USA Biology Olympiad (USABO) Practice Exam

In angiosperms, a distinctive feature of fertilization is the process known as double fusion. During this process, one sperm nucleus fuses with the egg nucleus to form the diploid zygote, while another sperm nucleus fuses with two polar nuclei to form the triploid endosperm. This endosperm serves as a nutrient-rich tissue that supports the developing embryo. This dual role of the sperm nuclei is what sets angiosperm fertilization apart from fertilization in other plant groups. The process ensures that the developing seed has the necessary nourishment to support its growth after fertilization. This unique double fusion is a key component of angiosperm reproduction, facilitating the efficient development of seeds in flowering plants. Other aspects listed, such as the reliance on male and female gametes meeting or the dependence on wind for successful fertilization, don't capture the specific mechanism of double fusion that is fundamental to angiosperm fertilization.