Plants with floating leaves likek the water lily (Nymphaea sp.) obviously do not lack water. Their stomata of importance for the exchange of gases are present at the top only. Large air cavities provide buoyancy for the leaf. The large intercellular spaces continue into the stem and root and they provide the necessary oxygen to these organs. Leaves floating on a water surface are exposed to the sun and therefore often show a thick cuticle to avoid radiation damage. The species can capture the ample sunlight thanks to their well-developed palisade parenchyma that contains many chloroplasts.
Floating water plant: example - the water lily
Cross-section through a leaf of the waterlily. Labels in the left image: 1 upper epidermis, 2 stomata, 3 intercellular space below stomata, 4 palisade parenchyma, 5 spongy parenchyma, 6 air cavities, 7 sclereids, 8 xylem, 9 phloem, 10 lower epidermis
Links to the image-gallery: click on the thumbnails