Although the “alternation of generation” is a widely used term in botany, the correct interpretation of this term is very important. It is known also as “alternation of phases” which means that the a haploid and a diploid phase can be generally distinguished in the life cycles of plants. These “generations” consequently do not correspond to the “generation” used in genetics. The plants’ diploid spore mother cells produce haploid spores via meiotic cell division. The haploid spores divide mitotically; this way a haploid organism develops from them. This haploid organism contains the sexual organs, in which gametes are produced via mitotic division. Since this haploid organism produces the gametes, this organism is called gametophyte (and the haploid phase is called gametophyte phase). The fusion of the gametes results in the production of zygote, which dividing with mitotic cell divisions produces a diploid organism. This diploid organism contains diploid spore mother cells which produce spores via meiotic divisions. This is the origin of the name of the diploid organism: it is called sporophyte (and the diploid phase sporophyte phase). The ratio of the gametophyte to the sporophyte is variable. The gametophyte can be very simple, it can contain only a few cells; in this case the sporophyte is dominant but we can find the opposite phenomenon: the sporophyton is represented only by the single cell of the zygote and the gametophyte is dominant in the life cycle of a given plant. There are examples for the intermediary stage, too, when the gametophyte and sporophyte organisms are morphologically similar (they are izomorph). On the basis of the ratio of the gametophyte and sporophyte , the life cycles are categorized into haplontic, diplontic, haplodiplontic or diplohaplontic types. Despite the evolutionary trends in the ratio of gametophyte to the sporophyte (i.e. the cell number and the importance of gametophyte gradually decrease and those of the sporophyte gradually increase) there is no absolute rule for the dominance of gametophyte in primitive plants and for the dominance of the sporophyte in developed plants. We can find unicellular and diploid diatoms (Bacillariophyceae) and giant diploid brown alga kelps (Phaeophyceae).