The successive appearance of the gametophyte stage and the sporophyte stage in the life cycle of a plant is called generation. There are two phases in the life cycle of Terris. Sporophyte Dasha and Gametophyte Dasha.
1. Sporophyte stage: The stage of plant life cycle in which spores are produced is called sporophytic 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 the sporophyte stage. The zygote is enveloped by the wall and becomes a zygospore. Zygospore germinates in favorable environment. The germinating zygospore divides repeatedly in the process of mitosis to form the sporophyte plant. Sporangia are produced along the margins of sporophyte plant leaves. Each sporangium contains 16 spore mother cells. Each spore mother cell divides by meiosis to produce four spores. This condition is called tetrad. As a result, 64 spores are produced in each sporangium. When the spore develops, the wall of the sporangium ruptures and comes out. The sporophyte stage ends with the formation of spores.
2. Gametophyte stage: The stage of plant life cycle in which gametes i.e. sperm and egg are produced is called gametophytic stage. In sporophyte plants, spores are produced inside the sporangium. The spore is the first cell of the gametophyte stage. Spores germinate in favorable environment to form 4-6 celled protonema. Cells on either side of the protonema divide repeatedly in the process of mitosis to form a green heart-shaped prothallus. Pteris is a companion plant. Flax-shaped archegonia below the anterior groove and spherical or ovoid antheridia below the thallus. Each archegonium produces one ovule and 20-50 spermatozoa in the antheridium. Sperm and egg unite to form a zygote. The gametophyte stage ends with the formation of the zygote.
The zygote germinates under favorable conditions to form the sporophyte. Spores are produced in sporophytes. Spores germinate to form gametophytes. Thus the sporophyte and gametophyte stages appear successively.