BRYOPHYTES

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BOTANY: SEM-I,PAPER-DSC1AT:BIODIVERSITY(MICROBES,ALGAE,FUNGI AND ARCHEGONIATE), UNIT-5:BRYOPHYTES.

BRYOPHYTES

The division bryophyta comprises of a large group of plants which represents one of the main four divisions of the plant Kingdom and phylogenetically occupies a position intermediate between thallophyta and bryophyta. Hence, bryophytes stand at an evolutionary level higher than that of thallophytes but lower than that of pteridophytes and spermatophytes.

Occurrence

They are generally restricted to moist and shady places on land such as damp rocks and trees, by the side of pools and streams. Except some members are epiphytic, while others are aquatic. A few many now grow even in dry areas but their growth and other activities are largely restricted to wet and cold seasons. All the members of bryophytes have adapted to land conditions still they require water for the act of fertilization. Hence due to their complete dependence on external water supply for completing their life cycle, bryophytes are also regarded as amphibious plants. A great number of species is found in temperate and even in very cold regions but some are also found in the tropics. Specially in tropical rainforest where the vegetation is loaded with moisture.

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Characteristic features of bryophytes

 Presence of distinct and well-defined heteromorphic alternation of generation in the life history. That is sexual gametophyte and sexual sporophyte generations or phases follow one another. The plant body represents the gametophytic generation and which is the highly developed, differentiated and longer live structure plant body. The sporophytic generation is always an independent structure. Gametophytic plant body is green in colour due to the presence of glass green plastid in the cells and the thallus is not differentiate into stem, roots and leaves.

Fig: Thallus of Marchantia sp.

 Unlike tracheophyta, that is vascular plants, true roots are absent among the members of bryophyta. Instead root like structure called rhizoids and scales perform the function.

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Fig: Phyllodes and Rhizoids of Bryophytes

 In the gametophyte body the presence of multicellular sex organ is called antheridium and the female sex organ is called the archegonium. The archegonium is more or less spherical. It consists of a swollen basal portion called the venter and a slender elongated upper region called neck. The egg is present in the venter of the archegonium.

 The antheridium is a spherical or oblong body and consist of a single layer of protective jacket enclosing a mass of cells called sperm mother cells or androcytes.

 All the members of bryophytes lack typical vascular tissue that is xylem and phloem.

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 Sexual reproduction takes place by the process of fertilization. Male gametes are smaller and mortile known as sperm or antherozoids. While female gametes are larger and non motile called egg or ovum.

 The embryo is multicellular. Development of the embryo takes place within the venter. With the developing embryo, the basal portion of the archegonium enlarges to form a protective layer and will uphold the sporophyte body, that is, sporogonium developed ultimately from the embryo.

 The sporogonium is a simple structure that is without stem, leaves and roots and is largely dependent on the gametophytic plant body for its water and mineral nutrition.

Characters of bryophyta resembling pteridophyta

In both bryophytes and pteridophytes fertilization is affected in presence of water.

In both spore mother cells represent the end of the sporophyteic generation while spores represent the initiation of the gametophytic generation basically the life cycle of a bryophyte is same like that of a homosporous pteridophyte.

characters of bryophytes resembling algae

Bryophytes resemble the members of chlorophyta in the following points :

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presence of photosynthetic pigments that is chlorophyll in the assimilatory tissues sometimes chloroplast are with pyrenoids and the mode of nutrition is autotrophic. In both metabolic product is starch..

Life cycle of bryophytes

The life cycle of a bryophyta possesses a regular alternation of generation. The plant body in bryophytes represents the gametophytic generation which starts with the formation of spores. This spore is the starting point of the gametophytic generation which is haploid in nature. The sporophyte that is asexual diploid generation. It starts from the fertilized egg, that is with the formation of the zygote.

Fig: The alternation of generation in Bryophytes

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Apospory

The production of a gametophyte from a sporophyte without the formation of spores takes place. This phenomenon is called apospory.

Apogamy

The production of sporophyte plant body directly from gametophytic plant body without any union of gametes is known as apogamy.

REFERENCE

Mitra.J.N, Mitra.D, Chauradhy.S.K (1966) Studies in Botany. Moulik Library. Sixth edition.