To find out the effect of water on growth direction of plant roots.
Slice across the stem of a healthy potted plant a few inches above the ground in the morning, and fix to it, by means of a rubber tubing, a T-tube. Pour some water into the tube and freely water the soil. Fill a U-tube with a long arm and a bulb (manometer) partially with mercury. Connect the U-tube to the T-tube through a rubber cork. Insert a cork fitted with a narrow glass tube to the apex of T-tube. Make all the joints air-tight by applying melted paraffin wax. Seal the bore of the narrow tube and note the level of mercury in the long arm of the manometer.
After a some hours note the rise of mercury-level in the long arm; also note the rise of water-level in the T-tube.
The rise of mercury is certainly due to accumulation of water in the T-tube, and the latter phenomenon is due to exudation of water from the cut surface of the stem, which shows that the water is forced up through the stem by root-pressure.