pollination

POLLINATION

 

In flowering plants, male and female gametes are produced in the pollen grain and embryo sac respectively. These gametes are non-motile, therefore, they have to be brought together for fertilisation. Pollination is the mechanism to achieve this objective. The transfer of pollen grains to the stigma is called pollination. Depending upon the source of pollen, pollination is of three types :

    (1)  Autogamy :

          If the pollen grains are transferred from anther to the stigma of the same flower, the process is called self pollination or autogamy.

   Ÿ      Contrivances for autogamy

   (i)    Bisexuality - Flower should be bisexual, e.g., Catharanthus

   (ii)   Homogamy - Male and female reproductive parts in a bisexual flower mature at the same time, e.g., Mirabilis.

   (iii)  Cleistogamy - Sometimes bisexual flowers remain closed and never open, such flowers are known as cleistogamous. In such flowers, the anthers and stigma lie close to each other. When the anthers dehisce, the pollen grains come in contact with stigma and pollination takes place. Thus, cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma. So one of the advantage of cleistogamy is, it ensures seed-set even in the absence of pollinators, e.g., Arachis hypogea (Groundnut). Some plants like Viola (common pansy), Oxalis and Commelina produce chasmogamous flowers (open flowers) as well as cleistogamous flowers.

   (iv)  Bud pollination -When self pollination occurs in the bud stage before the opening of flowers, e.g., Pisum, Wheat, Rice.

   (2)   Geitonogamy:

          Pollination taking place between the two flowers of the same plant (genetically self pollination but ecologically or functionally cross pollination), e.g., Maize.

   (3)   Xenogamy :

          When the pollen grains are transferred from the anther to the stigma of the flower of a different plant, it is called as xenogamy. (It is cross pollination, both genetically and ecologically).

 

   Outbreeding Devices (Contrivances for cross pollination)

          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 results in inbreeding depression, therefore angiosperms have developed following devices to, discourage self pollination and to encourage cross-pollination.

   (i)    Dichogamy: In bisexual flowers, the two sexes mature at different timings. When anthers mature first it is called protandry, e.g., Sunflower, Cotton, Salvia. When gynoecium matures first it is called as protogyny, e.g., Ficus, Aristolochia, Magnolia.

   (ii)   Incompatibility: It is the inability of certain gametes, even from genetically similar plant species, to fuse with each other. This is also called intra-specific incompatibility, self-sterility or self-­incompatibility.

 

   (iii)  Unisexuality or dicliny : It is the formation of unisexual flowers. Thus, cross pollination becomes obligatory. It can be seen in monoecious plants where both male and female flowers are produced on same plant, e.g., maize, castor as well as dioecious plants where male and female flowers are produced on different plants, e.g., Vallisneria, Papaya. Monoecious condition prevents autogamy but not geitonogamy while dioecious condition prevents both autogamy and geitonogamy.

   (iv)  Herkogamy : It is the presence of natural and physical barrier between androecium and gynoecium, e.g., in Calotropis, gynoecium is fused with pollinium (anthers) and form gynostegium.

   (v)   Prepotency : Pollen grain of one flower germinates more rapidly over stigma of another flower as compared to its own flower, e.g., Apple, Grape.

 

Agencies for cross pollination :

          Pollinating agency may be biotic (animals) or abiotic (wind and water). Majority of plants use biotic agents for pollination.

   A. Abiotic agents

   (a) Anemophily: (Pollination by wind).

          It is common amongst abiotic pollinations. It is a non-directional and wasteful process. The female flowers have large, feathery or brush like stigmas to catch the pollen grains. Anemophilous flowers are small and inconspicuous with long and versatile stamens. Pollen grain are dry, powdery, light and non-sticky. The flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence. Anemophily is quite common in grasses.

          e.g., Maize, wheat, sugarcane, bamboo, Pinus (winged pollen), Papaya.

   (b) Hydrophily : (Pollination by water).

          It is quite rare in angiosperms and is limited to about 30 genera, mostly monocotyledons. All aquatic plants are not hydrophilous. In a majority of aquatic plants like water hyacinth and water lily, the flowers emerge above the level of water and are pollinated by insects or wind.

