Question

The inference in Ganong’s potometer is that the
A. Rise in mercury in the tube indicates the suction created by transpiration
B. Water test by transpiration has created a suction force which pulls the water from the beaker and the bubble in the mercury
C. Both A and B
D. None of the above

Answer 1

In Ganong’s potometer, when transpiration occurs the suction force pulls the water and the bubble from the beaker.

Complete Answer:
- A potometer is a device which is used during transpiration to measure the water uptake by stem. It is also referred to as a transpirometer. The Ganongpotometer is used in the laboratory to determine the transpiration rate.
- The potometer of the Ganong is made up of a glass tube twice curved and a glass cylindrical with a wide mouth. In the horizontal glass bar a capillary tube is inserted attached to a bottle. Towards the front of the apparatus is a broad-mounded glass cylinder.
- On the glass cylinder mouth, a rubber cork is placed with a hole. In this hole a freshly cut twig is placed. There are marked readings in the horizontal bar. The bend end has an opening of the nozzle. Under this horizontal bar is a beaker containing coloured water inserted in the bent end. To store water, a reservoir is attached to a horizontal bar. The whole layout is placed on a flat surface.
- When transpiration occurs, the shoot produces a transpirational pull to compensate for the loss. As a result, the horizontal bubble continues to shift towards the twig. Taking the distance that the air bubble covers over a period, the transpiration rate is calculated. A frequent change of the air bubble to zero noted many readings. The average transpiration rate is determined by these readings.

Thus, the correct answer is option B. i.e., Water test by transpiration has created a suction force which pulls the water from the beaker and the bubble in the mercury.

Note: The plant produces a transpiration pull in Ganong's Potometer which causes a suction force. The driving force here is this suction force. The water and the bubble are pulled from the beaker. Only one bubble is carefully formed and the bubble's distance is scaled. The calculation of water intake by the plant is thus obtained.