A potometer  sometimes known as a transpirometer, is a device used for measuring the rate of water uptake of a leafy shoot. The causes of water uptake are photosynthesis and transpiration.

The instrument used for measuring the rate of transpiration is the Potometer.

1. Potometer:
A potometer is a device used to measure the rate of water uptake or transpiration in plants. It consists of a glass tube attached to a plant stem, with a graduated scale for measuring water loss. The potometer is designed to measure the amount of water lost by a plant through transpiration, which is the process by which water is evaporated from the surface of plant leaves.

2. Principle of Operation:
The potometer works on the principle of water potential gradient. As water is lost through the leaves of a plant during transpiration, it creates a negative pressure or tension in the xylem vessels of the plant. This negative pressure is transmitted to the water column in the potometer, causing it to move upwards. By measuring the distance moved by the water column over a given time period, the rate of transpiration can be calculated.

3. Components of a Potometer:
A potometer typically consists of the following components:
- A glass tube with a graduated scale: This is used to measure the movement of the water column.
- A rubber stopper or cork: This is used to seal the tube and prevent water loss.
- A capillary tube or porous pot: This allows water to enter the potometer from a reservoir or be taken up by the plant.
- A plant stem: The cut end of the plant stem is inserted into the potometer to allow water uptake and transpiration.

4. Procedure:
To measure the rate of transpiration using a potometer, the following steps are typically followed:
- The potometer is filled with water, and any air bubbles are removed.
- The cut end of the plant stem is inserted into the potometer, ensuring a tight seal.
- The apparatus is set up in a controlled environment, such as a closed chamber, to prevent water loss from sources other than transpiration.
- The movement of the water column is observed and measured over a specific time period.
- The rate of transpiration is calculated by dividing the distance moved by the water column by the time taken.

In conclusion, the potometer is the instrument used to measure the rate of transpiration in plants. It allows for the quantification of water loss through transpiration, helping in the study of plant physiology and water transport mechanisms.