A stick of celery is stood in a beaker of water and food dye and left until the dye is detected in the leaves. A careful incision on the inside of the stick allows the outside edge to be peeled back revealing the water transporting xylem. The xylem are clearly stained with the dye but the surrounding tissue is completely unaffected. The xylem tissue is a hermetic hollow tube. Water in the celery leaves evaporates through their surface and this lowers the pressure which draws water up the xylem to replace the transpired water. This is how plants transport water and nutrients from the soil up to the very highest leaves. The celery stick is, in fact, not a stem but the leaf stalk or petiole.