Tubercle: Thick and fleshy stem or root that acts as a storage organ. Some plants with tuberous roots can lose their leaves and stem in autumn, as the tubercle stores nourishment for new growth during the next spring. Example. Begonia tuberosa. Sometimes tubercles are produced in stalks. The tubercle is a globular structure provided with a reserve of nutrients; the potato is a typical tubercle, but not all tubercles are subterranean. (Compare Bulb, Corm, and Rhizome).