BSC 314 Lecture 52: A Typical Bryophyte Life Cycle
27 Jun 2018
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A Typical Bryophyte Life Cycle
Although individuals of the three bryophyte groups differ from one another
morphologically and in other details
As do all plants, bryophytes alternate a gametophytic generation with a
sporophytic one (a sporic meiosis, a life cycle in which meiosis gives rise
to spores, not gametes). Each of the haploid (1 n) spores is capable of
developing into a multicellular, haploid individual, the gametophyte. The first
structure formed from spores in most mosses and many liverworts is a
filamentous, algal-like, green protonema (plural,protonemata). In some mosses
the protonemata are long lived with rhizoids and aerial filaments and they
often form dense green mats in suitable sites. Cells in the protonema,
probably stimulated by red light and kinetin, give rise to shoots, which
enlarge and become the mature gametophytes. In the bryophytes, these are
the dominant, independent (photosynthetic) plants.
The gametophytes initiate gametangia on special branches or at the tip of the
main shoot. In these structures the gametes— eggs and sperms—are
produced during the sexual portion of the cycle. The female gametangium—
called an archegonium—and the male antheridium may be produced on the
same plant or on different plants. In both kinds of gametangia, a protective
layer of non-reproductive tissue—a sterile layer—surrounds the inner
reproductive cells. (A sterile layer is absent in algal gametangia and is
considered an upward evolutionary step towards the protective seed coats of
flowering plants.) Mature sperm, released from the tip of the antheridia
when dew or rainwater is present on the surface of the plants, swim to the
archegonia and down the necks to reach the eggs. One fuses with the single
egg in each archegonium—the process of fertilization—thus combining the
sperm and egg nuclear and cytoplasmic material. The resulting cell, a zygote,
has a diploid (2 n) chromosome number and is the beginning of the
sporophytic generation. This reproduction is termed oogamy—a large,
nonmotile egg is fertilized in the archegonium by a small, motile sperm that
swims to the egg. In the bryophytes, an external film of water on the
surface of the plant is the passageway for the biflagellate sperm; in more
advanced plants, sperm move internally within special structures (pollen
tubes) to reach the eggs.
After fertilization, the zygote remains in the archegonium and divides by
mitosis repeatedly to form a multicellular, diploid embryo, the young
sporophyte. Sugars and other materials are translocated from gametophyte
to the developing sporophyte through placental tissue, a type of nutrition
called matrotrophy. (No plasmodesmata connect the gametophyte and
sporophyte; movement of material is along the cell wall, that is, it
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Document Summary
Although individuals of the three bryophyte groups differ from one another morphologically and in other details. As do all plants, bryophytes alternate a gametophytic generation with a sporophytic one (a sporic meiosis, a life cycle in which meiosis gives rise to spores, not gametes). Each of the haploid (1 n) spores is capable of developing into a multicellular, haploid individual, the gametophyte. The first structure formed from spores in most mosses and many liverworts is a filamentous, algal-like, green protonema (plural,protonemata). In some mosses the protonemata are long lived with rhizoids and aerial filaments and they often form dense green mats in suitable sites. Cells in the protonema, probably stimulated by red light and kinetin, give rise to shoots, which enlarge and become the mature gametophytes. In the bryophytes, these are the dominant, independent (photosynthetic) plants. The gametophytes initiate gametangia on special branches or at the tip of the main shoot.
