also a multicellular sporophyte.
Some algae , especially coleochaete, are monobiontic, having only one multicellular generation: A zygote undergoes only meiosis, producing more spores that will grow into new gametophytes.
The interpolation hypothesis, postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically.
The transformation theory, postulates that after the dibiontic life cycle originated both gametophyte and sporophyte became larger, more complex and vascularized in a life cycle with an alternation of isomorphic generations.
