Pollination types and vector

Pollination

         

Sexual reproduction involves fusion of male and female gametes to form a zygote which develops into an embryo. The gametes are produced in flowers, more precisely in stamens and carpels. The journey of male gamete (pollen grain) from the site of their formation –the anther to stigma (a part of carpel where female gametes or ovules are formed) is called pollination. It is basically of two types.

         

When pollen from an anther falls on stigma of the same flower - it is called self-pollination or Autogamy. E.g. Wheat.

         

 When pollen from an anther of one plant reaches to stigma of the flower of another plant - it is called cross -pollination or Allogamy. Eg.mango

          

A third condition, Geitonogamy, results when pollen from a flower of one plant falls on stigmas of other flowers of same plant.eg.,maize. But, genetically the results of geitonogamy are the same as those of autogamy.

         

The mode of pollination depends largely on the structure of flower. There are special  characteristic plants develop to promote self pollination or cross pollination

Floral characters

The floral characters promoting self-pollination-

Homogamy –

In some plants stamens and carpels of a flower mature at the same time    Therefore pollen grains of a flower can pollinate the stigma of the same flower e.g., Mirabilis

Cliestogamy –

In this case, flowers do not open at all. So, foreign pollen can not reach the stigma of the closed flower .E.g. Wheat, oats.

Chasmogamy –

In these cases, flowers open only after the pollination has taken place. Since the flowers open, some cross-pollination may occur. E.g. Rice.

In some plants, the stigmas are closely surrounded by anthers. Pollination generally occurs after the opening of flower but the position of anthers in relation to stigmas ensures self – pollination. E.g. Tomato.

In some species, flowers open but the stamens and the stigma are hidden by other floral organs. For eg.In the flowers of Papilionatae , the stigmas and the stamen are enclosed by the two petals forming keel.

In Compositae, stigmas become receptive and elongate through staminal columns. This ensures predominant self – pollination. 

The floral characters promoting cross - pollination-  

Dicliny – Dicliny or unisexuality is a condition in which the lowers are either staminate (male) or pistillate (female).

Monoecy – Staminate and pistillate flowers occur in the same plant, either in same inflorescence, e.g., castor, mango or in separate inflorescence e.g., maize.

Dioecy – The male and female flowers are present on different plants. E.g. Papaya.

Dichogamy – Stamens and pistils of bisexual lowers may mature at different times.

Protogyny - Pistils mature before stamens e.g., bajra.

Protandry - Stamens mature before pistils e.g., sugarbeets.

Herkogamy –

In some flowers, there is a physical barrier between anther and the style. In members of Cruciferae and Caryophyllaceae, style is long and carries stigma far beyond the stamens. Such a stigma would not receive pollen from the same flower. In Gloriosa, on the other hand, stamens are extrose and their dehiscence is not likely to allow pollen grains to land on the stigma. In Calotropis and some orchids, pollens occur in special structures called pollinia. These can be transferred only with the help of insects.

Heterostyly –

In some plants the flowers are of two (dimorphic) or three (trimorphic ) forms with anthers and stigmas at different levels. One form has short stamens and long style and the other has long stamens and short style. Based on difference in length, the phenomenon is called heterostyly and heteroanthy. Dimorphic heterostyly is found in primrose and trimorphic heterostyly in Lythrum.

In Alfalfa stigmas are covered with a waxy film the stigma does not become              receptive until this waxy film is broken by the visit of honey bees which also effect cross – pollination.

 Mechanism

         

The pollen grains are destined to reach to the stigma. Their mode of transport may be simple as in self- pollinated flowers or they may involve several vectors to reach to the stigma. The mechanisms used for this are specific to the vector involved. So, we’ll study these mechanisms with the vectors.

           

Vectors

         

Cross – pollination involves external agents for the transfer of pollen grains of one flower to another flower.

There are two types of vectors-

Biotic agents – which include animals of different types such as insects( Entomophily ), birds( Ornithophily), bats( Cheiropteriphily ), snails (Malacophily), etc.,

Abiotic agents -   like wind( Anemophily) and water(Hydrophily).     

Entomophily: 

pollination by insects is known as entomophily. A large majority of flowers are insect pollinated. The common insects are bees, flies, wasps, drones, moths, butterflies etc.

The insect  pollinated flowers develop certain adaptations to attract them. These are- arge sized flowers Bright  colored petals  and perianth members Scent and odors Nectar in special glands Edible sap Edible pollen

Other special mechanisms.

                 

The pollen grains of entomophilous flowers are sticky and spinous. The stigma is also sticky.

Ornithophily :

There are only a few bird pollinated flowers. The androecium’s and parts of gynoecium are often protruded out of the corolla tube in these flowers. A large amount of nectar is also produced by them. Birds come to flower to suck the nectar and pollinate them.

 Cheiroptiphily:

 Flowers pollinated by bats are large sized, have long pedicels and produce large amount of nectar. The flowers generally open at night and also emit scent. The bats visit several flowers to collect the nectar and in the process transfer pollen grains.

Malacophily :

In these cases pollinating agents are snails and slugs. Land plants like Chrysenthemum leucanthemum and water plant like Lemna show malacophily.

Anemophily :

Wind pollinated flowers are small and inconspicuous. These are not showy and lack scent, nectar, etc. But, these flowers produce a large amount of pollen grains. The pollen grains are often dry, small and light in weight. They may contain wings

The style and stigma are generally branched to catch the pollen grains, floating in the air. In cereals, the styles are feathery appearing as tufts of   fine, long and silky threads.

       

Hydrophily : The aquatic plants depend upon water for transfer of pollen grains

E.g. Ceratophyllaceae. Pollination may take place in flowers occurring below the water level or in flowers floating on the surface.