Flowers by nature are mostly cross pollinated. There are various adaptations for this process. In unisexual and bisexual flowers, certain devices are present for effective cross pollination and avoiding self pollination. Some of these devices which may be called contrivances of cross pollination are as follows.
(i) Dicliny or unisexuality
Here, the flowers are unisexual. It may be monoecious bearing male and female flowers in the same plant or it may be dioecious, in which case male and female flowers are borne in two separate plants. In monoecious plants although cross pollination takes place by several agents, self pollination or geitonogamy may occur. Some of the such monoecious plants are Cucurbita, Ricinus, Zea etc. In dioecious plants, cross pollination is the rule. The examples are Piper, Cannabis, Morus etc.
(ii) Self sterility –
It is the condition when a flower cannot be pollinated by the pollen grains of the same flower or from any flower of the same plant.
It is found that stigma of some orchids wither away if the pollen grains from the .same flower are deposited on it. Many plants of Solanaceae (Solanum, Nicotiana) and tea plant are self sterile and are pollinated by cross polliation.
(iii) Dichogamy –
It is found in bisexual flowers where stamens and carpels mature at different times hence, the self pollination is prevented. There are two conditions for dichogamy. When the gynoecium matures earlier than the anthers, the stigma receives the pollen grains from another flower.
This condition is known as protogyny. Common examples are Anona, Polyalthia, Magnolia, Michelia etc. The other condition of dichogamy is protandry where the anther matures earlier than the stigma. Hence, the pollen grains are carried over to the stigma of another flower in which gynoecium is matured.
(iv) Herkogamy In some flowers, there are certain adaptations of floral parts which act as barriers to self pollination and thereby, favouring cross pollination. In many cruciferous and caryophyllaceous plar I s, the stigma is exerted far beyond the stamens, preventing pollens to reach the stigma. In flowers of Gloriosa, the panthers are extrose-facing outwards with respect to stigma.
In Iris, the panthers are sheltered or hooded by petals or by the petaloid style favouring cross pollination. The pollenia of Orchids and Calotropis develop in a position where from they cannot reach stigmas by themselves. The peculiar arrangement of stamens and pistils in Salvia achieve cross pollination only by insects. In Viola, the stigma is guarded by a flap or lid which insect closes during its visit, thus, prevents self pollination.
(v) Heteromorphism Plants may have two (dimorphic) or three (trimosphic) or different forms of flowers, based on the position of anthers and stigmas at differnt levels. Such heteromorphous flowers may be heterostyly (styles of different length) or heteroanthy (different types of anthers). One form has short stamens and long style while the other has long stamens and short style (Example – Primula)
In this case, the short style will be cross pollinated by pollens from low anthers and vice versa by insects having the capacity to enter to a particular depth of the flower. Dimorphism is observed in Jasminum, Linum etc. Some species of Oxalis, Linum, Lathyrus show trimorphism. Three types of flowers show three different positions of anthers and stigmas. It results in cross pollination only. Agents and types of Cross pollination:
Cross pollination is brought about by external agents, as it involves two separate plants of the same or closely allied species. These agents can be categorised as biotic agents (insects, birds, bats, snail etc.) and abiotic agents (wind, water). Accordingly the types of pollination are entomophily, zoophily, anemophily and hydrophily. (1) Entomophily
Insect pollinated plants are entomophilous. In these cases, the flowrer attract the insects in a variety of ways and the sticky pollens easily adhere to the body or body parts of the insects. Similarly, the stigma is also sticky to receive the pollen grains. The flowers develop the following adaptations to attract the insects, (i) Conspicuous and coloured flowersHere corollas are large sized, irregular and beautifully shaped to attract insects. Bracts, sepsal, or even stamens become coloured in some plants. Example : Mussaenda, Bougainvillea, Musa.
(ii) Nectar Nectary glands secrete the nectar which attract the bees. Nectar provides nutritio to these insects. Example – Oranges.
(iii) Scent Flowers that are nocturnal emit good scent which attracts many nocturnal insec (Example- Nyctanthes, Cestrum) Flowers with offensive smell and nauseating to hum beings attract swarm of carrion flies (Example mature inflorescence of Amorphophall Rafflesia and some aroids)
(iv) Edible sap there are certain plants which do not have nectaries to attract insects, Edib sap secreted by such plants attracts insects. Example – Some Orchids
(v) Edible pollens Wax on the pollen is utilized to build the honey comb and pollen may required to nourish the young insects. (Example – the flowers visited by insects)
(vi) Special mechanisms
Pollination in Saliva : (A-B) L.S. of flower showing immature gynoecia and movement of stamen when pressed by insect (as shown by arrow); (C) bee entering the corolla and getting dusted with pollen; (D) Bee entering another flower and transferring the pollen grains to stigma.