This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
As these contain little (or no) woody matter in their tissues the rise of sap is of a feeble character, but when wholly submerged there is no transpiration current at all. When the roots are in soil, food would be brought in that way, while the oxygen for breathing purposes and the carbon dioxide absorbed by the leaves are taken directly from the surrounding water. The leaves and stems have no cuticle, so that each cell comes in direct contact with water holding food in solution. Floating plants get all their food directly from the water, except the carbon dioxide of the air available to the exposed leaves.