The New Student's Reference Work/Hydrotropism
Hydrotropism (hī̇-drŏt′rō̇-pĭz’m), the sensitiveness of a plant to the presence of moisture in greater amount on one side, to which it responds by so altering the rate of growth as to curve toward or away from the source of moisture. Thus the mycelium or nutritive part of fungi grows towards moisture; the reproductive parts generally away from it. In the higher plants this sensitiveness is shown most strikingly by roots. If roots of corn are growing straight downward by geotropism (which see), they will be deflected in a few hours toward a sheet of wet blotting paper hung near them. Growth is accelerated on the side where the air is drier, but is retarded on the moister side. As this is exactly the reverse of the direct influence of moisture, it is clear that the result must be an indirect one, due to irritability (which see).