The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration.
A simple potometer is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance.
Aim of the experiment
To find the rate of water uptake of a plant.
Care must be taken when inserting the plant shoot into the rubber tubing attached to the capillary tube and with the scissors used to cut the plant shoot.
|Time in minutes||Distance moved by bubble in mm|
Plot a line graph of the data.
The rate of water uptake is best found using the gradient of the graph.
You need to calculate the gradient of the graph - and not take values from the table of results.
Choose any two points on straight line:
The most accurate results will be obtained if these points are as far apart as possible. Here, the two points on the line, labelled P and Q, are at either end of the line.
Calculate the gradient:
P Value of x = 0 minutes
P Value of y = 0 millimetres
Q Value of x = 30 minutes
Q Value of y = 96 millimetres
The distance moved by the bubble is 3.2 mm/minute.
Note that rate is always a compound measurement It is made up of two - or more - measurements. In this case, millimetres per minute.
When will the water uptake of a plant not equal the rate of transpiration?
When it is short of water, eg it has wilted.