To find out the effect of water on growth direction of plant roots.
Fill a trough with slightly moist soil and place a porous pot in the centre of it. Select about six seedlings germinated in moist blotting paper so that the roots and shoots are straight, and plant them vertically at a fixed distance away from a central porous pot. After a day or two, and before the seedlings begin to show the need for water, partly fill the porous pot with water.
After several days, dig out some of the seedlings and observe the nature of their roots. It will be found that the vertical roots have bent towards the porous pot. Use a control experiment, set up in the same way, but put no water in the pot; instead, water evenly all over the box. In the control there is no root curvature.
Roots grow towards water and are said to be positively hydro-tropic.