PLant Kingdom 3 Flashcards
give an introduction to gymnsoperms
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
which is the tallest tree in the forests?
One of the gymnosperms, the giant
redwood tree Sequoia is one of the tallest tree species.
explain the roots and stems of gymnsoeprms
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).
explain the organisation of leaves in gymnsoperms
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.
explain strobili of gymnsoperms
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.
how are pollen grains formed in gymnsoerpsm
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.
explain formation of female gamete
The cones bearing megasporophylls
with ovules or megasporangia are called
macrosporangiate or female strobili. 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.
explain pollination in gymnosperms
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.
describe the most unique feature of angiosperma
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
describe the variety of angiosperms
he 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 (Figure 3.5). The dicotyledons are characterised by
having two cotyledons in their seeds while the monocolyledons have only
one.
describe the male sex organ
The male sex organ in a flower is the stamen. Each stamen consists of
a slender filament with an anther at the tip. The anthers, following meiosis,
produce pollen grains.
describe the female sex organ
. The female sex organ in a flower is the pistil or the
carpel. Pistil consists of an ovary enclosing one to many ovules. Within
ovules are present highly reduced female gametophytes termed embryosacs.
describe the components of ebryosacs
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.
what is pollination
Pollen grain, 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.
explain double fertilisation in angiospermae
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.
