3. PLANT KINGDOM
BRYOPHYTES
- They are called amphibians of the
plant kingdom because they can live in soil but need water for sexual
reproduction.
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They occur in damp, humid and shaded
localities.
- Their body is more differentiated than
that of algae. It is thallus-like and prostrate or erect, and attached to the
substratum by unicellular or multicellular rhizoids.
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They lack true roots, stem or leaves. They
may possess root-like, leaf-like or stem-like structures.
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The main plant body is haploid. It
produces gametes, hence is called a gametophyte.
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The sex organs in bryophytes are
multicellular.
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The male sex organ (antheridium)
produces biflagellate antherozoids. The female sex organ (archegonium)
is flask-shaped and produces a single egg.
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Antherozoids are released to water and meet
archegonium. An antherozoid fuses with the egg to form zygote.
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Zygotes do not undergo meiosis immediately.
They produce a multicellular body called a sporophyte.
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Sporophyte is not free-living but attached
to the photosynthetic gametophyte and derives nourishment from it. Some cells
of the sporophyte undergo meiosis to form haploid spores. They germinate to form
gametophyte.
Importance of Bryophytes:
☺
Some mosses provide food for herbaceous
mammals, birds and other animals.
☺
Species of Sphagnum (a moss)
provide peat. It is used as fuel. It has water holding capacity so that used as
packing material for trans-shipment of living material.
☺
They are ecologically important because of
their role in plant succession on bare rocks/soil. Mosses along with
lichens decompose rocks making the substrate suitable for the growth of higher
plants.
☺
Since mosses form dense mats on the soil,
they can prevent soil erosion.
The bryophytes are divided
into liverworts and mosses.
Liverworts
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They grow usually in moist, shady habitats
such as banks of streams, marshy ground, damp soil, bark of trees and deep in
the woods.
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Their plant body is thalloid. E.g. Marchantia.
Thallus is dorsi-ventral and closely appressed to the substrate. The leafy
members have tiny leaf-like appendages in two rows on the stem-like structures.
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Asexual reproduction: By
fragmentation of thalli, or by the formation of gemmae (sing.
gemma). Gemmae are green, multicellular, asexual buds that develop in small
receptacles (gemma cups) on the thalli. Gemmae are detached from the
parent body and germinate to form new individuals.
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Sexual reproduction: Male
and female sex organs are produced on the same or different thalli. Sporophyte
is differentiated into a foot, seta and capsule. After meiosis, spores
are produced within the capsule. These spores germinate to form free-living
gametophytes.
Mosses
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The predominant stage of the life cycle of
a moss is the gametophyte. It consists of two stages.
o
Protonema stage:
The first stage which develops directly from a spore. It is a creeping, green,
branched and frequently filamentous stage.
o
Leafy stage: The
second stage which develops from the secondary protonema as a lateral bud. They
consist of upright, slender axes bearing spirally arranged leaves. They are
attached to soil through multicellular and branched rhizoids. This stage bears the
sex organs.
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Vegetative reproduction: By
fragmentation and budding in the secondary protonema.
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Sexual reproduction: The
antheridia & archegonia are produced at the apex of leafy
shoots. After fertilisation, zygote develops into a sporophyte, consisting of a
foot, seta and capsule. The sporophyte in mosses is more elaborate than that in
liverworts. The capsule contains spores. Spores are formed after meiosis. Mosses
have an elaborate mechanism of spore dispersal.
- E.g. Funaria, Polytrichum and Sphagnum.
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