01 Reproduction in Lower and Higher Plants - part 05 - pollination Pollination :

01 Reproduction in Lower and Higher Plants - part 05 - pollination

Pollination : 

• Pollen grains being non motile, angiosperms have evolved the strategy to use -

1. Abiotic agents (wind, water) and biotic agents (birds, insects, snails) to their flowers, 
2. Feeding the visitors and 
3. Exploiting their mobility for pollination and also seed dispersal. 

• Pollen grains are non-motile and they are usually carried from flower to flower by means of external agents. 
• Pollination is the transfer of pollen grains from anther to the stigma of the flower. 
• It is the prerequisite for fertilization because both the male and female gametes are non-motile. 
• Moreover gametes are produced at two different sites.

Self pollination :

• It is a type of pollination which occurs in a single flower or two flowers on a single plant. 
• It results in inbreeding or selfing. 
• In contrast cross pollination is the transfer of pollen grains from the anther of one flower to the stigma of another flower of different plants of same species. 
• Pollination can be further divided into three types on the basis of source of pollination.

1. Autogamy 
2. Geitonogamy
3. Xenogamy 

a. Autogamy (self pollination) :

•  It is a type of pollination in which bisexual flower is pollinated by its own pollen grains. 
• Offsprings are genetically identical to their parents e.g. pea.

b. Geitonogamy : 

• It is the transfer of pollen grain to a stigma of a different flower produced on the same plant. 
• It is functionally similar to cross pollination as it involves pollinating agents, but it cannot bring about genetic variations and is only of ecological significance 
• e.g. Cucurbita maxima. 
• It is similar to antogamy as pollen grains come from same plant.

 c. Xenogamy (cross polination/ out breeding) : 

• It is a type of cross pollination when pollen grain of one flower is deposited on the stigma of a flower of different plant belonging to same species, with the help of pollinating agency.
•  It generates genetically varied offsprings. 
• Majority of flowering plants depend on the transfer of pollen grains. 
• Virtually all seed plants need to be pollinated. 
• Most of the food and fibre crops grown throughout the world, depend upon pollinators for reproduction. 

Categories of Pollinsting agents - 

1. Abiotic agents 
2. Biotic agents 

1. Abiotic Agents : 

• These are non-living agents which include  - 

                      I. Pollination by wind and 
                      II.Pollination by water. 

I. Pollination by wind (Anemophily) : 

• Most of the important crop plants are windpollinated. 
• These includes wheat, rice, corn, rye, barley and oats. 
• Palms are also wind pollinated. 

Adaptations in anemophilous flowers : 

• The flowers are small,inconspicuous, colourless, without nectar and fragrance (odour). 
• The pollen grains are light in weight, dry and produced in large numbers to increase chances of pollination considering wastage of pollengrains. 
• Stigma is feathery to trap pollens carried by wind currents.
• Stamens are exserted with long filaments and versatile anthers. 
• Stamens and stigmas are exposed to air currents.

Always Remember :

• The pollens of wind pollinated plants are most frequently associated with symptoms of hay fever among people those are sensitive to pollens.
•  It is caused by hypersensitivity to pollen.

 II.  Pollination by water (Hydrophily) : 

• Found only in some 30 genera of aquatic monocots. 
• E.g. Vallisneria, Zostera, Ceratophyllum etc. 

Adaptations in hydrophilous flowers : 

• Flowers are small and inconspicuous. 
• Perianth and other floral parts are unwettable. 
• Pollen grains are long and unwettable due to presence of mucilage. 
• Nectar and fragrance are lacking in flowers. 
• Hydrophily is of two types- 

              i. Hypohydrophily 
              ii.  Epihydrophily

i. Hypohydrophily : 

• Pollination occurs below the surface of water. 
• Here the pollen grains are heavier than water, sink down and caught by stigmas of female flowers, 
• e.g. In Zostera (sea grass) the pollen grains are long, ribbon like and without exine. 

ii. Epihydrophily : 

• The pollen grains float on the water surfaceand reach the stigma of female flower. 
• e.g. Vallisneria is a submerged dioecious, fresh water aquatic plant in which female flowers reach the water surface temporarily to ensure pollination and male flowers float on the surface of water. 
• Specific gravity of pollen grain is equal to that of water. 
• That is why they float on surface of water. 
• Some aquatic plants are anemophilous e.g. Potamogeton, Halogaris, etc. 
• Some aquatic plants are entomophilous e.g. Lotus, water hyacinth, waterlily, etc.

B. Biotic Agents : 

• It includes living agents. 
• About 80% of plants require the help of other living, moving creatures such as insects, birds, bats, snails to transfer their pollens from one flower to another. 
• These also sustain our ecosystems and produce natural resources by helping plants to reproduce.

1. Pollination by insects (Entomophily) : 

• It occurs in Rose, Jasmine, Cestrum, etc. 

Adaptations in entomophilous flowers : 

• They are large, showy and often brightly coloured. 
• The flowers produce sweet odour (smell) and have nectar glands.
• The stigma is rough due to presence of hair or is sticky due to mucilaginous secretion. 
• The pollen grains are spiny and surrounded by a yellow sticky substance called pollen kit. 
• Some plants have special adaptations for the insect visitor to help in cross pollination, e.g. lever mechanism or turnpipe mechanism in Salvia.
  

Do u know ?

• In biotic pollination, plants are adapted to encourage the specific pollinators they need. 
• They are said to have developed pollination contrivance. 
• Plants and pollinators have co-evolved physical characteristics that make them to interact successfully. 
• Such characteristics are considered pollination syndromes.

 2. Pollination by birds (Ornithophily) : 

• Only a few types of birdsare specialised for pollination. 
• They usually have small size and long beaks e.g. Sun birds and humming birds.
• Some ornithophilous plants are Bombax, Callistemon (Bottle Brush), Butea, etc.

Adaptations in ornithophilous flowers : 

• Flowers are usually brightly coloured, large and showy. 
• They secrete profuse, dilute nectar. 
• Pollen grains are sticky and spiny. 
• Flowers are generally without fragrance, as birds have poor sense of smell.

3. Pollination by Bats (Chiropteryphily) : 

• Bats can transport pollens over long distance, some times several kilometers.

Adaptations in Chiropterphilous flowers : 

• Flowers are dull coloured with strong fragrance.
• They secrete abundant nectar. 
• Flowers produce large amount of edible pollen grains.
• e.g. Anthocephalous (kadamb tree), Adansonia (Baobab tree), Kigelia (Sausage tree).