Celery stalk. Light micrograph of a cross-section through a stalk from a celery plant (Apium graveolens). Red eosin dye has been used to stain the xylem tissues. Each area of xylem tissue (red) is part of a structure called a vascular bundle. The outer part (curved) of the vascular bundles consist of phloem tissue. The eosin dye (dissolved in water) demonstrates that it is the xylem vessels that transport water around a plant. The rest of the stalk includes the central pith, and various supporting tissues. Magnification: x14 when printed at 10 centimetres across.