The fusion of male gamete with female gamete (egg) is called fertilisation. In Angiosperm a specific form of fertilisation takes place which is known as double fertilisation. In this two sets of fertilisation take place or we can say female gametophyte (embryo sac) fuse with two male gametes. One male gamete fertilised with egg cell and other with polar nuclei. The process was discovered by S.G. Nawaschin and Guignard. Fertilisation occurs in four steps:
- Germination of pollen on stigma
- Growth of pollen tube and entry into ovule
- Pollen tube entry into embryo sac
- Fusion of gametes.
Germination of pollen on stigma
- In gymnosperms, the pollen grains usually land directly on the nucellus, while in angiosperms, they fall on the stigma.
- After being deposited on the stigma, the pollen grain absorbs liquid from the moist surface of the stigma, expands in size.
- The stigma plays an important role in the germination of pollen grain.
- The stigma, scretes fluid containing liquids, gums, sugar and resins. The chief function of the stigmatic secretion is to protect the pollen as well as the stigma from desiccation.
- The intine form a tube like structure through a germ pore. This tube like structure are known as pollen tube.
- It continues to elongate.
Growth of pollen tube and entry into ovule
- Each pollen tube grows through the stigma and style to the ovary.
- Each pollen tube has two nuclei, a vegetative or tube nucleus and a generative nucleus.
- The tube nucleus is at the growing tip of the pollen tube and disintegrates.
- The generative nucleus divides to give rise to two male gametes or nuclei.
- After reaching ovary, the pollen tube enters the ovule. Pollen tube may enter the ovule by any one of the following routes:
- Progamy: Entrance of the pollen tube through the micropyle is known as progamy. It is the most common type.
- Chalazogamy: Entrance of the pollen tube through chalazal region is known as chalazogamy.
- Mesogamy: Entrance of the pollen tube through the middle part or integument or through funicle is known as Mesogamy.
Pollen tube entry into embryo sac
- The pollen tube enters the embryo sac only from the micropylar end irrespective of its mode of entry into the ovule.
- The pollen tube either passes between a synergid and the egg cell or enters into one of the synergids through filiform apparatus.
- This filiform apparatus of synergids direct the grwoth of pollen tube by secreting some chemical substances (chemotropic secretion).
- After entrance of pollen tube in embryosac, synergid starts degenerating.
- The tip of pollen tube enlarge and ruptures to release both male gametes.
Fusion of gametes (Double Fertilisation)
The fusion of male gamete with female gamete is called fertilisation. After pollination, pollen germination takes place. Male gametes enter into embryo sac by pollen tube. At this stage there are two male gametes.
- First Fertilisation: One of the male gametes moves towards the egg cell and fuses with its nucleus and completing syngamy. This results in the formation of a diploid cell, zygote. This fertilisation is known as generative fertilisation.
- Second Fertilisation: The other male gamete moves towards the two polar nuclei and fuses with them to produce a triploid primary endosperm nucleus (PEN). This involves the fusion of three haploid nuclei hence it is termed as triple fusion. It is also knwon as vegetative fertilisation.
Since two types of fusion, syngamy take place in an embryo sac the phenomenon is termed double fertilisation. PEN develops into endosperm and the zygote develops into an embryo. Total number of nuclei involved in double fertilisation is five, 2 in syngamy + 3 in triple fusion.
Significance of Double Fertilisation
- Female gametophyte stops its growth at eight nucleate stage, further growth continues after double fertilisation.
- Syngamy leads to diploid zygote, which later on changes into embryo.
- Triple fusion makes endopserm, which provide nutrition to the developing embryo. So it is necessary for the formation of viable seeds.
- Secondary nucleus stops its division before fertilisation. So triple fusion induces this dormant nucleus to regain its division power.
Question: What is siphonogamy?
Answer: The fertilisation involving formation of pollen tube is called siphonogamy.