In this article, we will discuss about the contrivances for cross pollination and self pollination.
In order that a seed may be set, the male and the female elements must come together. The male element is contained within the microspore (pollen) which develops as the male gametophyte while the female element is contained within the embryosac which is the female gametophyte developed within the megaspore and is located within the ovule or the megasporangium.
So, the next biological phase is pollination which mean’s the transference of the pollen from the anther to the receptive stigma whether of the same flower or of a different flower. As the pollen is not capable of locomotion, this act either involves some agent for this transference or the anther must be placed in the flower right above the stigma so that pollens may drop directly on the stigma by sheer gravity.
If a stigma be pollinated by the pollen of the same flower, it is a case of self-pollination or autogamy. When the pollen of one flower pollinates the stigma of a different flower but on the same plant, it is called geitonogamy. But, from the biological point of view, there is little difference between geitonogamy and autogamy as all the flowers on a plant have the same genetic constitution.
When the pollen of a flower pollinates the stigma of another flower located on a different plant, whether of the same kind or not, it is called cross-pollination or allogamy. Cross-pollination within a species (this may be inter- varietal) is called xenogamy. Cross-pollination involving different strains of plants yields hybrids. Self-pollination is possible only in hermaphrodite flowers while only cross-pollination or geitonogamy is possible in unisexual flowers. Self-pollination ends in self-fertilization and cross-pollination in cross-fertilization.
There is some confusion in the use of the terms ‘self-pollination’ and ‘cross- pollination’ in many books on Botany. Strictly speaking from the genetic point of view, ‘cross-pollination’ means only those cases where pollination involves flowers of different genotypes (pure lines or strains).
This excludes not only ‘geitonogamy’, but even those cases where the flowers involved are from different plants but of the same genotype. In considering the ‘contrivances for cross-pollination’, which are of some classical importance, the term ‘cross-pollination’ is, however, used somewhat loosely, even in this book.
As to the merits and demerits of self-pollination and cross-pollination, the question assumed great importance when Darwin published his book on ‘Cross- and Self-pollination’. Darwin was greatly impressed by the effect of cross-pollination and concluded that a species could not survive without it.
Modern researches, however, have shown that, while the immediate effect of crossing is striking because of hybrid vigour and continuous self-pollination may sometimes be injurious because of the isolation of recessives, species may well survive without cross-pollination and continuous cross-pollination is not at all necessary.
As a result, it is found that in nature there are not only flowers particularly adapted for cross-pollination as Darwin emphasized but there are also flowers particularly adapted for self-pollination.
Contrivances for Cross-Pollination:
It seems that Nature favours cross-pollination as opposed to self-pollination. It is a study of this partiality on the part of Nature that so greatly impressed Darwin. All unisexual flowers and a large majority of bisexual flowers are naturally cross-pollinated.
Special contrivances ensuring cross-pollination as noted below are very conspicuous:
1. Dicliny:
Cross-pollination is the rule among diclinous plants, i.e., those bearing unisexual flowers. In dioecious plants nothing else can take place. In monoecious plants the only alternative is geitonogamy which, however, has the same effect as self-pollination.
2. Self-sterility:
This is the condition when a flower cannot be fertilized by the pollen of the same flower or, sometimes, from a flower of the same strain of plants. In some orchids, flowers wither away if pollinated by its own pollen.
Many species of Solanum (potato, tobacco, etc.) and the tea plant are self-sterile because of genetic reasons. Cross-pollination is obligatory in such plants.
3. Dichogamy:
When stamens and carpels of a bisexual flower mature at different times, pollination between them becomes ineffective. Sometimes, however, it is found that self-pollination may take place at a later stage if cross-pollination fails. Dichogamy may be of two types:
(a) Protandry:
The anthers ripen first as in most Compositae (Fig. 399), many Umbelliferae, Malvaceae, etc. As a result, when the anther bursts, it pollinates stigmas of other flowers but not its own stigma which is not yet ripe.
(b) Protogyny:
The carpel matures first as in many members of Annonaceae (e.g., Annona, Polyalthia)and Magnoliaceae (e.g., Magnolia, Michelia) as well as in Arum maculatum. When the stigma is receptive, its own pollen is riot ripe so that it has to depend on foreign pollens.
4. Herkogamy (herkos = barrier):
In some flowers there may be some physical barrier between the anther and the style so that pollination between them is rendered difficult or even impossible.
