The contrivances for cross pollination
The special devices or adaptations which favour cross pollination are as follows:
Dicliny or unisexuality:
Flowers are unisexual. The plants may be monoecious or dioecious. Monoecious plants bear both male and female flowers for example, maize, pumpkin, cucumber, castor etc.
In dioecious condition, male and female flowers are borne by two different plants, i.e., the staminate flowers are born by male plants and the pistillate flowers by the female plants. For example, mulberry, betel, papaya, cannabis, vallisneria etc.
In monoecious plant, self-pollination or cross pollination takes place whereas in dioecious plant, only cross pollination takes place.
In this condition, pollen grains from the same flower or any flower of the same strain does not have any effect on the stigma. In contrast, the stigma of some orchids withers if the pollen grains from the same flower are deposited on it. This phenomenon is called self-sterility.
Pollen grains of a flower do not germinate on the stigma of the same flower due to the presence of similar self sterile gene. Cross pollination becomes necessary in such plants. Self-sterility is observed in potato, tobacco, tea, crucifers etc.
The stamens and carpels of a bisexual flower mature at different times, thereby preventing self-pollination. Such condition is known as dichogamy. Dichogamy manifests itself in two ways.
a) Protandry: In such flowers, the anthers mature earlier than the stigma. The pollen grains of these flowers are transferred to the mature stigma of another flower. Protandry is commonly observed in cotton, salvia, coriander, sunflower etc.
b) Protogyny: In such flowers the carpels mature earlier than the anthers. Hence, the stigma receives pollen grains from another flower. Protogyny is observed in Mirabilis, Gloriosa, Magnolia, Michelia, custard apple, banyan etc.
It is a mechanical device to prevent self-pollination and promote cross pollination. In some flowers, there is a physical barrier between the style and the anther which prevents self-pollination.
In members of family Caryophyllaceae and Cruciferae, the style is long and carries stigma far beyond the stamens. Such a stigma is unable to receive pollen grains from the same flower.
In Gloriosa, on the other hand, stamens are extrorse and their dehiscence is not likely to allow the pollen grain to land on the stigma. In calotropis and some orchids, pollen grains occur in some special structure called pollinia. The pollen grains can only be transferred with the help of insects.
In pansy, stigma lies inside a flap while in Kalmia, the anthers lie inside the corolla pockets. In cypripedium, the stigma lies on the route of insect entry while anthers occur near the exit. Aristolochia has a foul-smelling pit fall protogynous flower where the insect gets entrapped and can come out only when the anthers mature.
Cross pollination is also promoted in plants in which length of the style differs in the individuals of the same species. This condition is known as heterostyly. There are two to three types of flower with different heights of styles and stamens.
a) Diheterostyly (dimorphic heterostyly): There are two types of flowers, pin-eyed and thrum-eyed. The pin-eyed flowers consist of long style and short stamens and the thrum-eyed possess short style and long stamens, for example, primrose and jasmine. While visiting such plants, the mouth parts of an insect thrust into a short-styled flower, come in contact with the anthers and get covered with pollen.
When the insect enters a long-styled flower the pollens are deposited on the stigma (which is located approximately the same level as the anthers in a short-styled flower). Similarly, the pollen adhering to the mouth parts of an insect that has visited a long-styled flower will be deposited on the stigma of a short-styled flower which is at the same level in the corolla tube as the anthers of the long-styled flower.
b) Triheterostyly (trimorphic heterostyly): There are three types of flowers with different heights of style and stamens. Pollination occurs between anthers and stigma of same height present in different flowers.
Significance of cross pollination:
- Cross pollination produces genetic recombinations. It results in the formation of individuals with new useful characters.
- Cross pollination increases the adaptability of the offspring to new environments.
- Offsprings produced usually have healthier seeds and better yield. This phenomenon is known as hybrid vigour.
- Cross pollination have been used to produce disease-resistant and high-yielding varieties of many economically important plants.
- The defective characters of the race are eliminated and replaced by better characters.
- Many economically important plants are self-sterile, i.e., the pollen grains cannot complete their growth on the stigma of the same flowers due to mutual inhibition or incompatibility, for example, members of cruciferae and solanaceae. Several plants are prepotent, i.e., the pollen grains of another flower germinate more readily and rapidly over the stigma than the pollen grains of the same flower, for example, apple, grape etc. In such cases, cross pollination proves to be a better option.
- It is highly wasteful because plants have to produce a large number of pollen grains and other accessory structures in order to attract various pollinating agents.
- Cross pollination is dependent on external agency, therefore, a chance factor is always involved.
- Since cross pollination involves recombination of genes, sometimes few undesirable characters may be introduced in the race.
- Good characters of the race are likely to be spoiled.
- It is less economical.