Water-lily leaf stem. Light micrograph of a cross- section through the leaf stem (petiole) of a white water-lily (Nymphaea alba). This shows the air canals (empty spaces, blue) in the tissue, which make the water-lily buoyant. This type of air- filled tissue is known as aerenchyma, and also improves respiration and photosynthesis. The air spaces are lined by thick wall cells, called sclereids, that have an irregular shape.