As part of an experiment, you grow some plants in an environment including carbon dioxide with radioactive carbon isotope 14C as a tracer. After a few days you take the plants out and look for the tracer. You find it primarily in the roots. How did it get there? Why is it in the roots?

Plants make their own food with the help of photosynthesis, in which the organic carbon is formed from the inorganic carbon. Organic carbon is used as a source of energy and helps in various cellular functions.

Organic carbon mostly stored in the form of starch and other carbohydrates and also helps in forming the structure of the plants. The starch is stored mainly in the roots, fruits and stems. The main source of starch is in roots in tubers of the plants. The radioactive carbon that is found in the plant roots in the experiment is the excess carbon that stored in the roots of the plants