PLANT KINGDOM-ANGIOSPERMS
ANGIOSPERMS
Unlike the gymnosperms where the ovules are naked, in the
angiosperms or flowering plants, the pollen grains and ovules are developed in
specialised structures called flowers. In angiosperms, the seeds are enclosed
by fruits.
- The angiosperms are an
exceptionally large group of plants occurring in wide range of habitats. They
range in size from tiny, almost microscopic Wolfia to tall trees of Eucalyptus
(over 100 metres).
- They provide us with food,
fodder, fuel, medicines and several other commercially important products.
- They are divided into two
classes: the dicotyledons and the monocotyledons.
- The dicotyledons are
characterised by having two cotyledons in their seeds while the monocolyledons
have only one.
Reproduction
·
The male sex organ in
a flower is the stamen.
·
Each stamen consists
of a filament with an anther at the tip. The anthers following meiosis produce
pollen grains.
·
The female sex organ
in a flower is the pistil or the carpel.
·
Pistil consists of an
ovary enclosing one too many ovules.
·
Within ovules are
present highly reduced female gametophytes termed embryo sacs.
The embryo-sac formation is
preceded by meiosis. Hence, each of the cells of an embryo-sac is haploid.
·
Each embryo-sac has a
three-celled egg apparatus one egg cell and two synergids, three antipodal
cells and two polar nuclei.
·
The polar nuclei
eventually fuse to produce a diploid secondary nucleus.
·
Pollen grains, after
dispersal from the anthers, are carried by wind or various other agencies to
the stigma of a pistil. This is termed as pollination.
·
The pollen grains
germinate on the stigma and the resulting pollen tubes grow through the tissues
of stigma and style and reach the ovule.
·
The pollen tubes enter
the embryo-sac where two male gametes are discharged.
·
One of the male
gametes fuses with the egg cell to form a zygote (syngamy). The other male
gamete fuses with the diploid secondary nucleus to produce the triploid primary
endosperm nucleus (PEN).
·
Because of the
involvement of two fusions, this event is termed as double fertilisation, an
event unique to angiosperms.
·
The zygote develops
into an embryo (with one-or two cotyledons) and the PEN develops into endosperm
which provides nourishment to the developing embryo.
·
The synergids and
antipodals degenerate after fertilisation. During these events the ovules
develop into seeds and the ovaries develop into fruit.
Size:
(1) Smallest angiosperm is Wolffia. The plant body of
Wolffia consists of tiny flat oval green stem (phylloclade) having a few small
roots. The-plants are about 1 mm in diameter and found free floating in aquatic
habitats like ponds, etc.
(2) The tallest angiosperm is Eucalyptus. Their trees
may attain a height upto 100 meters or more.
(3) Banyan (Ficus bengalensis)
tree covers a large area. It's slanting aerial
branches spread in all directions. The tree spreads with the help of prop or
pillar roots.
Classification
The plants of
Angiosperms are divided into two major groups as - Dicotyledons and
Monocotyledons.
Dicotyledons
They are show
following distinguished characteristics.
(i) Tap roots found in
the members of this group.
(ii) The leaves in
members of these class exhibit reticulate (net like) venation.
(iii) The flowers are
tetramerous or pentamerous having four or five members in the various floral
whorls, respectively.
(iv) The vascular
bundles arranged in a ring, numbering 2-6, open and with cambium.
(v) The seeds of dicotyledons are with two cotyledons as the name indicates.
They are show
following distinguished characteristics:
(i) Adventitious
roots found in the members of this group.
(ii) The leaves are
simple with parallel venation.
(iii) The flowers are
trimerous having three members in each floral whorl.
(iv) The vascular
bundles scattered in the ground tissue, many in number, closed and without
cambium.
(v) The seeds of
monocotyledons are with one cotyledons as the name indicate. e.g., Cereals,
bamboos, sugarcane, palms, banana, lilies and orchids
Longevity
(i) Ephemerals: This category includes the plants which live only for a few weeks because of a very short growing season. Such plants are found near deserts or in very cold countries. For example, Arabidopsis species have a life span of 20–28 days.
(ii) Annuals: The plants of this category live and
complete their life-cycle in a single favourable season. During this period,
they grow in size, produce flowers, shed their seeds, undergo senescence and
die. They pass the unfavourable period in the form of seeds. Many crop plants
(e.g., wheat, rice, maize, etc.) are annuals. The smallest angiosperm – Wolffia
is an aquatic annual.
(iii) Biennials: The plants of this category
complete their life-cycle in two favourable seasons (i.e., in two years). They
grow vegetatively in the first season and produce flowers and set seeds in the
next. Often they produce some storage organs, as in the sugar beet, where food
is stored in their swollen roots.
