hydrotropism

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Growth or movement in a sessile organism toward or away from water, as of the roots of a plant.

from The Century Dictionary.

• noun A state induced in a growing organ by the influence of moisture, in which under certain conditions it turns toward the moisture, and under other little understood conditions it turns away from the moisture.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A tendency towards moisture.
• noun (Bot.) In a broader sense, any curvature or turning induced in certain growing plant organs under the influence of moisture.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology The movement of a plant (or other organism) either towards or away from water