THE FERTILIZATION PROCESS IN PLANTS

Pollination occurs when pollen is deposited on the stigma (in angiosperms) or the ovule (in gymnosperms ).

• Gymnosperms are a group of plants that share one common characteristic: they bear seeds, but their seeds do not develop within an ovary.
• Angiosperms are vascular seed plants in which the ovule (egg) is fertilized and develops into a seed in an enclosed hollow ovary. 

The pollen germinates through a weakening in the pollen wall by forming a slender pollen tube that elongates through its "tip extension" to penetrating between cells of the host parent. Within the pollen tube, two non-motile sperm cells are ultimately formed and are conveyed through the tube, keeping pace with tip growth. The pollen tube uses chemotropic signals to determine the final pathway to the egg cell, deep within the ovule. In angiosperms, pollen tubes penetrate the stigma, style, and ovary until they are amid the ovules. In gymnosperms, pollen germinates directly on the ovule. Pollen tubes enter ovules through a tiny pore called the micropyle and then elongate into the female gametophyte (called the embryo sac in angiosperms). In gymnosperms, the pollen tube directly penetrates the egg cell, but in angiosperms, there are sterile cells in the embryo sac, called synergids, that initially receive the sperm.

At this point, one sperm cell is discharged from the pollen tube and fuses, with the egg cell to form the zygote (the immediate fusion product) and subsequent embryo, which will become the offspring plant. In angiosperms, the second sperm fuses with the central cell to form a nutritive endosperm during double fertilization . The endosperm is needed for successful embryo development.