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Enlist advantages and disadvantages of cross pollination. Add a note on pollination mechanism is salvia.

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Advantages of cross pollination are :
(i) Involves genetic recombination and brings variations.
(ii) The undesirable can be eliminated and the desirable characters can be introduced in the offspring .
(iii) Improved varieties like hybrids can be produced by hybridisation method.
(iv) The offspings formed are strong, disease resistant and better adapted to climatic variations.
(v) Offsprings show better vigour and vitality.
(vi) It favours the process of evoltion .
Disadvantages of cross pollination are :
(i) As it depends on the external carriers therefore, chances of failure are more.
(ii) It is not an economical method as lot of plant energy is wasted for attraction of carriers.
(iii) Pollen grains are wasted .
(iv) Due to genetic recombinations , some desirable characters can be eliminated and undesired ones may get introduced in the offspring.
(v) Genetic purity is not maintained in the offspring.
image
(A) flower with mature ,closed stigma and short style.
(B) Shedding of pollen grains on the back of entering insect.
(c) Flower with mature stigma and withering anthers.
(D) stigma receiving pollen grains from the back of entering insect.
Few plants develop specical mechanism to favour this process e.g. Salvia has developed lever mechanism in its flower.
In salvia, flower is bisexula and protandrous. The two stamens of flowers have long bifurcated connective. Upper branch of connective bears fertile anther lobe whle the lower has sterile anther lobe. An insect when enterns in the flower. It pushes the lobe bends down. It comes in contact with back side of insect body and pollen grains are dusted there. The same iinsect when visits another flower with mature gynoecium , the pollen grains are picked up by the respective stigma. such mechanism is called lever-mechanism.

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