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In this essay we will discuss about the types of pollination.
Essay # 1. Self-Pollination or Homogamy or Autogamy:
When the pollen grains are transferred from an anther to the stigma of the same flower, the process is called self-pollination. viz. Commelina benghalensis of Commelinaceae, Argemone mexicana of Papaveraceae, Mirabilis jalapa of Nyctaginaceae etc.
[On the other hand, the process of transfer of pollen grains from anther of one flower (unisexual or bisexual flower) to the stigma of another flower (unisexual or bisexual) occurring on the same plant is called Geitonogamy.]
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From the biological point of view, little difference exists between autogamy and geitonogamy, as all the flowers of a plant are genetically same.
Contrivances for Self-Pollination:
Self-pollination occurs only in bisexual flowers and never in unisexual ones.
There are several devices which ensure self- pollination:
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1. Cleistogamy:
Some plants produce small and closed bisexual flowers, in addition to normal flowers. These small flowers remain either on or under the ground and never open and so self-pollination is obvious.
These flowers are said to be cleistogamous or cleistogenes, e.g., Commelina benghalensis (Fig. 2.146) of Commelinaceae, sundew, Drosera burmanii of Droseraceae, Oxalis corniculata of Oxalidaceae, Impatiens balsamina of Balsaminaceae, Polygala polygama of Polygalaceae etc.
It is of two types:
a. Obligate or Habitual Cleistogamy:
The flowers are typically cleistogamous and never open, e.g., small underground flowers of Commelina benghalensis of Commelinaceae etc.
b. Facultative or Pseudo-Cleistogamy:
The flowers remain open for a short period of time and then they become closed permanently, e.g., sundew, Drosera burmanii of Droseraceae, Oxalis corniculata of Oxalidaceae, Portulaca oleracea of Portulacaceae etc.
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2. Homogamy:
When anthers and stigmas become matured almost at the same time, the self-pollination takes place. This condition is called homogamy.
The chasmogamous flowers (i.e., the flowers normally open during anthesis) are commonly cross-pollinated, but due to failure of cross-pollination they may undergo self-pollination by one of the following processes:
a. The style and filament get spirally coiled, e.g., Clerodendrum viscosum of Verbenaceae, Mirabilis jalapa of Nyctaginaceae etc.
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b. The style elongates to receive the pollen, e.g., Gardenia florida of Rubiaceae.
c. The stigma either contracts or bends to come in close contact with the anthers, e.g., Grewia subinaequalis of Tiliaceae.
d. The stigma curls back to reach the anthers, e.g., sunflower, Helianthus annuus of Asteraceae etc.
Essay # 2. Cross-Pollination or Allogamy:
When the pollen grains are transferred from an anther of one flower to the stigma of another flower borne on different plants of same or different species, the process is called cross- pollination. Viz. Michelia champaca of Magnoliaceae, Leonurus sibiricus of Lamiaceae etc.
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[The cross-pollination taking place between two different plants of the same species is called Xenogamy and between two different species is called Hybridism.]
Contrivances for Cross-Pollination:
There are several devices which ensure cross-pollination:
1. Dicliny or Unisexuality:
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When unisexual (diclinous) flowers i.e., staminate or male flowers and pistillate or female flowers are borne on the different plants of the same species (dioecious plant), then the cross- pollination is a must, e.g., Bryonia dioica and Trichosanthes dioica of Cucurbitaceae; Carica papaya of Caricaceae; Borassus flabellifer of Arecaceae etc.
In monoecious plants, however, geitonogamy may take place, e.g., maize, Zea mays of Poaceae; gourd, Cucurbita maxima of Cucurbitaceae etc.
2. Dichogamy:
In some bisexual flowers, the stamens and carpels do not mature at the same time, thereby the self-pollination becomes hindered.
It is of two types:
a. Protandry or Proterandry:
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The anthers of a flower mature earlier than the carpels, e.g., Leonurus sp. of Lamiaceae (Labiatae), Helianthus annuus of Asteraceae (Compositae), Hibiscus sp. of Malvaceae etc.
b. Protogyny or Proterogyny:
The carpels of a flower mature earlier than the anthers, e.g., Magnolia grandriflora and Michelia champaca of Magnoliaceae, Pedilanthus tithymaloides and Poinsettia pulcherrima of Euphorbiaceae etc.
3. Self-sterility or incompatibility:
In this condition, the pollen grains are not able to fertilise the ovule of the same flower as they failed to germinate on the stigma, e.g., Reseda odorata of Resedaceae, a few fruit trees of Rosaceae, Solanum tuberosum of Solanaceae, Passiflora sp. of Passifloraceae etc.
4. Herkogamy:
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In some flowers, due to some physical barriers between anther and style, the pollination becomes impossible. In orchids and members of Asclepiadaceae like Calotropis procera, where the pollens are aggregated in pollinia, the pollination is completely depends on the mercy of the insects. Due to the extrorse anthers in Gloriosa superba of Liliaceae (Fig. 2.120C), the pollens become out of reach of its own stigma.
