Question

Papaya plants exhibit xenogamy only. Why?

Answer 1

Hint:- In flowering plants, the first cell of male gametophyte is called the microspore or pollen grain. Partial germination or development of pollen grain starts before dehiscence of the anther (before pollination). At the beginning of this process, the nucleus of pollen grain divides by unequal mitotic division, resulting in two unequal sized nuclei. Both the nuclei are surrounded by cytoplasm and it becomes dense, then followed by unequal cytokinesis, resulting in two cells of unequal size. Now pollen grains come in a two-celled stage.

Complete Answer:- The two-celled stage of the pollen grain is called a partially developed male gametophyte stage or mature pollen grain. Pollination takes place in this stage in most of the angiosperms. The process of transfer of pollen grains from the anther to the stigma is called pollination. Flowering plants have evolved an amazing array of adaptation to achieve pollination.
On the basis of the source of pollen, pollination is of different types- self-pollination (autogamy), geitonogamy and cross-pollination (xenogamy). If the pollen grains are transferred from anther to the stigma of the same flower then it is called self-pollination or autogamy. When pollination takes place in between the two flowers of the same plant, then it is called geitonogamy.
If both male and female flowers are present on the same plant but flowers are unisexual, then the plant is called a monoecious plant. If both male and female flowers are present on different plants and flowers are unisexual, then the plant is called a dioecious plant.
Papaya is a dioecious plant, thus in papaya plant, both autogamy and geitonogamy are not possible. When the pollination takes place in between the two different flowers of two different plants (dioecious plants) of the same species, then it is called xenogamy (cross-pollination).
Thus papaya exhibits true cross-pollination (xenogamy) genetically as well as ecologically.

Note:- Monoecious plants prevent autogamy but not geitonogamy while dioecious plants prevent both autogamy and geitonogamy. Pollen grains of papaya are transferred to the stigma through the wind (anemophily). As the pollen grains of papaya are very small and lightweight, they are easily transferred to the stigma of another flower. Also, the stigma of anemophilous plants is large, hairy and sticky that easily traps the pollen grains.
Further development of pollen grain takes place after pollination on the stigma of the carpel. Pollens absorbs moisture and sugar content from the stigma and ultimately pollen tube is produced and pollens are transferred to the ovule.