Xylem

Celery is a plant that retains a lot of water. One of the biggest reasons that it's able to keep this much water is the amount of xylem in it. Xylem is the transportation system for water in the stem of a plant. The xylem will move water and other necessary minerals upward through the plant's stem. The hollow circles that you can see on the stem of celery is where the xylem is transferring those nutrients and water to the leaves of the plant.