Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
In Gymnosperm, endosperm is formed by
(a) fusion between a male gamete and two polar nuclei.
(b) fusion between a male gamete and a polar nuclei.
(c) fusion between egg and male gamete.
(d) germination of megaspore.
Answer:(d) germination of megaspore.
In gymnosperms, three of the four haploid spores produced in meiosis typically degenerate leaving one surviving megaspore inside the nucellus. The megagametophyte consists of around 2000 nuclei and forms archegonia, which produce egg cells for fertilization. Fertilization occurs within the archegonia produced by the female gametophyte. While it is possible that several egg cells are present and fertilized, typically only one zygote will develop into a mature embryo as the resources within the seed are limited.
Gymnosperm
👉More than 70 per cent of
all the species recorded are animals, while plants (including algae, fungi,
bryophytes, gymnosperms and angiosperms) comprise no more than 22
per cent of the total. Among animals, insects are the most species-rich
taxonomic group, making up more than 70 per cent of the total. That
means, out of every 10 animals on this planet, 7 are insects.
👉Loss of Biodiversity
While it is doubtful if any new species are being added (through speciation)
into the earth’s treasury of species, there is no doubt about their continuing
losses. The biological wealth of our planet has been declining rapidly
and the accusing finger is clearly pointing to human activities. The
colonisation of tropical Pacific Islands by humans is said to have led to
the extinction of more than 2,000 species of native birds. The IUCN Red
List (2004) documents the extinction of 784 species (including 338
vertebrates, 359 invertebrates and 87 plants) in the last 500 years. Some
examples of recent extinctions include the dodo (Mauritius), quagga
(Africa), thylacine (Australia), Steller’s Sea Cow (Russia) and three
subspecies (Bali, Javan, Caspian) of tiger. The last twenty years alone
have witnessed the disappearance of 27 species. Careful analysis of records
shows that extinctions across taxa are not random; some groups like
amphibians appear to be more vulnerable to extinction. Adding to the
grim scenario of extinctions is the fact that more than 15,500 species
world-wide are facing the threat of extinction. Presently, 12 per cent of
all bird species, 23 per cent of all mammal species, 32 per cent of all
amphibian species and 31per cent of all gymnosperm species in the world
face the threat of extinction.
👉Earlier
classification systems included bacteria, blue green algae, fungi, mosses,
ferns, gymnosperms and the angiosperms under ‘Plants’. The character
that unified this whole kingdom was that all the organisms included had a
cell wall in their cells. This placed together groups which widely differed in
other characteristics. It brought together the prokaryotic bacteria and the
blue green algae ( cyanobacteria) with other groups which were eukaryotic.
It also grouped together the unicellular organisms and the multicellular
ones, say, for example, Chlamydomonas and Spirogyra were placed together
under algae. The classification did not differentiate between the heterotrophic
group – fungi, and the autotrophic green plants, though they also showed
a characteristic difference in their walls composition – the fungi had chitin
in their walls while the green plants had a cellulosic cell wall.
👉Plantae includes algae,
bryophytes, pteridophytes, gymnosperms and angiosperms.
👉The plantae includes all eukaryotic chlorophyll-containing organisms. Algae,
bryophytes, pteridophytes, gymnosperms and angiosperms are included in this
group. The life cycle of plants exhibit alternation of generations – gametophytic
and sporophytic generations.
👉GYMNOSPERMS
The gymnosperms (gymnos : naked, sperma : seeds) are plants in which
the ovules are not enclosed by any ovary wall and remain exposed, both
before and after fertilisation. The seeds that develop post-fertilisation, are
not covered, i.e., are naked. Gymnosperms include medium-sized trees
or tall trees and shrubs (Figure 3.4). One of the gymnosperms, the giant
redwood tree Sequoia is one of the tallest tree species. The roots are
generally tap roots. Roots in some genera have fungal association in the
form of mycorrhiza (Pinus), while in some others (Cycas) small specialised
roots called coralloid roots are associated with N2- fixing cyanobacteria.
The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves
may be simple or compound. In Cycas the pinnate leaves persist for a few
years. The leaves in gymnosperms are well-adapted to withstand extremes
of temperature, humidity and wind. In conifers, the needle-like leaves
reduce the surface area. Their thick cuticle and sunken stomata also
help to reduce water loss.
