Plumeria ensures cross-pollination between flowers by having specific reproductive structures. Each flower typically has both male (stamen) and female (pistil) parts. The stamen produces pollen, while the pistil contains the ovary, where the seeds develop. When insects visit a flower to collect nectar, pollen from the stamen can be transferred to the pistil of another flower, facilitating cross-pollination. The structure of Plumeria flowers and the behavior of pollinators play a vital role in promoting cross-pollination in the plant.