Final answer:
Angiosperms dominate terrestrial ecosystems primarily due to their unique reproductive structures, flowers and fruit, which facilitate pollination and seed dispersal, respectively.
Step-by-step explanation:
The two structures that allow angiosperms to be the dominant form of plant life in most terrestrial ecosystems are flowers and fruit. Flowers enable the reproductive process by facilitating pollination, often through evolutionary relationships with animals, especially insects. These modified leaves, with their sepals and petals, protect the reproductive organs like stamens and carpels, while inviting specific pollinators with their colors and fragrances. After fertilization, the ovary's walls thicken to form the fruit, which plays a critical role in seed dispersal, through various means such as wind, water, or animal ingestion, thus promoting wide distribution and growth of the angiosperm species.