Explain the Difference Between Self Pollination and Cross Pollination
Pollination means transfer of pollen grains from anther of a flower to the stigma of the same or another flower of the same or allied species through various agencies.
Types of Pollination
There are two types of pollination
1. Self-pollination or autogamy: Self-pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower. Self-pollination also takes place between different flowers of the same plant. Such self-pollination is called geitonogamy. Self-pollination can take place only in bisexual flowers.
2. Cross-pollination or allogamy: Cross pollination is the transfer of pollen grains from the anther of one flower to the stigma ma of flowers located in another plant. This process is also called xenogamy. Cross-pollination when takes place between two flowers belonged to allied species or allied genera is called hybridism.
MECHANISMS OF SELF-POLLINATION
There are certain adaptations or contrivances (favouring points) met with in flowers to achieve self-pollination. These are :
1. Cleistogamy: In certain plants like Portulaca, Commelina,Viola etc., the flowers in bud condition remain underground.The flowers never open before pollination. As a result of which pollen from other flower cannot reach the stigma. The flowers are also homogamous, that is, the androecium and the gynoecium mature at the same time facilitating self-pollination within the flower.
2. Homogamy: In many flowers the androecium and the gynoecium mature at the same time which facilitate self-pollination. Cross-pollination may however take place.
In some flowers even if the cross-pollination is the rule, self-pollination may take place. In Mirabilis jalapa the matured anther touches the matured stigma by recoiling the filament. in Vinca, lxora etc., the anthers which are located near the throat of the corolla mature first and the stigma which mature later pushes up through the throat of the corolla and thereby collect pollen grain from already ripe anther. In flowers of Asteraceae family also the anthers mature first and if cross-pollination is not affected the stigma which matures late coils back to receive the left over pollens from the matured epipetalous anthers.
3. Bud pollination: In pea, wheat, rice etc. the anther and stigma mature before the flower buds open. Self-pollination is therefore the rule in such plants.
4. Geitonogamy: In this type the pollen-grains are transferred from one flower to the stigma of another flower borne on the same plants.
MECHANISM OF CROSS-POLLINATION
Cross-pollination occurs in both unisexual and bisexual flowers. In unisexual flowers self-pollination never takes place and as such cross-pollination must takes place in such flowers. In nature majority of flowers are cross-pollinated. Nature favours cross-pollination and therefore the majority flowers are adapted for such pollination.
Adaptation or Contrivances for Cross-pollination
Flowers adapt themselves variously or create contrivances (favourable situation) so that instead of self-pollination cross-pollination can take place. The contrivances are of the following types :
1. Dicliny or unisexuality: In unisexual flowers the stamens and carpels are found in separate flowers. So self-pollination can never take place in unisexual flowers. Flowers bearing stamens and flowers bearing carpels may be borne by the same plant (monoecious) or by different plants (dioecious).In both the cases the pollen must be transferred from the stamen of one flower to the stigma of another flower.
2. Self-sterility: Pollen grains of some flowers cannot fertilize the ovules of the same flower. The same pollen grains again can fertilize the ovules of another flowers. Example-Orchid, Solanum tuberosum,Nicotiana etc.
3. Dichogamy: In many flowers anthers and stigma of the same flower mature at different time and thus prevent self-pollination. In such flowers the anther may mature earlier than stigma and in some others the stigma may mature earlier. The former condition is called protandry and the later condition is called protogyny. China rose, cotton, sunflower etc. are protandrous flowers. Ficus, Mirabilis, Magnolia etc., are protogynous flowers.
4. Heterostyly: Some flowers show dimorphism with respect to the relative length of the stamens and the styles. In some of the flowers the style is longer than the stamens and in some of the flowers the stamens are longer than the style. Cross-pollination takes place between long stamens and long stigma and short stamens and short stigma. Thus self-pollination does not take place in such dimorphic flowers. Example – Primrose.
5. Herkogamy: In some flowers some barriers exist which prevent self-pollination. In orchid and Calotropis the pollen grains are held together in anther lobe as a group called pollinium. The two pollinia of adjacent anther lobes joined together to a sticky disc called corpusculum. The pollinia cannot move out unless these are taken out by insects and deposited in another flower.
In some flowers again the stamens and the stigma may lie at some distance from each other, the anther may be inserted within the corolla tube and the stigma far exerted or the anther far exerted and the style inserted. In certain flower (bleeding heart) the style bend downward and move away when the anthers mature. Then afterward when the pollen grains have been removed by insects the stamens roll down and the style stands erect. In such flowers therefore self-pollination cannot takes place.
Differences between Self-pollination and Cross-pollination
|1||Pollen-grains are transferred from the anther to an stigma of the same flower (homogamy) or to another flower borne on the same plant (geitonogamy).||Pollen-grains are transferred from anther of one flower to the stigma of another flower borne on a different plant.|
|2||It takes place because, anther and stigma mature at a time in a flower.||It takes place because anther and stigma mature at different times in a flower.|
|3||It takes place even when the flowers are closed or remain underground (cleistogamy).||It takes place when flowers open.|
|4||As pollen-grains are not wasted, less pollen-grains are produced.||As most pollen grains are wasted in transit a huge quantity of pollen grains are produced.|
|5||No genetic variation among offsprings takes place because no gene from other plants can migrate.||Input of new gene leads to development of newer characters.|
|6||It can maintain useful character but cannot eliminate harmful one.||It can eliminate harmful characters but cannot preserve useful characters.|
|7||It does not contribute towards evolution as variation does not occur and therefore new species seldom occurs.||It has a great role in evolution as it leads to variation of characters and this in turn gives rise to new species.|
|8||Quality and yield of plants gradually decline.||Quality and yield gradually improve.|
|9||Plants are less resistant to diseases and pests.||Plants are better resistant to diseases and pests.|
|10||Mechanism of pollination is simple with few adaptations to ensure pollination.||Mechanism is extensive with many types of adaptation to ensure pollination.|