Features of the plant kingdom  (Page 3/13)

Alternation of generations

Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages ( [link] ).

Most plants exhibit alternation of generations, which is characterized by alternating between two generations: the gametophyte generation followed the sporophyte    . The gametophyte    gives rise to the gametes, or reproductive cells, by mitosis. It can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain in the higher plants (the collective term for the vascular plants). The sporophyte stage is barely noticeable in lower plants (the collective term for the plant groups of mosses, liverworts, and hornworts) whereas the root, shoot and leaf system that you see when looking at towering trees such as sequoias and pines is the sporophyte structure in more derived groups of plants.

Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered from desiccation and other environmental hazards. In both seedless and seed plants, the female gametophyte provides protection and nutrients to the embryo as it develops into the new generation of sporophyte.

Sporophytes in the seedless plants

The sporophyte of seedless plants is diploid and results from the fusion of two gametes ( [link] ). The sporophyte bears the sporangia (singular, sporangium), organs that first appeared in the land plants. The term “sporangia” literally means “spore in a vessel,” as it is a reproductive sac that contains spores. Inside the multicellular sporangia haploid spores are produced by meiosis, which reduces the 2 n chromosome number to 1 n . The spores are later released by the sporangia and disperse in the environment. After germinating from a spore, the gametophyte produces both male and female gametes, usually on the same individual.

When the haploid spore germinates, it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte or vegetative form, and the cycle begins anew ( [link] and [link] ).

Gametophytes in the seedless plants

The gametophytes of seedless plants in which gametes are produced by mitosis. The male gametophyte: the antheridium, releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia, the female gametophyte. The embryo develops inside the archegonium as the new sporophyte.