Where does sexual reproduction happen in a plant?
the stem, the roots, the flowers, or the capillaries?

In order to create a new plant the plant has to have both male and female gametes (gametes are sex organs) so the plant transfer them to other to fertilize and the ovules (ovules:the part of the ovary of seed plants that contains the female germ cell and after fertilization becomes the seed) and they grow into seeds.