Sexual Reproduction in Flowering Plants 4 Flashcards
what happens when pollen lands on stigma
Pollination does not guarantee the transfer
of the right type of pollen (compatible pollen of the same species as the
stigma). Often, pollen of the wrong type, either from other species or from
the same plant (if it is self-incompatible), also land on the stigma. The
pistil has the ability to recognise the pollen, whether it is of the right type
(compatible) or of the wrong type (incompatible). If it is of the right type,
the pistil accepts the pollen and promotes post-pollination events that leads to fertilisation. If the pollen is of the wrong type, the pistil rejects the
pollen by preventing pollen germination on the stigma or the pollen tube
growth in the style. The ability of the pistil to recognise the pollen followed
by its acceptance or rejection is the result of a continuous dialogue
between pollen grain and the pistil. This dialogue is mediated by chemical
components of the pollen interacting with those of the pistil.
explain growth of pollen tube
following compatible pollination, the pollen grain
germinates on the stigma to produce a pollen tube through one of the
germ pores (Figure 1.12a). The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and
style and reaches the ovary (Figure 1.12b, c). You would recall that in
some plants, pollen grains are shed at two-celled condition (a vegetative
cell and a generative cell). In such plants, the generative cell divides and
forms the two male gametes during the growth of pollen tube in the stigma.
In plants which shed pollen in the three-celled condition, pollen tubes
carry the two male gametes from the beginning. Pollen tube, after reaching
the ovary, enters the ovule through the micropyle and then enters one of
the synergids through the filiform apparatus
what is pollen pistil interaction
All these events–from
pollen deposition on the stigma until pollen tubes enter the ovule–are
together referred to as pollen-pistil interaction. As pointed out earlier,
pollen-pistil interaction is a dynamic process involving pollen recognition
followed by promotion or inhibition of the pollen. The knowledge gained
in this area would help the plant breeder in manipulating pollen-pistil
interaction, even in incompatible pollinations, to get desired hybrids.
explain double fertilisation
After entering one of the synergids, the pollen tube releases the two male
gametes into the cytoplasm of the synergid. One of the male gametes moves
towards the egg cell and fuses with its nucleus thus completing the
syngamy. This results in the formation of a diploid cell, the zygote. The
other male gamete moves towards the two polar nuclei located in the central
cell and fuses with them to produce a triploid primary endosperm nucleus
(PEN) (Figure 1.13a). As this involves the fusion of three haploid nuclei it
is termed triple fusion. Since two types of fusions, syngamy and triple
fusion take place in an embryo sac the phenomenon is termed double
fertilisation, an event unique to flowering plants.
what happens to central cell after double fertilisation
The central cell after
triple fusion becomes the primary endosperm cell (PEC) and develops
into the endosperm while the zygote develops into an embryo
what are post fertilisation events
Following double fertilisation, events of endosperm and embryo
development, maturation of ovule(s) into seed(s) and ovary into fruit, are
collectively termed post-fertilisation events.
why does endosperm development precede embryo development
This is because endosperm acts as the nutritive tissue required for the growth of embryo. So endosperm develops first to facilitate that.
The
primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of this tissue are filled with
reserve food materials and are used for the nutrition of
the developing embryo.
describe development of endosperm into free nuclear endosperm
In the most common type of
endosperm development, the PEN undergoes successive
nuclear divisions to give rise to free nuclei. This stage of
endosperm development is called free-nuclear endosperm.
development of cellular endosperm
Subsequently cell wall formation occurs and the
endosperm becomes cellular. The number of free nuclei
formed before cellularisation varies greatly. The coconut
water from tender coconut that you are familiar with, is
nothing but free-nuclear endosperm (made up of
thousands of nuclei) and the surrounding white kernel is
the cellular endosperm.
is endosperm persistent or not persistent in seeds
Endosperm may either be completely consumed by the
developing embryo (e.g., pea, groundnut, beans) before seed
maturation or it may persist in the mature seed (e.g. castor
and coconut) and be used up during seed germination
describe embryogeny initial stages
Embryo develops at the micropylar end of the embryo sac where
the zygote is situated. Most zygotes divide only after certain
amount of endosperm is formed. This is an adaptation to
provide assured nutrition to the developing embryo. Though
the seeds differ greatly, the early stages of embryo development
(embryogeny) are similar in both monocotyledons and
dicotyledons. Figure 1.13 depicts the stages of embryogeny in
a dicotyledonous embryo. The zygote gives rise to the
proembryo and subsequently to the globular, heart-shaped
and mature embryo.
describe the parts of a dicotyledonous embryo
A typical dicotyledonous embryo (Figure 1.14a), consists
of an embryonal axis and two cotyledons. The portion of
embryonal axis above the level of cotyledons is the epicotyl,
which terminates with the plumule or stem tip. The cylindrical
portion below the level of cotyledons is hypocotyl that
terminates at its lower end in the radicle or root tip. The root
tip is covered with a root cap.
describe the parts of a monocot embryo
Embryos of monocotyledons (Figure 1.14 b) possess only
one cotyledon. In the grass family the cotyledon is called
scutellum that is situated towards one side (lateral) of the
embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called
coleorrhiza. The portion of the embryonal axis above the level of
attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a
few leaf primordia enclosed in a hollow foliar structure, the coleoptile.
what is the final pdt of sexual reproduction
In angiosperms, the seed is the final product of sexual reproduction. It is
often described as a fertilised ovule. Seeds are formed inside fruits. A
seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.
describe cotyledons
The cotyledons (Figure 1.15a) of the embryo are simple structures,
generally thick and swollen due to storage of food reserves (as in legumes).
