- Stamen, Microsporangium and Pollen Grain
- The pistil, Megasporangium (ovule) and Embryosac
The union of haploid male and female gamete is known as fertilization.
Fertilization result in the formation of the zygote (2n) which undergoes division to form an embryo. Siphonogamy is the phenomenon in which male gamete are brought to the egg containing female gamete by the pollen tube.
The different steps in fertilization:-
- Germination of pollen grain on stigma.
- The Path taken up by the pollen tube.
- Entry of the pollen tube into female gametophyte or embryo sac.
- Path of male gametes.
- Fusion of gametes and double fertilization.
Germination of pollen grain on stigma
when pollen reaches to the stigma, for germination it requires hydration. Pollen absorbs secretion of stigma and swells up and because of this swelling the exine ruptures and immediately intine comes out in the form of the pollen tube. The pollen tube pierces through stigmatic papilla into the tissue of style.There are two important conditions of for the formation of the pollen tube.
- when one pollen tube arises from one pollen grain, the condition is called Monosiphonous.
- when many pollen tubes arise from one pollen grain, the condition is called Polysiphonous.
Path taken up by pollen tube
when pollen tube emerges by rupturing of exine it pierces through stigmatic papilla into the tissue of style and makes a way up to ovary region. when it reaches in ovary region, it enters into the ovule in either of the three ways:-
- Through micropyle – This is the most common mode in angiosperms. In this, the pollen tube makes its entry through micropyle into the ovule and this is known as Porogamy.
- Through chalaza – In chalazogamy, the pollen tube reaches into ovary through chalaza. It is first reported in Casuarina.
- Through integuments or funiculus – When pollen tube reaches into ovary through integuments of funiculus it known as Mesogamy.
Entry of the pollen tube into female gametophyte or embryo sac
Pollen tube can follow any of the path described above to reach ovule, but the pollen tube always enters into the embryo sac at the micropylar end. The pollen tube entry may be between egg and one synergid or between the embryo wall and a synergid or entering the embryo sac directly through one synergid. So it is clear that one synergid always is degenerated by the pollen tube.
Path of male gametes
When pollen tube reaches inside the embryo sac, the two male gametes are discharged through a sub-terminal pore in pollen tube. The content of the pollen tube is discharged in the synergid and the pollen tube does not grow beyond in the embryo sac.
Fusion of gametes and double fertilization
The pollen tube always enters the embryo sac at the micropylar at one of the synergids and release two gametes into the cytoplasm of the synergid.
- One of the male gametes moved towards the egg and fused with it and called as Syngamy ( the real act of fertilization). This syngamy form a diploid cell, the zygote (2n).
- The second male gamete moves towards the two polar nuclei located in the central cell and fuse with them to form triploid primary endosperm nucleus. As this involved the fusion of three haploid nuclei, it is called triple fusion.
Since the two type of fusion (syngamy and triple fusion) simultaneously take place in an embryo sac, the phenomenon is termed as double fertilization (Unique and universal feature of angiosperms). Total five nuclei involved in double fertilization, i.e., 2 in syngamy and 3 in triple fusion.
Sometimes due to the entry of more than one pollen tube inside embryo sac or due to the presence of more than two male gametes in a single pollen tube, the two male gametes or sperms may reach inside a single embryo sac, which is called polyspermy.
When the egg is fertilized with the male gamete of one pollen tube and polar nuclei are fertilized with the male gamete of another pollen tube this phenomenon is called hetero- fertilization.