The successive appearance of gametophyte and sporophyte stages in the life cycle of a plant is called generation. Pteris has two stages in its life cycle. Gametophyte state and sporophyte state.
1. Gametophyte stage: The stage of plant life cycle in which gametes i.e. sperm and egg are formed is called gametophyte stage. Spores are produced in sporophyte plants. Spore is the first cell of gametophyte. Spores germinate in favorable environment. The germinated spore divides by mitosis to form a green, flat and cordate prothallus. Fern prothallus is a gametophyte plant. Under its anterior groove the pitcher or flux-shaped female genital archegonia and below the thallus the round or ovoid male genital antheridia grow. Eggs are produced in the archegonium and spermatozoa in the antheridium. Sperm and egg unite to form zygote. The gametophyte phase ends with the formation of the zygote.
2. Sporophyte stage: The stage of plant life cycle in which spores are produced is called sporophyte stage. Gametophytes produce sperm and ovules in plants. A zygote is formed by the union of sperm and egg. Zygote is the first cell of sporophyte. The zygote becomes enveloped by the wall and becomes a zygospore. Zygospore germinates in favorable environment. The germinating zygospore divides repeatedly by the process of mitosis to form the sporophyte plant. The sorus grows on the leaf margin of the sporophyte. The sorus contains sporangia. Inside the sporangium is the spore mother cell. Each spore mother cell divides by meiosis to produce four spores. The sporophyte stage ends with the formation of spores.