Color-enhanced Scanning Electron Micrograph (SEM) showing a celery stalk in cross section. A celery (Apium graveolens var. dulce) stalk is a special part of the leaf structure called a petiole. It serves many of the same functions as a stem. The "pipes" that conduct water and nutrients in a stalk of celery are shown in the micrograph. They contribute to the crunchiness of the stalks. Image width: 320 micrometers. Magnification: 312x if the image is printed 10 cm wide. Bar: 50 micrometers.