Bryophyta - Mosses, Liverworts and Hornworts

Bryophytes are a type of plants. Plants can be organized into four major groups based on the types of tissues and reproductive structures they have. These four major groups are:

  • • Bryophytes- Bryophytes are plants such as mosses. Bryophytes do not have a vascular system as well as they are incapable of producing flowers or seeds.

  • • Ferns- Ferns have vascular tissue, but they do not produce seeds.

  • • Gymnosperms- Gymnosperms is also known as conifers. Vascular tissue is present in them. They are capable of producing cones and seeds but, incapable of producing flowers.

  • • Angiosperms- Angiosperms is also known as flowering plants. They have vascular tissue and are capable of producing both flowers and seeds. There are two distinct groups exist among flowering plants: monocots and dicots. Corn and lilies are examples of monocots while beans, oak trees, and daisies are examples of dicots. Monocots have seeds with one cotyledon while dicots have seeds with two cotyledons.

  • Bryophytes are a group of plants consisting of three divisions of non-vascular land plants. They are mosses, liverworts, and hornworts. In this class of plants, there are about 24,700 bryophytes. The bryophytes are very simple but highly adapted organism that can survive in a quite diverse environment including terrestrial environments (even deserts also). They have three quite distinct phyla of relatively unspecialized plants. Gametophytes of bryophytes are photosynthetic. Gametophytes are attached to the sporophytes. Some bryophytes require water to reproduce sexually. Bryophytes are especially common in moist places such as tropic region and temperate regions. 

    Most bryophytes are small in size but few bryophytes exceed 7 cm in height. The gametophytes of bryophytes are clearly visible than the sporophytes. Within gametophyte tissue, some of the sporophytes are completely enclosed.
    As discussed earlier, there are three major phyla of nonvascular plants. They are- mosses, liverworts (hepaticophyta), and hornworts (anthocerotophyta). All these plants are relatively unspecialized; however, they are well suited for diverse terrestrial environments. The details of each class are as follow.

  • 1. Mosses (Bryophyta)

  • The gametophytes of mosses have typical leaf-like structures. They are arranged spirally or alternatively around a stem-like axis. This axis is anchored to its substrate by means of rhizoids. These rhizoids are particularly important in the absorption of water. They have cells that are capable of absorbing water. However, specialized vascular plant roots are absent. The leaves of moss are completely different than compared to the true leaves, except for the superficial appearance of the green, flattened blade and slightly thickened midrib. Mosses are only one cell thick except at the midrib. All cells present in mosses are haploid. They generally lack vascular strands and stomata.

    Water may be absorbed through a strand of the specialized cells in the center of a moss gametophyte axis. However, the most required quantity of water used by the plants travels up from the outside of the plant. There are specialized food conducting cells surrounding those water conducting specialized cells. This mechanism is also present in some mosses. 

    Process involved in sexual reproduction of mosses (Life cycle of a typical moss)

    At the tips of the leafy gametophytes, multicellular gametangia are formed. The female gametangia are known as archegonia while male gametangia are known as antheridia. The female gametangia may be developed on the same gametophyte as the antheridia or it may be developed on the separate plants. In the swollen lower part of archegonium, a single egg is produced while in an antheridium, numerous sperms are produced. The sperms are released from the antheridium and then, they swim with the help of flagella to the archegonia. This is mediated by the presence of dew or rainwater. The haploid sperm unites with a haploid egg and ultimately forms a diploid zygote.

    After the formation of diploid zygote, it divides. This step is mediated by mitosis and it leads to the development of the sporophyte including a slender, basal stalk with a swollen capsule which is known as sporangium. This takes place at its tip. After proper development of sporophyte, its base inserts in and attached to the gametophyte tissue. This step is very important as this gametophyte tissues are their nutritional sources. The shape of sporangium is often cylindrical or sometimes it is club-shaped. Within the sporangium, the spore mother cells undergo meiosis and as a result, four haploid spores will be produced and the maturation process starts. The top of the sporangium kicks off at the time of complete maturity. Now, the germination of spore depends on favorable conditions. A spore could germinate if it lands in a suitable damp location followed by growth into a thread like structure. This thread like structure, now, branches to form rhizoids and buds and that grow upright. Each bud has the capability to develop into a new gametophyte plant consisting of a leafy axis.

    The major portion of the life cycle of moss is spent in the haploid state. The leafy gametophyte is photosynthetic. However, the smaller sporophyte is not photosynthetic and it is not able to produce energy on its own. Hence, it is nutritionally dependent on the gametophytes. Water is an important component required to carry the sperm to the egg.

    Abundance of Mosses

    Mosses are the most abundant plants in the Arctic and the Antarctic area. They are only the largest number of individuals and the largest number of species in these harsh regions. Even though drought-like conditions are not so common in deserts, many mosses are capable of withstanding prolonged periods of drought. Most of the mosses are quite sensitive to the pollution present in the air, and hence, they are not present in abundance in or near cities or the areas with the high levels of air pollution. 

    Utility of Mosses with an example of Sphagnum

    The mosses of peat are known as sphagnum. They are very valuable commercially as a soil conditioner. It is possible due to their capability of absorbing a high amount of water. They absorb majorly up to 25 times their weight in water. They are also useful as fuel when they are in a dry condition.

  • 2. Liverworts (Hepaticophyta)

  • Liverworts are also known as hepaticophyta. They are considered as a part of Bryophyta. The word liverworts can be split into two words. They are liver and wyrt. Wyrt is an old English word and it means plant or herb. Additionally, some liverworts have flattened gametophytes with lobes looks like those of liver- hence, the combination liverwort. The lobbed liverworts are the best-known representatives of this phylum. However, they constitute only about 20% of the species. The remaining part of this phylum is leafy and superficially resembles mosses. The physiological and biological systems are less complex in the liverworts as compare to the mosses. The rhizoids of liverworts have unicellular structure and the gametophytes are prostrate instead of erect. 

    Some of the liverworts have air chambers in the upright position with branching rows of photosynthetic cells. Each chamber has a pore at the top in order to facilitate the gas exchange. This pore is capable to remain open always, cannot be closed, unlike stomata. 

    In liverworts, both, sexual and asexual, reproduction can take place. The sexual reproduction in liverworts is similar to that present in mosses. Gametangia in umbrella-like structure are characteristics of lobed liverworts. Asexual reproduction takes place when lens-shaped pieces of tissue are released from the gametophyte to grow new gametophytes. An example of common liverwort is Marchantia. It belongs to the phylum Marchanciophyta. In this liverwort, the sporophytes are borne within the tissue of umbrella-shaped structures. It arises from the surface of the flat, green, and creeping gametophyte.

  • 3. Hornworts (Anthocerotophyta)

  • The hornworts belong to the phylum which is known as anthocerotophyta. The origin of hornworts is still a question mark. Hornworts are most likely among the earliest land plants, and yet, the earliest fossil spores date from the Cretaceous period (i.e. 65 to 145 million years ago, when angiosperms were entering into the picture.

    The sporophyte of hornworts is small in size and it resembles with tiny green broom handles rising from filmy gametophytes. They are usually less than 2 centimeters in diameter. The base of the sporophyte is entered in to- and attached with gametophyte tissue. This attachment is quite important as sporophyte tends to absorb some nutrients from the gametophyte tissue. The hornworts have stomata and it is also capable of carrying out a photosynthesis reaction. Hence, it provides much of the energy needed for the growth and also for reproduction. The cells of hornworts usually have a single chloroplast and it is important during the energy production by photosynthesis. The capability of carrying out the photosynthetic reaction by hornwort sporophyte is a quite distinguishing property.