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Q.19. State and explain any
three devices found in flowers
which favour cross-pollination.
Your answer
Answers
Answer:
When the pollen of a flower pollinates the stigma of another flower located on a different plant, whether of the same kind or not, it is called cross-pollination or allogamy.
Structural contrivances which favor cross-pollination are listed below:
1. Dicliny:
Cross-pollination is the rule among diclinous plants, i.e., those bearing unisexual flowers.
2. Self-sterility:
This is the condition when a flower cannot be fertilized by the pollen of the same flower or, sometimes, from a flower of the same strain of plants.
3. Dichogamy:
When stamens and carpels of a bisexual flower mature at different times, pollination between them becomes ineffective.
4. Herkogamy (herkos = barrier):
In some flowers there may be some physical barrier between the anther and the style so that pollination between them is rendered difficult or even impossible.
Explanation:
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Answer:
Cross-Pollination. In this type of pollination, the pollen is transferred from the anthers of one flower to the stigma of another flower. In this case, the two flowers are genetically different from each other. Cross-pollination is always dependant on another agent to cause the transfer of pollen.
Explanation:
Pollination can be of two types:
Self- Pollination
Cross-Pollination
When the pollen is transferred from the anthers of a flower to the stigma of the same flower, it is called as self- pollination. This form of pollination is common in hermaphrodite or dioecious plants which contain both male and female sexual parts on the same flower.
In self-pollinating plants, there is less dependence on the external factors to cause pollination. These plants depend on wind or other smaller insects that visit the flower regularly. In self- pollinating flowers, the anthers, and stigma are of similar lengths to facilitate the transfer of pollen. Self – pollination can be further divided into two types:
Autogamy– In this type of self-pollination, the pollen is transferred from the anthers of one flower to the stigma of the same flower.
Geitonogamy– In this type of self- pollination, the anthers are transferred from the anthers of one flower to the stigma of another flower but on the same plant.
Advantages of self – pollination
In self- pollination, there is no diversity in the genes and therefore the purity of the race is maintained.
The plants do not depend on external factors for pollination and even smaller quantities of pollen grains produce have a good success rate in getting pollinated.
Self- pollination ensures that recessive characters are eliminated.
Disadvantages of self- pollination
Since there is no mixing up of genes, there are no new characters or features that are introduced into the lineage of the offsprings.
Self- pollination is said to reduce the vigor and vitality of the race as there are no new features introduced.
Without new characters introduced, the resultant offsprings’ immunity to diseases reduces.
Cross-Pollination
In this type of pollination, the pollen is transferred from the anthers of one flower to the stigma of another flower. In this case, the two flowers are genetically different from each other. Cross-pollination is always dependant on another agent to cause the transfer of pollen. The agents of pollination include birds, animals, water, wind, and insects. Based on the agent of pollination, cross-pollination can be of different types:
Hydrophilous Flowers-These flowers are pollinated by water means. The flowers are often very small and inconspicuous to other agents. They do not have any fragrance or too much color on their petals. The pollen is adapted to be able to float in water.
Zoophilous flowers– In this type of pollination, the pollinating agents are animals like human beings, bats, birds etc. The zoophilous flowers have pollen that is designed to stick on to the body of the animal so that they can be easily carried from one flower to another.
Anemophilous flowers– These flowers are pollinated by the agency of wind. These flowers, like zoophilous flowers, are small and inconspicuous. Another important feature of flowers that are wind pollinated is that they are very light so that they are easily carried by the wind. The pollen grains are very light, non-sticky and sometimes winged.
Entomophilic flowers– These flowers are pollinated by insects. These flowers are often attractive to look at with bright petals and are fragrant to attract the insect visitors to them. They often have broad stigmas or anthers to allow the insect to perch on it. Many of the insect-pollinated flowers also secrete nectar which attracts bees, butterflies or other similar insects to the flowers. The pollen grains in these flowers are often spiny or have extensions that help them to stick on to the body of the insects.
Ornithophilous flowers– These flowers are pollinated by birds. Very few flowers and birds show this form of pollination.
Advantages of cross-pollination
Cross-pollination is beneficial to the race of the plant as it introduces new genes into the lineage as a result of the fertilization between genetically different gametes
Cross-pollination improves the resistance of the offsprings to diseases and changes in the environment.
The seeds produced as a result of cross-pollination are good in vigor and vitality.
If there are any recessive characters in the lineage, they are eliminated as a result of genetic recombination.
It is the only way unisexual plants can reproduce.
Disadvantages of cross-pollination
There is a high wastage of pollen grains that need to be produced to ensure fertilization occurs.
There are high chances that the good qualities may get eliminated and unwanted characteristics may get added due to recombination of the genes.