Transpiration

Introduction

Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. Plants absorb water through their roots. This water can come from deep in the soil. In the desert plants roots can penetrate as deep as 20 meters underground.

This is a simple experiment to prove that water is evapourating from the plant.

Equipment required

• DrDAQ educational data logger
• Humidity sensor
• Temperature sensor
• Plant
• Polythene bag

Experiment set up

The apparatus is set up as shown in the diagram. The humidity and temperature sensors are connected to the DrDAQ data logger, which is turn is plugged into the PC.

One branch of the plant, the humidity sensor and the temperature sensor are placed into the polythene bag and the end tied up with string.

PicoLog is set to record at one sample per second, with a maximum of 1200 samples (i.e. 20 minutes worth of data). The graph is set to show humidity. The spreadsheet is set to show both humidity and temperature.

Carrying out the experiment

Once the experiment is setup, leave the humidity sensor to log data for approximately 20 minutes. The temperature sensor is there to prove the temperature is kept constant.

Questions and discussion of results

1. What is the name of the small pores found on the underside of leaves?
2. What percentage of evapourating water is attributed to transpiration?
3. Plants pump up water from the soil. What purpose does this water serve?
4. Name some environmental stimuli that would effect the rate of transpiration. Explain whether they would increase or decrease the rate of transpiration.
5. What is the name of the cells that control the opening and closing of the small pores (mentioned in question 1) in response to various environmental stimuli?

Further study

Further study could involve comparing the transpiration of a control leaf to one smeared in Petroleum jelly.