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General characters of Bryophytes Posted on : 24-11-2017
Posted by : Admin
Bryophytes are chlorophyllous, autotrophic, embryophytic, atracheophytic, archegoniate and amphibious cryptogams which show invariable heteromorphic alternation of generations in their haplodiplontic life cycle. They include liverwort, hornworts and mosses.
General characters of Bryophytes
Bryophytes are primitive land plants that grow on moist shady places. They prefer moist, cool and shady places to grow. Few of them grow in water and others in bogs, moist walls, rocks and tree trunks.
Though they started land life, they require presence of water to complete their life cycle for movement of motile male gametes (antherozoids).
They are predominantly amphibious in nature, hence called “amphibians of the plant kingdom”
Bryophytes show “heteromorphic alternation of generations”. The gametophytic and sporophytic generations alternate with each other regularly in the life cycle. In this the haploid plant body is gametophyte and it is dominant phase. Diploid sporophyte is physically and nutritionally dependent on the gametophyte.
In primitive bryophytes the gametophyte is dorsoventral, dichotomously branched green prostrate structure. In few advanced forms gametophyte is differentiated into stem, leaves and rhizoids.
Bryophytes are autophytes and lead autotrophic mode of nutrition.
Bryophytes lack true roots. In primitive forms there are unicellular rhizoids, while in advanced forms the rhizoids are branched and multicellular. These rhizoids help in anchorage and absorption.
The entire thallus is leaf like in primitive bryophytes while in advanced forms leaves are spirally arranged and are called microphyll. The microphyll is a small leaf with median midrib.
The plant body of bryophytes consists of simple parenchymatous tissue with NO vascular tissues like xylem and phloem.
Bryophytes reproduce vegetatively with the help of tubers, bulbils, protonemal branches, fragmentation etc.
Sexual reproduction is oogamous type. The sex organs of bryophytes are called gametangia. Gametangia are multicellular with sterile jacket. Female gametangium is known as archegonium and male gametangium is known as antheridium.
Antheridium is club shaped. It shows a basal stalk and a dome shaped body. The body is covered by jacket enclosing androcytes which develop spermatozoids. Spermatozoids are small, slender and bicilate.
Archegonium is flask shaped. It has a basal small stalk, median swollen venter and a terminal long neck. On the bod axis archegonium has axial row of neck cells, ventral canal cell and egg. The venter wall enlarges with the developing embryo to form protective envelop called calyptra.
Fertilization is possible in the presence of water. The egg is fertilized by the actively swimming motile spermatozoids while it is still within the archegonium.
The fertilized zygote develops into sporophyte. Sporophyte is diploid, multicellular and not well defined. Sporophyte has a distinct foot, seta and capsule. Sporophyte remains attached to the gametophyte throughout its life and is dependent on it. Reduction division takes place in the developing sporophyte which results in the formation of haploid spores. All the spores produced in a sporophyte are morphologically alike.
Life cycle of bryophytes is haplodiplontic with heteromorphic alternation of generation of multicellular generations.