(iv) Perennials: Plants of this category live for more than two years. Generally they live for many years and bear the flowers and fruits during specific seasons. Some perennials continue their vegetative growth for several years and produce fruits and seeds only once in their life time, e.g., Agave, Bamboos, etc. They are called monocarpic.
Depending upon' the" habit
of plants,' the angiosperms belong to following categories:
(1) Herb: These are small, soft, non-woody plants
without persistent parts above ground. The height of plants usually reaches up
to 1 m. The plants may be annual (Brassica), biennial (Sugar beet) or perennial
(Canna). The perennial herbs usually possess underground rhizomes which form
the new aerial shoots every year. The plants of banana are perennial herbs.
(2) Shrubs: These are woody plants of relatively low
height (1-4 m). They typically branch at or near the base and do not have a
main trunk, e.g., Rose. They are mostly perennial.
(3) Trees: These are perennial woody plants with
one main trunk. The trunk mayor may not be branched. These are of the following
types:
(i) Caudex: The stem is unbranched and usually bears
a crown of leaves at the apex. e.g., date-palm.
(ii) Excurrent: The lower part of stem is thicker which
gradually tapers above. Branches arise from the main stem in acropetal
succession and plant appears conical e.g., Pinus.
(iii) Deliquescent: The apical bud of the main stem dies
after some time and branches and sub-branches spread in different directions.
e.g., Tamarindus, Ficus.
(4) Culms: In these plants, nodes and internodes
are extremely clear. Internodes of such plants are usually hollow. These plants
a.re grasses but cannot be considered as herb or shrub or tree. e.g., Bambusa
(Bans).
Habitat
Warming (1895) divided
the plants, on the basis of their adaptation to water, into four major groups –
hydrophytes, mesophytes, xerophytes and halophytes. A fifth group epiphytes can
also be included.
(i) Hydrophytes : The plants which grow in aquatic
habitats are called hydrophytes.
They are further grouped as
(a) Submerged (e.g., Hydrilla)
(b) Attached floating
(e.g., Nymphaea)
(c) Free-floating (e.g., Eichhornia, Wolffa)
(d) Amphibious or
partly emerged hydrophytes (e.g., Sagittaria).
(ii) Mesophytes : These are the plants which grow under
moderate moisture and temperature conditions. They have no special adaptations
to grow either in very dry or in very wet conditions (e.g., Sun flower, Brassica).
These plants do not possess special adaptations to reduce transpiration.
(iii) Xerophytes : The plants which grow in dry or xeric habitats (i.e., under deficient supply to available water) are called xerophytes. These plants face acute shortage of water and therefore, develop morphological, structural and physiological adaptations in order to survive under such habitats. The adaptations in plants are mainly to check the transpiration and survive under acute shortage of water. e.g., Cynodon (Doob grass), Casuarina, Euphorbia tirucalli, Asparagus, etc.
(iv) Halophytes : Halophytes are those plants which
grow in saline habitats, i.e., in salt
marshes, alkaline soils, river estuaries, saline ponds near seashore or sandy
and heavy soils having excess of salts. In such habitats, the water is present
in sufficient amount but due to high osmotic concentrations it is
physiologically not available to normal plants. Such conditions are said to be physiologically
dry. e.g., Spartina, Atriplex, Portulaca etc.
(v) Epiphytes : These are the plants which grow on
other plants for space only. The plants are autotrophic and occur both in
aquatic and terrestrial habitats. e.g., Vanda (an
orchid).
• In plants, both haploid and diploid cells can divide by mitosis. This ability leads to the formation of different plant bodies haploid and diploid.
• The haploid plant body produces gametes by mitosis. This plant body represents a gametophyte.
• Following fetilisation the zygote also divides by mitosis to produce a diploid sporophytic plant body.
• Haploid spores are produced by this plant body by meiosis. These in turn, divide by mitosis to form a haploid plant body once again.
• Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between, gamete producing haploid gametophyte and spore producing diploid sporophyte.
Different plant groups, as well
as individuals, representing them, differ in the following patterns:
1. Sporophytic generation is represented
only by the one-celled zygote, There are no free-living sporophytes, Meiosis in
the. zygote results in the formation of haploid spores. The haploid spores
divide mitotically and form the gametophyte. The dominant, photosynthetic phase
in such plants is the free-living gametophyte. This kind of life cycle is
termed as haplontic. Many algae such as Volvox, Spirogyra and some species of
Chlamydomomas represent this pattern.
2. The type wherein the diploid sporophyte is the
dominant, photosynthetic, independent phase of the plant. The gametophytic phase
is represented by the single to few-celled haploid gametophyte. This kind of
lifecycle is termed as diplontic. All seed-bearing plants i.e. gymnosperms and
angiosperms, follow this pattern.
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