Life Cycle of Bryophytes
Bryophytes show a marked advance over thallophytes in the method of sexual reproduction. It is highly oogamous. Bryophytes have multicellular, jacketed sex organs. Each sex organ consists of an outer protective wall of sterile cells surrounding the cell or group of cells which produce the gametes. The male sex organ is called the antheridium and the female archegonium. A single egg is produced in the swollen basal portion of each archegonium. When this egg matures, the central cells in the neck of the archegonium disintegrate, leaving an opening at the top and a column of fluid through which the sperms swim to the egg. Antheridium is made up of an outer layer of cells surrounding specialised cells that become the sperms.
Fertilisation takes place in the archegonium. The zygote develops into a young sporophyte, which marks the beginning of sporophyte generations in the life cycle. The zygote inside the archegonium develops into a young sporophyte having seta (stalk-like structures) and a sporangium (swollen capsule). After meiosis, haploid spores are produced. The sporophytes draw their food from the photosynthetic gametophytes on which they are attached. These spores mark the beginning of the new gametophyte generations. Each spore germinates under suitable conditions to give rise either to the main gametophyte plant directly or first to a juvenile, filamentous stage called protonema from which sooner or later arises the main gametophyte.
Sporangium is the end product of the sexual act. It is generally considered a second individual in the life cycle because it is with a different inheritance as it is developed from the diploid zygote. It is made up of cells having a diploid number of chromosomes unlike cells with a haploid number of chromosomes in the gametophytes. Besides it differs from the parent gametophyte in its function.
The life cycle of moss is summarised in the following figure: