The fusion of nuclei of male and female gametes is called fertilization. It was discovered by Strasburger in the year 1884. In seed plants, fertilization takes place in the embryo sac. The pollen grain received by the stigma germinates, produces pollen tube and carries the male gametes to the egg cell. This process is known as siphonogamy.
Germination of pollen grain on stigma and growth of pollen tube:
Germination of the pollen grain on stigma is the first step in the process of fertilization. Pollen grain after landing on the receptive stigma firmly adheres to the stigmatic surface. The stigma exudes sticky oily or mucilaginous substances which help in the process of adhering pollen grains on the stigma. The stigma first recognizes the pollen and if it is compatible, only then it germinates. Otherwise, it is rejected (pollen-pistil interaction).
The time taken by the pollen grain to germinate on a receptive stigma depends upon their longevity which varies from few hours to several months in different species. Besides this, availability of moisture and favourable temperature is also essential for the germination of pollen grains. The pollen grains after absorbing moisture from the stigmatic surface slightly increase in size, which leads to the activation of their cytoplasm.
The pollen tube emerges from the germ pore irrespective of the position of the pollen grain on the stigma. A pollen grain usually produces a single pollen tube (monosiphonous), but in some plants, i.e., members of Malvaceae and Cucurbitaceae pollen grains are known to produce 10 pollen tubes (polysiphonous). However, in polysiphonous condition, only one pollen tube carrying the pollen nuclei grows up to the ovule and others degenerate.
The germination and growth of the pollen tube is facilitated by the enzymes which are present on the surface of the stigma. For example the enzyme, cutinase dissolves the cutin present on the stigma and facilitates the entry of the pollen tube into the style.
Flowers such as Amaryllis, Gladiolus, Linum, Vigna etc., .have hollow style whereas some flowers have solid style, such as Solanum, Petunia, Lycopersicon. In case of hollow styles, the mucilaginous secretion lining of the stylar canal helps in the passage of pollen tube through the style. In case of solid style, the styles have a specialized tissue called, conducting tissue or transmitting tissue along which the pollen tube grows.
The entire content of the pollen grain moves into the tube as the growth of the pollen tube starts. The growth of the pollen tube is restricted to the tip only. Close to the tip of the tube is present a dark-coloured callous plug, which is helpful in restricting the cytoplasm to the tip region. Successive plugs are formed just behind the tip as the tube grows.
The pollen tube consists of a vegetative nucleus or tube nucleus and two male gametes. By the time tube reaches the ovule, the vegetative nucleus degenerates.
Entry of pollen tube into the ovule:
Pollen tube may enter into the ovule by any one of the following routes:
- Chalazogamy: The entry of pollen tube into the ovule from chalazal region is known as chalazogamy, e.g., Casuarina.
- Porogamy: The entry of pollen tube into the ovule through micropyle is known as porogamy. This method of entry is observed in most of the angiosperms.
- Mesogamy. The entry of pollen tube into the ovule through funicle or integument is known as mesogamy, i.e., Cucurbita, Pistacia, Populus.
Entry of pollen tube into the embryo sac:
The pollen tube enters into the embryo sac from the micropylar end. Due to the disintegration of the integuments at the micropylar end, a mucilaginous substance is formed. The growth of the pollen tube towards the embryo sac through micropyle is guided chemotropically by this mucilaginous substance.
In the embryo sac, the pollen tube either passes between the synergid and the egg cell or enters into one of the synergids through the filiform apparatus. The synergid which is destined to receive pollen tube shows signs of disintegration immediately after the pollination.
The growth of pollen tube in the stylar canal, its entry into the embryo sac and fusion of male and female gametes are also determined by the pollen-pistil interaction. The contents of the pollen tube are discharged into the degenerating synergids either through an apical or sub-apical pore. The tube does not grow beyond this.
Syngamy and triple fusion or double fertilization:
The male gametes are released in the embryo sac. Out of the two male gametes, one fuses with egg or oosphere resulting in the formation of diploid zygote. The process is called syngamy or generative fertilization or true fertilization.
The other male gamete enters the central cell and fuses with two polar nuclei or the secondary nucleus. This results in the formation of a triploid primary endosperm nucleus. Since, this process involves fusion of three haploid nuclei (two polar nuclei and one male gamete), it is known triple fusion.
In angiosperm or flowering plants, two acts of fertilization occur in the same embryo sac, one generative and other vegetative. The fusion of one male gamete with the egg and that of other male gamete with polar nuclei or secondary nucleus is called double fertilization. Thus, it involves both syngamy and triple fusion. This unique feature of angiosperm was first demonstrated by Nawaschin (1898.) in Fritillaria and Lilium.
The diploid zygote develops into an embryo and the primary endosperm nucleus forms the endosperm.