Vascular plants have a life cycle referred to as alternation of generations, characterized by alternating asexual and sexual phases. The asexual generation reproduces through spores. The spores develop into the sexual generation, which produces gametes. fertilization creates a zygote which develops into a new asexual generation, and the life cycle starts again. During the life cycle, the zygote receives one set of chromosomes from two gametes. Thus the two haploid cells produce one diploid zygote. The zygote is the starting cell for the asexual (or sporophyte) generation; therefore, that generation is diploid. The diploid cells of the asexual generation produce haploid spores by meiosis (or reductive cell division). The spore develops into the haploid sexual (or gametophyte) generation.
First, let's follow the main phases in the life cycle of mosses.
The sporophyte generation develops on the moss plant; it consists of a seta and a sporangium. The sporangium produces haploid spores by meiosis (or reductive cell division). Later, it opens, spores fall on the soil and develop into moss plants. The moss plant consists of haploid cells; this is the sexual (or gametophyte) generation. It absorbs water and minerals through its entire surface. Mosses are thallophytes; they do not have differentiated tissues or real organs. They have root-like, stem-like and leaf-like structures. Gametes are produced by so-called 'reproductive organs' (which are not real organs either) situated on top of the plant. In the presence of water, sperm cells fertilize the eggs and thus diploid zygotes are produced. The zygote is the starting cell for the new asexual generation; it develops into a seta and a sporangium. Thus the life cycle starts again. The sexual generation of mosses is more developed; therefore, it is the dominant generation. The asexual generation is limited to a short phase. In the life cycle of other land plants, however, the asexual generation is dominant.
The fern plant is the diploid sporophyte generation. Its main organs are the rhizome, which consists of the subterranean stem and the roots, and the leaves. On the back of the leaves there are several clusters of sporangia, called sori. Sporangia produce haploid spores by meiosis (or reductive cell division). These spores fall onto the soil when the sporangia open. The spores in wood ferns develop into a characteristic heart-shaped prothallium. The bottom side of the prothallium contains thread-like structures and reproductive organs. Sperm cells swim to the female reproductive organs, where they fertilize the eggs and thus zygotes are formed. Zygotes then develop into fern plants. In the life cycle of ferns, as with angiosperms and gymnosperms, the asexual generation is dominant. In the evolution of plants, the reduction of the gametophyte generation is an important tendency. While ferns have a relatively developed prothallium, the gametophytes of the most developed plants consist of a few short-lived cells within the pollen and the ovule.