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LIFE CYCLE AND PLANT BODY OF BRYOPHYTES

The term bryophyte is applied to a group of plants in which the plant body is simple flat thallus, still in the process of developing, resembling to some of thallophyta in vegetative structure, but showing an advance in the method of reproduction. They exhibit distinct geteromorphic alternation of generations. The principal phase is gametophyte. It is conspicuous and nutritionally independent. The sporophyte is simple and comprises of foot, seta and capsule. It never becomes independent, but remain attached to the gametophyte throughout its life.
Habitat: About 960 genera and 24000 species of bryophytes existing in the universe. They range from few millimeters to 30 centimeters or more in size. Most of the species are inconspicuous. With the exception of few aquatic forms they are truly land plants.
They are found in humid and shady places. The bryophytes are abundant in moist mountain forests of tropical and sub tropical regions. Species like Ricca, Marchntia, Anthoceroso and Funaria are common in the plains. A few of them, for example Riccia fluitan, Riella and Ricciocarpus are strictly aquatic. Most of the bryophytes are autotrophic but Buxbaumia hornum are more or less saprophytic and grow upon organic matter such as rotten wood.
Water is required at the time of fertilization; therefore they are commonly called amphibians of the plant kingdom.
Plant   Body: The gametphyte is haploid and an independent plant at maturity. In many cases the gametophyte is athallus, i.e., plant body is without differentiton into roots, stem and leaf, for example in Marchantia and Anthqeeros. In other it is externally differentiated into stem and leaves, however there are no roots. The absorbing organs are rhizoids. The thallus may be dorsiventrally flattened (Marchantia, Anthoceros) or erect (Funaria).
The sporophyte is diploid and dependent upon gametophyte for its nutrition and remains attached to the gametophyte. It consists of foot, embedded in the gametophyte and absorbs nutrition from it; seta which raises the sporophyte above the level of gametophyte, and conducts food absorbed by the foot; and capsule, the spore producing structure. The sporophyte of the bryophytes is generally called a sporogonium.
In thalliod bryophytes (Riccia, Marchantia, Anthoceros) the plant body is composed of thin walled parenchymatous cells. The upper and lower surfaces are bounded by epidermis. The outer walls of epidermal cells are cutinized. The bryophytes lack typical vascular system, i. e., xylem and phloem from their gametophyte and sporophyte.
Reproduction: The bryophytes reproduce by vegetative, sexual and asexual modes of reproduction. The gametophyte reproduces sexually by formation of gametes whereas the sporophyte reproduces by the formation of spores, therefore the mode of reproduction is asexual.

Vegetative Reproduction: In some species asexual reproduction takes place by the vegetative methods of fragmentation and genome. However asexual spores like endospores or akinetes produced in algae are absenr.
Sexual Reproduction: The sexual reproduction is oogamous type. The gametes are produced in  multicellular sex organs. The male gametes are small and motile, while the female gametes are large and non motile.
The sex organs are antheridia and erchegonia. These are multicellular and each sex organ is protected by an outer wall of sterile cells surrounding sex cells. Both kinds of sex organs may develop on the same plant (monoeious or homothallus) or on different plants (dioecious or heterothallic).
Each antridium is usually club shaped. It contains androcytes or antherozoid mother cells surrounded by a single layer of sterile jacket cells. Each androcyte metamorphoses into a motile biflagellate antherozoid or male sperms. The archegonia are usually flask-shaped. Each archigonium consists of a basal awollen portion, the venter and an elongated part, the neck. An axial row of cells surrounded by jaclet of sterile cells is present in the neck and venter. It consists of few neck canal cells, a ventral canal cell and an egg or oosphere.
Fertilization:  Water is essential for fertilization. The jacket if mature antherifium ruptures librating the sperms. The motile flagellated sperms swim in the film of water and reach the neck of an archigonium. In the meantime axial row of the neck canal cells disorganize and the tip of the archigonium ruptures opens. The antherozoids enter the neck and swim down to the egg. One of these sperms fertilizes the egg to produce zygote.
Asexual Reproduction: A mature sporogonium consists of a basal swollen foot, a stalk or seta, and a spore-producing structure, the capsule. The capsule comprises of sporogenous cells surrounded by a layers of sterile cells. These cells undergo divisions repeatedly and last generation of cells is called spore mother cells which undergo meiosis to produce tetrads of haploid spores. The spores are non motile, cutinized, wind disseminated and alike, therefore the bryophytes are homosporous. In most cases, e. g., Riccia and Marchantia, sterile cells called elators are also produced along with spores.

Germination of Spores: Each spore upon falling on a suitable place germinates to a gametophyte plant directly or into a filamentous or thalloid green protonema which give rise to the thallus (gametophyte).
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