Hydrophyte	Type of pollination
1. Alisma	Entomophily
2. Lotus	Entomophily
3. Potamogeton	Anemophily
4. Myriophyllum	Anemophily

 

   Ÿ      Hydrophily is of two types:

   (1)   Epihydrophily : Pollination at the surface of water

          Example : Vallisneria (Tape grass, Ribbon weed)

          Vallisneria is a dioecious rooted submerged aquatic plant in which male flowers are small and light weight. Female flowers have very long coiled pedicels which uncoil when they become mature. Male flowers float at the surface of water. As soon as the male flowers touch the female flowers, anther lobes burst, stigma receives the pollen grains and pedicels coil again.

   (2)   Hypohydrophily : Pollination inside the water.

          Examples: Zostera (sea grass), Ceratophyllum

          Zostera is a marine water plant. Female flowers remain submerged in water and the pollen grains are long, ribbon like and they are carried passively inside the water, some of them reach the stigma and achieve pollination.

   (i)    Pollen grains are protected from wetting by a mucilaginous covering in most of the hydrophilous species.

   (ii)   Anemophilous as well as hydrophilous flowers are not very colourful and do not produce nectar.

   (iii)  Pollen grains coming in contact with the stigma is a chance factor in both wind and water pollinating plants. To compensate for these uncertainities and associated loss of pollen grains, the flowers produce enormous amount of pollen when compared to the number of ovules available for pollination.

   B. Biotic Agents

   (a) Entomophily: (Pollination by insects)

   (1)   80% pollination occurs by insects (chief pollinators) and honey bee is main pollinator among insects. All the flowers pollinated by bees are brightly coloured, have a sweet smell and produce nectar. Entomophilous flowers produce a small amount of pollen which have a spinous and may have a sticky exine due to presence of pollenkitt. The stigmas of such flowers are long, rough and sticky. If the entomophilous flowers are small, a number of flowers are clustered into an inflorescence to make them conspicuous. Moth pollinated plants are white flowered and fragrant. The flowers pollinated by flies and beetles secrete foul odours to attract these animals.

   (2)   When petals are not conspicuous, other parts may become coloured or showy to attract the insects, e.g., bracts in Bougainvillea, leaves in Euphorbia pulcherrima (Poinsettia), one sepal in Mussaenda acts like advertisement flag, anthers are coloured in Mimosa.

   (3)   To sustain insect visits, the flowers have to provide rewards to the insects. Nectar and pollen grains are the usual floral rewards. In plants like Papaver, Rosa, Clematis, etc., edible pollen grains are produced. Some of the pollen grains stick to the back of insects while feeding on the edible pollen grains. Some species provide safe place to insect for laying eggs, e.g., Amorphophallus (the flower is 6 feet in height), Yucca.

   (4)   Some special cases of entomophily are as follows:

   (i)    Yucca : The flowers at Yucca and pollinating moth, i.e., Pronuba yuccasella / Tegaticula show a very close relationship. The insect punctures the ovary for egg laying. The insect creeps on the style and deposits pollen ball in between the stigmatic lobes, hence bringing about pollination. The larvae of the moth come out of the eggs as the seeds start developing.

   (ii)   Salvia or Sage plant : It shows lever mechanism or turn pipe mechanism. Anthers are distractile; lower lobe is sterile and upper lobe is fertile. The flower is bilabiate and protandrous. The insect sits on the lower lobe of the corolla; the upper fertile lobe of anther touches the body of insect. It ruptures and pollen grains are shed on the back of the insect. When this insect visits the other flowers, pollination is affected.

   (iii)  Ficus sp. : Trap Door mechanism in Ficus carica. The receptacle forms a cup having a cavity. The cavity has a pore called ostlole near which male flowers are present. Female flowers are present at the bottom and gall (sterile) flowers are present between these types. The pollinating insect is Blastophaga or gall insect. The insect lays eggs in the bottom, the larvae feed on the ovules of gall flowers. When the young insects crawl out of the inflorescence, their bodies are laden with pollen grains. They enter new hypanthodium and affects the pollination.

   (iv)  Calotropis : It shows translator or clip mechanism. Pollen grains are present in pollinia. The pollinia are attached to a rough and sticky disc called corpusculum. When the insect visits the flower, pollinia get entangled in the legs of the insect. When this insect visits other flowers, pollinia are transferred.

 

 

   (v)   Centaurea : It belongs to Compositae family and exhibits piston mechanism. It has syngenesious and epipetalous androecium. The anther lobes dehisce introsely. When filaments contract on being touched by the insect, the stigma pushes the pollen grains out of the anther tube which stick to the body of the insect. When the insect visits some other flower, pollination is affected.

   (vi)  Aristolochia : The upwardly directed protogynous flowers have downwardly directed stiff hairs in the middle of the corolla tube. These form the trap. This is called pit fall mechanism or fly trap mechanism.

