The life cycle of bryophytes has two alternations of generations, one is haploid gametophyte generation and the other is diploid saprophytic generation. In both the life cycle, the bryophytes possess different morphology and physiological functions. Between these two alternations of generation, the haploid gametophyte phase lasts longer or dominates the diploid saprophytic phase.
Gametophytes (male and female gametes) are developed by the heterosporous (two types) bryophytes, where one kind of spore develops a male gametophyte, while the other develops a female gametophyte.
Content: Life Cycle of Bryophytes
Meaning of Bryophytes
Bryophytes are the terrestrial, non-vascular plants that grow in damp or humid habitats and reproduce via spore generation instead from seed germination. Mosses, hornworts, and liverworts are the three common plants that come under the bryophyte group. There is one thing in common among the non vascular plants (mosses, liverworts and hornworts) that all possess small leaves, but lacks flowers, wood and true root system.
In addition, the bryophytes possess dual generations (gametophytic and sporophytic phase) in their life cycle. Unlike vascular plants, bryophytes can survive in variety of habitats (rocks, roof , bare soil etc.) as these lack true roots for the water and nutrient absorption.
Certain bryophytes are only capable of asexual reproduction or cannot achieve fertilization, due to loss of functional sexuality. Asexual reproduction occurs vegetatively via small, reproductive structures gemmae that disperse easily. Gemmae comprise either a single cell or cluster of cells that are having undifferentiated growth. A gemma can bring about the formation of a new vegetative structure or gametophyte when it dissociates from the parent plant body.
Fragmentation is another way of asexual reproduction, by which bryophytes can regenerate via leaf and stem fragments. The mechanism of spore dispersal and fragmentation is aided by water, wind, and animal movement.
Its life cycle exists two phases that are explained below:
This stage begins, when the dormant spores form a germ tube. The spores first develop a green, filiform complex called a protonema. A protonema goes through several maturation phases, after which it starts developing rhizoids and aerial filaments. Red light and kinetin stimulate the cells that promoting the growth of shoots, which later enlarge to modify into mature gametophytes.
Then, the gametophytes develop a specialized reproductive structure (gametangia) at the apex of main shoot that holds the gametes (eggs and sperms). The leafy gametophyte develops separate branches for the male gametangium (antheridium) and the female gametangium (archegonium).
The male antheridia produce on the upper branches that hold antherozoids or sperms, while the female archegonium carries a single egg or archaegonia. Before fertilization, the anthrezoids of the male gametangium of same or different plant goes down to the archaegonium via water droplets. There is a unique feature, where both the gametangia of bryophytes are enclosed by a sterile layer of non-reproductive tissue that is absent in algal gametangia.
This stage begins with the fusion of a biflagellate sperm with a single ovum inside the archegonium. After fusion, nuclear and cytoplasmic exchange occurs between anthreaoids and archegonia, resulting in the formation of the diploid zygote (2n), and the process is called fertilization. In bryophytes, a biflagellate sperm goes down to the archegonium via a passage of water film on the plant surface.
A diploid zygote in the archegonium develops into a multicellular, diploid embryo by undergoing repeated mitotic divisions. An embryonic sporophyte highly relies on the gametophyte that provides nutrition sources like sugars and minerals etc.
An embryonic sporophyte differentiates into three distinct structures, namely foot, stalk, and capsule. The foot provides a base to anchor the young sporophyte. A stalk appears as a slender filament that attaches to the sporangium and originates from the foot cell.
A spore capsule appears as a tight cap (calyptra) that comprises a layer of sterile protective cells around an embryonic sporophyte, at the opposite end of foot. A spore capsule encloses a sporangium sac that holds the heterospores, which function as sexual dispersal units. The spore-producing cells go through meiotic cell division to form spores (haploid).
The spore formation occurs, when the foot cell uptakes the nutrients from the gametophyte, and then conducts it to the spore capsule via long stalk or seta. A spore capsule comprises a layer of sterile cells, second layer of elaters that are hygroscopic or absorbs moisture from the surrounding.
The hydrogroscopic movement of the cells within the spore capsule results in flicking of the spores upward and outward. Unfortunately, the spore capsule ruptures, as the spores attain maturity and move freely in the environment as dormant spores. The life cycle of bryophyte continues once the dormant spore starts germinating into protonema.
Therefore, we can conclude that the gametophyte phase contains haploid cells that possess an unpaired number of chromosomes. In contrast, the saprophyte phase includes diploid cells that possess two sets of paired chromosomes. Gametophyte phase occurs predominantly in all the bryophytes and lasts longer.