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56 MTBIOLOGY TODAY | JULY    ‘15

Advantages of cross pollination


• The advantages for cross pollination are:



  • Cross pollination introduces genetic recombinations
    and hence variations in the progeny.

  • Cross pollination increases the adaptability of the
    offspring towards changes in the environment.

  • The plants produced are usually larger and the offspring

    have characters better than the parents due to the
    phenomenon of hybrid vigour.

  • The plants produced are more resistant to diseases.

  • New and more useful varieties can be produced through
    cross pollination.

  • The defective characters of the race are eliminated and
    replaced by better characters.


Disadvantages of cross pollination


• The disadvantages of cross pollination are:



  • It is highly wasteful because plants have to produce
    a large number of pollen grains and other accessory

    structures in order to suit the various pollinating

    agencies.

  • Some undesirable characters may creep in the race.

  • The very good characters of the race are likely to be
    spoiled.

  • As external agency is involved so the chance factor

    is always there.


agents of pollination


• The transfer of pollen grains from plant to plant in cross

pollination takes place through biotic agencies (living
agencies) or through abiotic agencies (non-living

agencies).


Anemophily


• The pollination taking place by means of wind is known as

anemophily. E.g., Zea mays (maize), grasses, mulberry,
date palm, Salix, Cannabis etc.


• Main features of wind pollinated flowers are :



  • Non-essential whorls such as calyx, corolla, bracts and
    bracteoles are not showy.

  • Flowers are devoid of scent and nectar.

    • In case of unisexual flowers, the male flowers are more

      in number than female flowers and in case of bisexual

      flowers, the stamens are generally numerous.

    • Both the stigmas and anthers are exserted.

    • Plants produce a very large quantity of dusty pollen

      grains.

    • Pollen grains are dry, very small sized, non-sticky and
      unwettable, so that they can be carried by wind to

      long distances.

    • Stigma is hairy, feathery (grasses) or branched to catch

      the wind borne pollens.

    • Anemophily is associated with reduction in the number
      of ovules per ovary. Biological justification for this
      reduction is that chances for windborne pollen reaching

      the right stigma are remote. If the flowers of wind-
      pollinated plants were to produce many ovules most

      of them would have remained unfertilized. In grasses
      there is just one ovule per ovary.




Hydrophily
• Pollination which occurs with the help of water is called
hydrophily.
• Main features of water pollinated flowers are :


  • Inconspicuous and small flowers.

  • Odour, nectar and colour is absent in flowers.

  • Flower whorls, if present then coated by wax.

  • Pollen grains are small, light weighted, non-sticky and
    coated by wax.

  • Stigma is unwettable but sticky.
    • Hydrophily is of two types— hypohydrophily and

    epihydrophily.
    • Hypohydrophily occurs below the surface of water, e.g.,
    Ceratophyllum, Najas, Zostera.
    • Zostera is submerged marine perennial plant where the
    pollens are exceptionally long (upto 250 mm) and needle-
    like. They look more like pollen tubes. The specific gravity
    of these pollens is almost the same as that of sea water.
    Consequently, they can freely float in water at any depth.
    The stigma is also very long in this plant. If a pollen comes

    in contact with the stigma, it coils around the latter.
    • Epihydrophily takes place over the surface of water, e.g.,
    Potamogeton, Vallisneria, Myriophyllum, Ruppia maritima.
    • Vallisneria (ribbon weed) is a classic example of this type. It

    is a submerged dioecious plant. The flowers are borne under

    water. Upon attaining maturity the male flowers get detached

    from the parent plant and float on the surface of water. The
    anthers project beyond the margins of the sepals.
    • The pistillate flowers also develop under water. At the time

    of pollination they are brought to the surface by thin, long

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