The roots move water, minerals and nutrients to the plant. However, the energy for regrowth comes from the above ground parts of the plant, not the root system. The food from the root system goes to support the root system itself. One researcher planted an acorn in a known weight of soil. After five years, he dug up the resulting plant, carefully washed away the soil, then weighed the soil and the plant.
Plants reproduce several ways.
Seed production is the obvious way. Seeds such as threeawn, beggars' lice and grassbur can spread by attaching themselves to animals. Juniper, mesquite, and sumac can be spread through animal dropping. Water lilies are spread through the water, and dandelions are spread through the air. Another way plants reproduce is by underground stems called rhizomes such as sideoats grama, switchgrass, and ragweed.