In many Cruciferae and Caryophyllaceae, the stigma extends far beyond the stamens so that pollens from the latter are not likely to reach the former. The extrorse anthers of Gloriosa (Fig. 335) dehisce the anthers out of reach of its own stigmas. In Calotropis and orchids, where the pollens are aggregated in pollinia, the pollination (described later) is entirely at the mercy of insects.
5. Heteromorphism:
In certain plants there are flowers of two (dimorphic) or three (trimorphic) different forms with anthers and stigmas at different levels. This dimorphism or trimorphism usually involves beterostyly (styles of different lengths) and heteroanthy (i.e., different types of anthers).
The primrose (Primula sinensis of Primulaceae—Fig. 389) shows an interesting case of dimorphism. In the first form, the anthers are placed deep down in the corolla lube and the stigma lies at the entrance.
The pollens of this type are smaller and the stigma papillae larger. In the second type the anthers arc placed at the entrance while the stigma is deep down. Moreover, in this case the pollens are larger and the stigma papillae smaller.
Different types of insects moving about these flowers will naturally touch floral organs at the same level because of the difference in the lengths of their organs (proboscis, legs, etc.), so that the short style will be cross-pollinated by pollens from low anthers and vice versa.
This will involve pollens and stigmas of similar luxuriance and growth. Such dimorphism is also shown by jasmine and other sweet scented flowers of Oleaceae, by Linum, by Fagopyrum (buckwheat) of Polygonaceae, etc. Some species of Oxalis, Linum and Lythrum (Lythraceae) show trimorphism (Fig. 390).
The three types of flowers show three positions of anthers and stigmas so that there is cross-pollination involving three heights. According to Tischler, dimorphic or trimorphic conditions may be altered by changed nutrition.
Contrivances for Self-Pollination:
Although cross-pollination seems to be favoured by Nature, there are cases where self-pollination is ensured.
1. Cleistogamys:
In these cases the flowers never open (kleistos= closed) as opposed to most flowers which show chasmogamy (i.e., flowers open normally during anthesis). In cleistogamous flowers the pollens are shed within the closed flowers so that self-pollinaion is obligatory.
Cleistogamy is seen in the underground flowers of Commelina benghalensis (Fig. 391) which are small and inconspicuous. This plant also bears normal chasmogamous blue flowers above.
Such plants bearing normal as well as cleistogamous flowers are called chasmocleistogamous. Many rice varieties also are cleistogamous in the sense that the anthers in them shed their pollens and pollination is complete before the flowers open. Cleistogamy or chasmocleistogamy is also seen in balsam (Impatiens balsa- mina), pansy (Viola tricolor), Oxalis, Portulaca, etc.
2. Homogamy:
Homogamy, as opposed to dichogamy, simply means that the stamens and carpels of a flower mature at the same time. So, there is a greater chance of self- pollination although that is not obligatory.
Some homogamous flowers, however, show special mechanisms for self-pollination. Thus, in Mirabilis jalapa, when the stamens mature the filaments recoil and bring the anthers near to the stigma so that when they burst self-pollination is achieved.
Somewhat similar adaptations are seen in Argemone mexicana. Grewia subtnaequaus, etc.
3. Dichogamous Flowers Showing Adaptations for Self-pollination:
(a) Flowers of Ixora, Gardenia, Vinca, etc., have their anthers placed at the mouth of the corolla tube (throat). As the stigma elongates from below, it pushes out through these anthers which are already ripe and are pollinated in so doing.
(b) Compositae flowers are protandrous. In sunflowers the bilobed stigma, is still young and hidden within the syngenesious tube of anthers when the latter ripen and shed the pollens within the tube.
At this stage self-pollination is not possible as the receptive surfaces of the stigmas are not only immature but also hidden (Fig. 392A). In the second stage the bifid stigma grows through the anther tube pushing out pollens and opens out above (Fig. 392B).
Cross-pollination usually takes place at this stage through the agency of insects. But, if cross-pollination fails, it is seen that the stigma lobes curl back so that the receptive surfaces brush against any pollen still sticking on its surface and are thereby self-pollinated (Fig. 392C).
It is seen in actual practice that a large number of agricultural crops are naturally self-pollinated. Among these are rice, wheat, barley, oats, potato, peas, beans, tobacco, linseed, tomato and most jute varieties (specially Corchorus capsularis).
