The process by which female gamete and male gamete are sexually united is called fertilization or fertilization. Sexual reproduction of herbaceous plants is ugamous in nature. The stages of fertilization process are described as-
- Pollen Germination: Inside the anther, the pollen nucleus divides to form two nuclei. Tubular nucleus and gonial nucleus. In the binucleate state, the pollen grains fall into the female endosperm through pollination. Pollen is trapped in the endosperm by the sticky sap secreted by the endosperm or with appendages. Pollen enlarges by absorbing fluid. This condition is called hydration. The anther is energized and pressure builds up inside it. Due to this pressure, the integument of the pollen tube is extruded through the reproductive pore. It is called pollen tube. Pollen does not germinate without its own species.
- Pollen tube travel to the uterus and sperm production: Cutinase enzyme degrades the cutin of the uterus and the pollen tube enters the uterus. Enzymes like pectinase, cellulase etc. are secreted from the tip of the pollen tube and dissolve the pistil cells to form the tube. The pollen tube continues towards the ovary under the influence of calcium (Ca++) ions released from the ovary and ovary. The tube nucleus enters the pollen tube first and the generative nucleus later. The germ nucleus divides to form two spermatozoa.
- Entry of the pollen tube into the embryo sac: The pollen tube enlarges and reaches the uterus. Penetrates the lining of the uterus and enters the ovum. The membrane of the ovary dissolves and the anther approaches the ovule. In most plants the pollen tube enters the ovary through the ovule.
- Ejection of sperm into the ovary: After the pollen tube enters the ovary, its tip bursts. Through the slit, the sperm enters the two follicles.
- Union of sperm and egg: In the embryo sac, one of the sperm moves towards the egg and the other towards the secondary nucleus like an amoeba. One of the two sperm fuses with the egg and the other fuses with the secondary nucleus. The union of sperm and ovum is called fertilization or syngamy and the union of sperm and secondary nucleus is termed triunion. Nishek and Trimilan together are called Bi-Nishek.