 

   (vii) Orchid : Pollination occurs by wasp (Pseudocopulation mechanism), e.g. , Ophrys (orchid) and Colpa (wasp). The flower of Ophrys resemble in shape, colour and odour to female of wasp Colpa aurea, thus showing mimicry. It is a case of co-evolution also.

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

          Pollen robbers create a hole at the base of corolla tube and draw nectar from a flower whose design is not suitable for them. Other insects often take advantage of the same.

 

   (b) Ornithophily: (Pollination by birds).

          Flowers are brightly coloured but odourless and produce plenty of nectar and large quantity of pollens, e.g., Bombax (red silk cotton), Callistemon (bottle brush), Sterlitzia, Erythrina (coral tree) etc. Honey bird, humming bird and sun bird are common pollinators.

 

   (c) Cheiropterophily: (Pollination by bats).

          Bats pollinate the flowers of tropical regions, e.g., Anthocephallus (Kadamb), Kigelia (Sausage tree), Adansonia (Baobab tree).

 

   (d) Malacophily: (Pollination by snails), e.g., Arum; Lilies, Arisaema, Lemna.

 

   (e) Ophiophily: (Pollination by snakes), e.g., Santalum, Michelia.

 

   (f)   Some larger animals have been reported as pollinators in some species, e.g., Lemur (Primate) in Ravenela plant, Lizard in Flax

 

 

POLLEN-PISTIL INTERACTION

   Ÿ      Pollination does not guarantee the transfer of the right type of pollen. 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.

   Ÿ      The pistil has the ability to recognise the pollen, whether it is of the right type (compatible) or the wrong type (incompatible). The ability of the pistil to recognise the pollen followed by its acceptance or rejection is the result of a continuous dialogue between pollen and the pistil.

   Ÿ      This dialogue is mediated by chemical components of the pollen interacting with those of the pistil. If the reaction is favourable, the pollen grain germinates on the stigma to produce pollen-tube through one of the germ pores.

   Ÿ      Plants in which the pollen grain are shed at 2-celled stage, the generative cell divides and forms the 2 male gametes during the growth of pollen tube in the stigma. If pollen grains are shed at 3-celled stage, pollen tubes carry the 2-male gametes from the beginning.

   (1)   Entry of the pollen tube into an ovule

   (A)  Pollen tube, after reaching the ovary, enters the ovule through the micropyle. It is called as porogamy as seen in most of the flowering plants.

   (B)  In some plants, e.g., Casuarina, the pollen tube enters an ovule through chalaza and it is called as chalazogamy.

(a) Pollen grains germinating on the stigma; (b) Pollen tubes growing through the

style; (c) L.S. of pistil showing path of pollen tube growth

   (C)  Some times the pollen tube enters into an ovule through integuments or funicle. It is called as mesogamy. e.g,. Cucurbita.

   (2)   Entry of the pollen tube into the embryo sac :

          Irrespective of the place of entry of pollen tube into the ovule, the tube invariably enters the embryo sac at micropylar end, i.e., degenerating synergid cell. Entry of pollen tube in the embryo sac is under chemotropic guidance (synergids have filiform apparatus and secrete some chemicals).

(A) enlarged view of an egg apparatus showing entry of pollen tube into a synergid;

(B) Discharge of malegametes into a synergid and the movements of the sperms,

one into the egg and the other into the central cell

   (3)   All these events from pollen deposition on the stigma until pollen tubes enter the ovule are together referred to as pollen-pistil interaction. This interaction is a dynamic process.

   (4)   Pollen germination can be studied by dusting pollen (e.g., pea, chick pea, Grotalaria, balsam, Vinca) on a glass slide containing a drop of 10% sugar solution with boric acid, Ca, Mg and K salts. After 15-30 minutes, pollen tubes will' be observed to come out of the pollen grains. So, this germination of pollen grain in laboratory is called hanging drop method.

   (5)   Artificial hybridisation : It is one of the major approaches of crop improvement in which crosses are made between different varieties, species and genera in order to combine their desirable characters in a single superior variety. Emasculation and bagging are two precautionary measures in this hybridisation. Emasculation is the removal of anthers from the floral buds of female parent bearing bisexual flowers. Bagging is the covering of both emasculated as well as non-emasculated flowers with butter paper or polythene to prevent contamination of its stigma with unwanted pollen. When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma and the flowers are rebagged, and the fruits allowed to develop.