Fertilization in flowering plants and how self-fertilization / pollination is avoided

Before fertilization occurs, pollination takes place which is the transfer of pollen from the anther to the stigma of the flower. This pollen maybe from the same flower or from another flower of the same plant in which case it is called self-pollination; the pollen may also come from a flower of another plant in which case it is referred to as cross-pollination. When transferred to the stigma of a plant, the pollen germinates. A sucrose solution is secreted by the epidermal cells of the stigma. This stimulates germination of the grain and possibly supplies food. A pollen tube emerges from one of the pores in the wall of the pollen grain and grows rapidly down the style to the ovary. This is because the female gamete is protected by within the carpel and the male gamete can only reach it via the pollen tube. Its growth is controlled by the tube nucleus of the pollen grain, which is found at the growing tip of the tube. The tube nucleus is only concerned with the growth of the pollen tube and plays no part in fertilization. Growth is stimulated by auxins produced by the carpel, and the pollen tube is directed towards the ovary by certain chemicals. Growth depends upon compatibility between the pollen and the style tissue. During growth of the pollen tube, the generative nucleus of the pollen grain divides by mitosis to produce two male nuclei that represent the male gametes. They cannot swim and depend on the pollen tube to reach the female gamete which is located in the embryo sac of the ovule. The secretion of pectases by the pollen tube may soften the middle lamellae of the cells in the style and so assist its growth towards the micropyle- the usual point of entry to the embryo sac. Once it enters, the tube nucleus degenerates and the tip of the tube bursts, releasing the male gametes near the embryo sac, which they enter. One nucleus fuses with the female gamete, forming a diploid zygote; this process is referred to as fertilization. The other fuses with the diploid nucleus forming a triploid nucleus known as the primary endosperm nucleus; this process is referred to as double fertilization which is unique to flowering plants. It leads eventually to the two structures found in seed, namely the embryo and the endosperm (food store). The ovule forms the seed and the whole ovary forms the fruit.

There can be no doubt that self fertilization occurs more or less regularly in some plants. However, there appears to be a general tendency to avoid self-pollination, since this is a form of inbreeding and would very quickly reduce the variability of a population and hence its potential for evolutionary change, and thus survival. Variation usually arises from crossing over, random assortment and mixing of two sets of genes from different parents (coming from two separate flowers). There are various methods of avoiding self-pollination/fertilization and thereby promoting outbreeding. These are:

• The stamens and stigma of a flower mature at different times. If the stamens ripen before the stigma is in a condition to receive pollen, the flower is protandrous. If the stigma and ovule ripen before the stamens, the flower is protogynous.
• Is a plant has separate male and female flowers it is said to be monoecious. This condition clearly limits the possibility of self pollination.
• Some flowers have a stigmatic flap which is exposed to the pollen on the back of a visiting insect. The insect collects pollen from the stamens and closes the flap as it withdraws from the flower, thus protecting the stigmatic surface from its own pollen.
• A dioecious species is one in which some individual plants have either all male or female flowers. This greatly prevents self-pollination/fertilization.
• In some flowers, the anthers hang outside the flower, thus reducing or preventing self pollination.
• Sometimes, the petals almost enclose the stigma so the pollen and the stigma have a lesser chance of meeting.
• The pollen grain size may differ to the size of the stigma, which would prevent it from sticking to the stigma.
• Lastly, in some flowers, such as pears, the pollen only becomes functional if the stigmatic surface on which it lands has a different genetic composition. This condition is known as incompatibility.