1.Plant hormones are special chemicals which regulate the growth of various plant parts.
2.The growth or movement of a plant or plant part in response to a chemical stimulus.
The growth or turning of plant roots towards or away from moisture.
3.Something that causes the reaction in an organ or cell.
1)Plant hormones are the chemicals that regulate plant growth.They are also required in their metabolic activities and maintainig the functions of the plant body.

2)Hydro-tropism is the movement of a plant-body part or growth-response to the stimulus of water.
Chemo-tropism is the movement of a plant body part or growth response to the stimulus of chemicals.

3)Stimulus is the detectable change in the internal and external environment.It can be said as a reaction of our body (internal or external) when something happens to us.