5. Heteromorphism:
In some plants, the flowers are of two (dimorphic) or three (trimorphic) different forms having anthers and stigmas developed at different levels. The di- or trimorphism usually involves heteroanthy (i.e., anthers of different types) and heterostyly (i.e., styles of different lengths). Thus, the cross-pollination takes place between stamens and stigmas of same length.
a. The dimorphism is found in Primula sinensis (Fig. 2.147) of Primulaceae, where one type having long stamen and a short style and the other having short stamen and a long style. It is also found in Polygonum orientale and Fagopyrum esculentum of Polygonaceae, Biophytum sensitivum of Oxalidaceae etc.
b. The trimorphism is found in Lythrum salicaria of Lythraceae (Fig. 2.148), where three different forms of flowers having short, long and medium sized stamens and styles. It is also found in Oxalis corniculata and Averrhoa carambola of Oxalidaceae, Woodfordia Moribunda of Lythraceae etc.
Agents for Cross-Pollination:
Different agents can transfer pollen grains from anther of one flower to the stigma of other flowers. The agents are: Wind (anemophily); water (hydrophily); insects (entomophily); slugs, snails and squirrels (malacophily); birds (ornithophily); bat (chiropteriphily).
There are different types of pollination based on the pollinating agents:
1. Anemophily or Anemogamy:
The pollination with the help of wind is called anemophily or anemogamy. The anemophilous plants produce enormous amount of pollen grains. The pollen grains are small, smooth, dry and light in weight. Pollens of such plants are blown off at a distance more than 1,000 km.
A single plant of Mercurialis annua has been estimated to produce more than 135 crores of pollen grains. The pollen grains of Pinus (Gymnosperm) are winged and help in wind pollination. In grasses, the flowers may be borne on long axis situated much above the leaves and the anthers are versatile, thereby the anther can oscillate in all directions at the tip of the filament.
On the other hand, the flowers have adequate device to catch the air-borne pollen grains. In grasses, the stigma is usually large and feathery, which helps to catch the pollen grains. In Typha, the stigma is brush-like which helps to catch more pollen grains.
2. Hydrophily or Hydrogamy:
The pollination with the help of water is called hydrophily or hydrogamy. It is of two types: a. Hypohydrogamy and b. Epihydrogamy.
a. Hypohydrogamy:
When the pollination takes place inside the water, it is called hypohydrogamy, e.g., Najas sp., Ceratophyllum sp. etc.
b. Epihydrogamy:
When pollination takes place on the water surface, it is called Epihydrogamy, e.g., Vallisneria spiralis (ribbon weed). In Vallisneria (Fig. 2.149), the flowers are borne under water. After maturation, the pistillate flowers are brought to the surface by their long stalk and form a cup-like depression.
The male flowers get detached from the parent plant and float on the surface of water. If male flower gets lodged into the depression, pollination occurs. After pollination, the elongated stalk of the pistillate flower undergoes spiral coiling, thereby it again comes under the water.
3. Zoophily or Zoogamy or Zoidiophily:
When pollination takes place with the help of animals, it is called Zoophily.
It is of the following types:
a. Entomophily or Entomogamy:
When pollination takes place with the help of insects, it is called entomophily. The insects those help in pollination are bees, flies, beetles etc. The flowers which open during sunrise are generally brightly coloured, acting as flag apparatus for attracting insects. But the flowers those open after sunset are white in colour, thus become visible in night.
In addition to colour, other devises to attract the insects are smell and/or nectar. The pollen grains are larger in size, the exine is pitted, spiny etc., so they can be adhered firmly on the sticky stigma. Approximately, 80% of the pollination done by the insects is carried by bees.
b. Malacophily or Malacogamy:
When pollination takes place with the help of other animals like slugs, snails, squirrels etc., it is called malacophily. During their visit from one tree to other, the slugs help in pollination. Like wise, snails and squirrels also pollinate some aroid flowers.
c. Ornithophily or Ornithogamy:
When pollination takes place with the help of birds, it is called ornithophily (Fig. 2.150). The ornithophilous flowers are very large and showy, those secrete profuse nectar within them, thereby some birds get attracted. Ornithophily is visible in some flowers like Bignonia capreolata of Bignoniaceae, Strelitzia reginae of Musaceae, Butea monosperma of Fabaceae, Bombax ceiba of Bombacaceae etc.
d. Chiropteriphily or Chiropterigamy:
When pollination takes place with the help of bats, it is called chiropteriphily. The flowers of Bombax ceiba of Bombacaceae, Anthocephalous cadamba of Rubiaceae etc., are pollinated by bat.
Advantages and Disadvantages of Self and Cross-Pollination:
Several advantages and disadvantages are there in nature in both self and cross-pollination:
A. Self-Pollination:
Advantages:
1. The purity of the race is maintained through self-pollination.
2. The wastage of pollen grain is negligible.
3. Chances of failure of pollination are very less.
Disadvantages:
1. The production of new species and varieties is inhibited.
2. Due to continued self-pollination for several generations, the progeny becomes weak.
B. Cross-Pollination:
Advantages:
1. Cross-pollination is useful in improving the crop and also for production of new varieties of different crops like fruits, vegetables etc.
2. The offsprings become more healthy and strong.
3. The production of more viable seeds is increased.
4. The plants are better adapted in the nature.
Disadvantages:
1. Pollination is uncertain, because it depends on external agencies like wind, insects, water etc.
2. The wastage of pollen is much more as it depends on external agents. Wastage of pollen is more in anemophily (wind pollination) than other types.
3. The wastage of energy is much more.
