The vascular system in the stem or trunk of a plant carries nutrients from the roots to the leaves and back again. It also stores food for the plant and supports the leaves. The cambium, the layer of tissue just under the bark that carries nutrients, acts much as our own skin does and protects the fleshy part of the stem, the xylem. Through photosynthesis leaves manufacture food, which is carried down to the roots. When you remove the main growth terminal to encourage a plant to grow bushy and full, you force growth at the leaf axils.
Flowers are plants' sexual organs.
Birds, bees and other pollinators do the job of carrying pollen to plants and fertilizing them.