Sexual reproduction of flowering plants. Fertilization
Fertilization is the fusion of sex cells from the pollen and the ovule; a new organism begins from it. A pollen grain has two cells: the larger is vegetative and the smaller generative. The sticky fluid on the stigma holds the pollen; the vegetative cell grows a long pollen tube through the stigma and style toward the ovary. The generative cell divides into two sperm cells. The fastest pollen tube reaches the ovule and delivers both sperms. In the ovule an egg cell and a central cell have ripened: one sperm fuses with the egg cell and becomes the embryo, the other fuses with the central cell and forms the endosperm. This double fertilization is characteristic of flowering plants only (it was discovered by the Russian scientist S. Navashin in 1898). As a result the ovule becomes a seed (embryo + endosperm + a coat formed from the ovule wall) and the ovary, with other flower parts, becomes the fruit. If the ovary has one ovule the fruit has one seed (apricot, cherry, peach); if it has many, the fruit has many seeds (tulip, cotton).
Role-play model: pupils play pollen grain, stigma, style, ovary and ovule; the “pollen” lands on the stigma and a “tube” (rope) runs down the style to the ovule, two “sperm” (balls) go to the egg and the central cell. Then draw a diagram. Safety: do not run during the role-play and avoid collisions.