POLLINATION

CHAPTER 2                                                   SEXUAL REPRODUCTION IN FLOWERING PLANTS

Pollination

(1)  The male and female gametes in flowering plants are produced in the pollen grain and embryo sac, respectively.

(2)  As both types of gametes are non-motile, they have to be brought together for fertilisation to occur. ( Pollination is the mechanism to achieve this objective ).

(3)  Transfer of pollen grains (shed from the anther) to the stigma of a pistil is termed pollination.

(4)  Flowering plants have evolved an amazing array of adaptations to achieve pollination.

(5)  They make use of external agents to achieve pollination.

Kinds of Pollination

Depending on the source of pollen, pollination can be divided into three types.

(1)  Autogamy

(1)  In this type, pollination is achieved within the same flower. ( Transfer of pollen grains from the anther to the stigma of the same flower ).

(2)  In a normal flower which opens and exposes the anthers and the stigma, complete autogamy is rather rare.

(3)  Autogamy in such flowers requires synchrony in pollen release and stigma receptivity and also, the anthers and the stigma should lie close to each other so that self-pollination can occur.

(4)  Some plants such as Viola ( common pansy ) , Oxalis , and Commelina produce two types of flowers – chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all. In such flowers, the anthers and stigma lie close to each other. When anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination.

(5)  Cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce assured seed-set even in the absence of pollinators.

(2)  Geitonogamy

(1)  Transfer of pollen grains from the anther to the stigma of another flower of the same plant.

(2)  Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.

(3)  Xenogamy

(1)  Transfer of pollen grains from anther to the stigma of a different plant.

(2)  This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma.

Agents of Pollination

(1)  Plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination.

(2)  Majority of plants use biotic agents for pollination. Only a small proportion of plants use abiotic agents.

(3)  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.

KINDS OF POLLINATION

(A)  WIND POLLINATION

(1)  Pollination by wind is more common amongst abiotic pollinations.

(2)  Wind pollination also requires that the pollen grains are light and non-sticky so that they can be transported in wind currents.

(3)  They often possess well-exposed stamens ( so that the pollens are easily dispersed into wind currents ) and large often-feathery stigma to easily trap air-borne pollen grains.

(4)  Wind pollinated 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.

(5)  Wind-pollination is quite common in grasses.

(B)  WATER POLLINATION

(1)  Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons.

(2)  Water is a regular mode of transport for the male gametes among the lower plant groups such as algae, bryophytes and pteridophytes.

(3)  It is believed, particularly for some bryophytes and pteridophytes, that their distribution is limited because of the need for water for the transport

of male gametes and fertilisation.

(4)  Some examples of water pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera.

(5)  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.

(6)  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; some of them eventually reach the female flowers and the stigma.

(7)  In another group of water pollinated plants such as sea grasses, 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.

(8)  In most of the water-pollinated species, pollen grains are protected from wetting by a mucilaginous covering.

(9)  Both wind and water pollinated flowers are not very colourful and do not produce nectar.

(C)  POLLINATION BY ANIMALS

(1)  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.

(2)  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.

(3)  Often flowers of animal pollinated plants are specifically adapted for a particular species of animal.

(4)  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.

(5)  The flowers pollinated by flies and beetles secrete foul odours to attract these animals.

(6)  To sustain animal visits, the flowers have to provide rewards to the animals. Nectar and pollen grains are the usual floral rewards.

(7)  For harvesting the reward(s) from the flower the animal visitor comes in contact with the anthers and the stigma.

(8)  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.

(9)  In some species 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).

(10)  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.

(11)  Many insects may consume pollen or the nectar without bringing about pollination. Such floral visitors are referred to as pollen/nectar robbers.