👉The gymnosperms are heterosporous; they produce
haploid microspores and megaspores. The two kinds of
spores are produced within sporangia that are borne
on sporophylls which are arranged spirally along an axis
to form lax or compact strobili or cones. The strobili
bearing microsporophylls and microsporangia are
called microsporangiate or male strobili. The
microspores develop into a male gametophytic
generation which is highly reduced and is confined to
only a limited number of cells. This reduced
gametophyte is called a pollen grain. The development
of pollen grains take place within the microsporangia.
The cones bearing megasporophylls with ovules or
megasporangia are called macrosporangiate or female
strobili. The male or female cones or strobili may be
borne on the same tree (Pinus). However, in cycas male
cones and megasporophylls are borne on different trees.
The megaspore mother cell is differentiated from one of
the cells of the nucellus. The nucellus is protected by
envelopes and the composite structure is called an
ovule. The ovules are borne on megasporophylls which
may be clustered to form the female cones. The
megaspore mother cell divides meiotically to form four
megaspores. One of the megaspores enclosed within the
megasporangium develops into a multicellular female
gametophyte that bears two or more archegonia or
female sex organs. The multicellular female gametophyte
is also retained within megasporangium.
👉Unlike bryophytes and pteridophytes, in
gymnosperms the male and the female gametophytes
do not have an independent free-living existence. They
remain within the sporangia retained on the
sporophytes. The pollen grain is released from the
microsporangium. They are carried in air currents and
come in contact with the opening of the ovules borne
on megasporophylls. The pollen tube carrying the
male gametes grows towards archegonia in the ovules
and discharge their contents near the mouth of the
archegonia. Following fertilisation, zygote develops
into an embryo and the ovules into seeds. These seeds
are not covered.
👉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.
👉On the other extreme, is 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 life cycle is
termed as diplontic. An alga, Fucus sp., represents
this pattern (Fig. 3.7b). In addition, all seed bearing
plants i.e., gymnosperms and angiosperms, follow
this pattern with some variations, wherein, the
gametophytic phase is few to multi-celled.
👉
👉Interestingly, while most algal genera are haplontic, some of them
such as Ectocarpus, Polysiphonia, kelps are haplo-diplontic. Fucus, an
alga is diplontic.
👉The gymnosperms are the plants in which ovules are not enclosed by any
ovary wall. After fertilisation the seeds remain exposed and therefore these plants
are called naked-seeded plants. The gymnosperms produce microspores and
megaspores which are produced in microsporangia and megasporangia borne on
the sporophylls. The sporophylls – microsporophylls and megasporophylls – are
arranged spirally on axis to form male and female cones, respectively. The pollen
grain germinates and pollen tube releases the male gamete into the ovule, where it
fuses with the egg cell in archegonia. Following fertilisation, the zygote develops
into embryo and the ovules into seeds.
👉When and where does reduction division take place in the life cycle of a liverwort,a moss, a fern, a gymnosperm and an angiosperm?Write a note on economic importance of algae and gymnosperms.Both gymnosperms and angiosperms bear seeds, then why are they classifiedseparately?
Match the following (column I with column II)
Column I Column II
(a) Chlamydomonas (i) Moss
(b) Cycas (ii) Pteridophyte
(c) Selaginella (iii) Algae
(d) Sphagnum (iv) Gymnosperm
Describe the important characteristics of gymnosperms.
👉Xylem functions as a conducting tissue for water and
minerals from roots to the stem and leaves. It also provides
mechanical strength to the plant parts. It is composed of four
different kinds of elements, namely, tracheids, vessels, xylem
fibres and xylem parenchyma. Gymnosperms lack vessels in
their xylem.
👉Phloem transports food materials, usually from leaves to
other parts of the plant. Phloem in angiosperms is composed
of sieve tube elements, companion cells, phloem parenchyma
and phloem fibres. Gymnosperms have albuminous cells and sieve cells.
They lack sieve tubes and companion cells.
👉Secondary growth also occurs in stems and roots of gymnosperms.
However, secondary growth does not occur in monocotyledons.
👉You also know that in dicotyledonous
plants and gymnosperms, the lateral meristems,
vascular cambium and cork-cambium appear
later in life. These are the meristems that cause
the increase in the girth of the organs in which
they are active.
👉Several organisms belonging to monera, fungi, algae and bryophytes
have haploid plant body, but in organisms belonging to pteridophytes,
gymnosperms, angiosperms and most of the animals including human
beings, the parental body is diploid.
👉In many terrestrial organisms, belonging to fungi, higher animals such
as reptiles, birds, mammals and in a majority of plants (bryophytes,
pteridophytes, gymnosperms and angiosperms), syngamy occurs inside
the body of the organism, hence the process is called internal fertilisation.
