Question

Darwin’s photometer was
A) ‘Y’ shaped
B) ‘T’ shaped
C) ‘L’ shaped
D) ‘C’ shaped

Answer 1

The photometer is a device that measures the rate of transpiration by measuring the rate of water absorption. All photometers are based on the assumption that the rate of water absorption is almost equal to the rate of the transpiration.

Complete answer: Darwin's Y-shaped photometer is the simplest type of photometer which is used to measure the rate of transpiration. It consists of a glass tube with a side tube. A fresh leaf is cut slightly under the water and is inserted in the side tube through the single hole cork. The upper end of the straight tube is also corked and at the lower end, a single tube cork with a capillary tube is fitted inside. The scale remains fitted on the capillary tube whose lower part is dipped in a beaker containing the water. An air bubble is inserted into the capillary tube through which the water is absorbed; hence the rate of movement of bubbles can be noted over a fixed distance. If we compare the bubble movement under different conditions, the rate of transpiration can be calculated. In most cases, a photometer does not measure the transpiration rate accurately because not all the water that is taken by the plant is used for transpiration.
Therefore, the correct answer is option A.

Note: A photometer can be used to measure the rate of transpiration that's proportional to the water uptake. The rate of the transpiration can be calculated by measuring the distance travelled by an air bubble in a capillary tube over a given time.