Final answer:
Pollination and fertilization are two separate but essential steps in plant reproduction; pollination must occur first for fertilization to happen thereafter. Pollination can be through self or cross-pollination, while fertilization entails the union of sperm and egg cells.
Step-by-step explanation:
Pollination and fertilization are significant steps in the reproductive cycle of plants, but they do not occur simultaneously. Pollination refers to the transfer of pollen from the male anthers to the female stigma, which can take the form of either self-pollination or cross-pollination. Self-pollination happens when pollen from the anther is deposited on the stigma of the same flower, or a different flower on the same plant. Cross-pollination, on the other hand, involves the movement of pollen from the anther of one flower to the stigma of a flower on a different individual of the same species, commonly facilitated by agents like wind, water, or animals.
Fertilization occurs after pollination and is the process where the sperm from the pollen unites with the egg to form a zygote. In angiosperms, a unique process called double fertilization takes place. One sperm cell fuses with the egg to form the zygote, while another fuses with the polar nuclei to form the triploid endosperm. Subsequently, the zygote develops into an embryo that grows into a new plant, and the endosperm nourishes this developing embryo. Therefore, while pollination can happen without fertilization, fertilization cannot occur without prior pollination.
To clarify, not all plants rely on birds or bees for pollination. Gymnosperms, for instance, use wind to facilitate the transfer of pollen from male to female cones. Upon landing on the female cone, the pollen germinates to form a tube through which the sperm travel to reach the egg for fertilization, resulting in the development of a zygote and then an embryo within a seed.