Sexual Reproduction in Flowering Plants 3 Flashcards
what are 2 types of agents of pollination
Plants use two abiotic (wind
and water) and one biotic (animals) agents to achieve
pollination. Majority of plants use biotic agents for
pollination. Only a small proportion of plants use
abiotic agents
describe certain general features of abitoically pollinated plants
Pollen grains coming in contact with
the stigma is a chance factor in both wind and water
pollination. To compensate for this uncertainties and
associated loss of pollen grains, the flowers produce
enormous amount of pollen when compared to the
number of ovules available for pollination.
Both wind and water pollinated flowers are not very colourful and do
not produce nectar.
describe wind pollination
Pollination by wind is more common
amongst abiotic pollinations. Wind pollination
also requires that the pollen grains are light
and non-sticky so that they can be
transported in wind currents. They often
possess well-exposed stamens (so that the
pollens are easily dispersed into wind currents,
Figure 1.10) and large often-feathery stigma
to easily trap air-borne pollen grains. Windpollinated flowers often have a single ovule in
each ovary and numerous flowers packed into
an inflorescence; a familiar example is the corn
cob – the tassels you see are nothing but the
stigma and style which wave in the wind to
trap pollen grains. Wind-pollination is quite
common in grasses.
describe water pollination
Pollination by water is quite rare in
flowering plants and is limited to about 30
genera, mostly monocotyledons.
Some
examples of water pollinated plants are Vallisneria and Hydrilla which
grow in fresh water and several marine sea-grasses such as Zostera. Not
all aquatic plants use water for pollination. In a majority of aquatic plants
such as water hyacinth and water lily, the flowers emerge above the level
of water and are pollinated by insects or wind as in most of the land
plants.
describe hypohydrophily. in which plant is seen
In another group of water pollinated plants such as seagrasses(zostera, hydrilla), female
flowers remain submerged in water and the pollen grains are released
inside the water. Pollen grains in many such species are long, ribbon like
and they are carried passively inside the water; some of them reach the
stigma and achieve pollination. In most of the water-pollinated species,
pollen grains are protected from wetting by a mucilaginous covering.
describe epidhydrophily
In Vallisneria, the female flower reach the surface of water by the
long stalk and the male flowers or pollen grains are released on to the
surface of water. They are carried passively by water currents (Figure
1.11a); some of them eventually reach the female flowers and the stigma.
what are soem animals which act as pollinators
Majority of flowering plants use
a range of animals as pollinating
agents. Bees, butterflies, flies,
beetles, wasps, ants, moths, birds
(sunbirds and humming birds) and
bats are the common pollinating
agents. (Figure 1.11b). Among the
animals, insects, particularly bees
are the dominant biotic pollinating
agents. Even larger animals such
as some primates (lemurs), arboreal
(tree-dwelling) rodents, or even
reptiles (gecko lizard and garden
lizard) have also been reported as
pollinators in some species.
what are the adaptations of insect pollinated flowers
Majority of insect-pollinated
flowers are large, colourful, fragrant
and rich in nectar. When the flowers
are small, a number of flowers are
clustered into an inflorescence to
make them conspicuous. Animals
are attracted to flowers by colour
and/or fragrance. The flowers
pollinated by flies and beetles
secrete foul odours to attract these
animals. To sustain animal visits,
the flowers have to provide rewards
to the animals. Nectar and pollen
grains are the usual floral rewards.
For harvesting the reward(s) from
the flower the animal visitor comes
in contact with the anthers and the
stigma. The body of the animal gets
a coating of pollen grains, which are
generally sticky in animal pollinated flowers. When the animal carrying
pollen on its body comes in contact with the stigma, it brings about
pollination
explain symbiosis of flowers and insects
floral rewards are in providing safe places to lay eggs;
an example is that of the tallest flower of Amorphophallus (the flower
itself is about 6 feet in height). A similar relationship exists between a
species of moth and the plant Yucca where both species – moth and the plant – cannot complete their life cycles without each other. The moth
deposits its eggs in the locule of the ovary and the flower, in turn, gets
pollinated by the moth. The larvae of the moth come out of the eggs as
the seeds start developing.
what is preferred outbreeding or in breeding
Majority of flowering plants produce hermaphrodite
flowers and pollen grains are likely to come in contact with the stigma of
the same flower. Continued self-pollination result in inbreeding depression.
Flowering plants have developed many devices to discourage selfpollination and to encourage cross-pollination.
Either the pollen is
released before the stigma becomes receptive or stigma becomes receptive
much before the release of pollen. In some other species, the anther and
stigma are placed at different positions so that the pollen cannot come in
contact with the stigma of the same flower. Both these devices prevent
autogamy. The third device to prevent inbreeding is self-incompatibility.
This is a genetic mechanism and prevents self-pollen (from the same flower
or other flowers of the same plant) from fertilising the ovules by inhibiting
pollen germination or pollen tube growth in the pistil. Another device to
prevent self-pollination is the production of unisexual flowers.
If both male
and female flowers are present on the same plant such as castor and maize
(monoecious), it prevents autogamy but not geitonogamy. In several species
such as papaya, male and female flowers are present on different plants,
that is each plant is either male or female (dioecy). This condition prevents
both autogamy and geitonogamy.
