When you are eating a celery stick, you are eating the tissue of a plant.
- vascular
- ground
- dermal

The answer would be dermal. This is the outermost part of the plant that serves as the protection from the outside environment. This is the part that creates cuticle, the waxy layer of the plant, and this helps the plant retain its water. Therefore, when you are eating a celery stick, you are eating the dermal tissue of the plant.