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.
While the roots of land plants can only absorb the film of water adhering to the particles of soil, the roots of water plants are able to absorb the free water with which they are surrounded. They are greatly elongated, much more branched than those of a land plant, and thin-walled, without cuticle or root hairs on their surface. A land plant may produce water roots, as when Hyacinths are grown in glasses of water. Another good instance in nature may often be seen where the roots of trees penetrate tile drains and actually choke them up. If the roots of a land plant are immersed in a vessel of water they continue to absorb water for a time, but soon develop true water roots and the earlier or original fibres die. They get their food and air dissolved in the water surrounding them. One peculiar form of air root may be seen in Orchids. The root is surrounded by a membrane of cells, several layers deep, more or less thickened, perforated with holes, and filled with air. They absorb rain containing plant food in solution, and deposited at first in the form of dust on or near the root. If such roots develop on the outside of a flower pot or basket they must not afterwards be buried either in soil or Sphagnum. The writer has seen a fine batch of Moth Orchids (Phalsenopsis) killed by placing the small baskets inside larger ones and filling the space between with Sphagnum. In many Aroids that produce aerial roots the surface is loose and spongy and more or less densely covered with root hairs which absorb moisture from the air.