Concept-
- The stem is the ascending part of the axis bearing branches, leaves, flowers, and fruits.
- It develops from the plumule of the embryo of a germinating seed.
- The stem bears nodes and internodes.
- The main function of the stem is spreading out branches bearing leaves, flowers, and fruits.
- It conducts water, minerals, and photosynthates.
Explanation-
- Some stems perform the function of storage of food, support, protection, and of vegetative propagation.
- The stem may not always be typically like what they are expected to be.
- They are modified to perform different functions.
- Underground stems of potato, ginger, turmeric, zaminkand, Colocasia are modified to store food in them.

Therefore turnip is not the stem.
Additional InformationTurnip
- Roots in some plants change their shape and structure and become modified to perform functions other than absorption and conduction of water and minerals.
- They are modified for support, storage of food, and respiration.
- Taproots of carrot, turnip, and adventitious roots of sweet potato, get swollen and store food.
