There are a number of methods for measuring transpiration. Some of these are:

1. Weighing method: A small light weight potted plant can be weighed before and after the end of a certain period of time. The soil surface and the pot should be fully covered to prevent evaporation from the surfaces other than the plant. The loss in weight by the plant during that time is due to the loss of water by transpiration.

An improvement in the weighing method can be made by using a glass bottle with a graduated side tube, filled with water and a tube fitted into it as shown in Figure below. This would indicate the volume of water loss that can be compared with the loss in weight with the help of a weighing machine (B) or by converting cc into grams (1 cc water weighs lg)

Another weighing experiment can be made by using a test-tube filled with water and inserting a leafy shoot (no roots) in it and pouring some oil on the surface to prevent loss of water by evaporation. Rest the test tube in a small beaker and weigh them together. Remove the intact test-tube and keep it straight in the test-tube stand for a few hours. Weigh it again by keeping it in the same beaker. Any difference in weight will indicate loss of water by the shoot (due to transpiration).

2. Potometer method: Potometer (potos : drink, meter : measure) means a device that measures the water taken in by a plant.

A. Ganong's Potometer: A twig of some suitable plant (e.g. coleus) cut with a sharp knife is fixed in an apparatus as shown in Figure below. The entire apparatus is filled with water so that no air spaces are present. An air bubble is introduced into the horizontal graduated capillary tube which is dipping into the beaker containing water. [This is done by lifting the bent capillary tube above the coloured water so that air will be sucked in due to suction pull and is again dipped into the water.] As the transpiration proceeds, i.e., as the water is lost from the twig, a suction force is set up which pulls the water from the beaker and the bubble in the capillary tube moves along. The readings on the capillary tube would give the volume of water lost in a given time. The air bubble can be brought back to its original position by releasing some more water into the capillary tube by opening the stop-cock.

B. Darwin's potometer: Above figure is the simplest type of potometer used to measure the rate of transpiration. It consists of a glass tube with a side tube. A twig (fresh) is cut obliquely under water, and is inserted in the side tube through a 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. A scale remains fitted on the capillary tube, whose lower part is dipped in a beaker containing water. An air bubble is introduced in the capillary tube through which water is absorbed, the rate of movement of bubble over a fixed distance is noted. The comparison of bubble movement under different conditions gives the rate of transpiration.

[Potometer does not measure water lost during transpiration but measures the water uptake by the cut shoot (L. potos : drink). Some of the water is used by the cells to carry out other processes, for example, manufacture of food (Photosynthesis). The potometer should be made completely water-tight and the twig should be cut obliquely (to allow larger surface for the water intake) and under water to avoid suction of an air bubble into the twig which will stop the absorption of water into the xylem.]

Limitations in the use of potometer:

• Introducing the air bubble is not very easy.
• The twig may not remain fully alive for a long time. Any changes in the outside air temperature may affect the position of the air bubble in the capillary tube.