Practical: Observing Vascular Tissue in Celery

Introduction:

We all know that plants need water – after all, it is an input for photosynthesis. But, in the scheme of things, most of the water that plants absorb from the soil is lost to the atmosphere via tiny pores in the leaves called stomata. This process is called transpiration, and it is vital for transportation of nutrients, cooling the plant, and prevention of cell damage and wilting. Transpiration works due to capillary action, cohesion of water molecules, and differences in pressure between different regions of the plant and the environment.

While transpiration is important, too much transpiration can lead to excessive water loss from the plant, and even plant death. Certain environmental factors such as high temperatures, wind, and low humidity can increase the rate of transpiration. Some plants have evolved mechanisms to help regulate transpiration and prevent water loss in these conditions. In this investigation, you will measure the rate of transpiration in celery that is in humid or standard environments. By doing so, you will discover if celery can regulate its rate of transpiration, and identify how quickly celery transpires water. You will also be able to visualise the process of transpiration and observe the structures through which water moves. In Part B of the investigation, you have the opportunity to improve on the original method outlined in Part A, and test if other factors influence the rate of transpiration in celery.

Aim:
To observe and record the movement of water through a celery stalk in a humid, closed environment compared to a stalk that is exposed to a less humid, open environment.

Hypothesis:
Hypothesis needs to be written in, 'If...then.. because' format. The because part needs to have clear links to content from last lesson.

Materials:

• 4 × celery stalks with leaves attached

• 2 × beakers (or glasses or cups)

• red or blue food colouring

• single-edge razor blades or scalpels

• cling wrap

• clear, sealable plastic container that is large enough to fit an upright celery stalk

• 1 × ruler

Method:

1. Fill two beakers with 150 mL of water and add 2-3 drops of food colouring to each.

2. Select four celery stalks of similar size and with a similar number of leaves (you can break the stalks off the celery bunch if necessary). Carefully use a scalpel to make a clean cut to remove the base of each stalk. After removing the base, the four stalks should be of a similar height.

3. Set two stalks into each beaker. Use the cling wrap to ensure that the stalks are upright and that the beaker is sealed (you could also pour a layer of oil over the top of the water to prevent evaporation). Be sure to support all stalks in the same fashion.

4. Add 50–100 mL of room temperature water to the bottom of the plastic container. Place one of the celery beakers into the container and seal the lid to create a humid, closed environment.

5. Place the two celery apparatuses next to each other. Make sure that both are exposed to the same amount of sunlight and temperatures.

6. Write a hypothesis for this investigation. How do you expect the independent variable to affect the dependent variable, and why?

7. Set up the apparatus again (but with only one, un-tampered celery stalk in each beaker) and leave overnight.

8. The next day, remove the last two celery stalks and cling wrap and record the volume of solution in each beaker. Carefully use a scalpel to slice the stalks open longitudinally. Once more, measure the distance that the dyed water has travelled from the base of the celery. Make observations about the colours of the leaves.

9. Take a very thin transect cut of each of the cut celery stalks. You should be able to see that the dye has travelled through certain parts of the stem, and avoided other parts. Prepare a scientific drawing of the transects and label all key structures.

Discussion:

Use the following questions to help you write a discussion response in paragraph format.

 Describe transpiration, including its purpose and mechanism.

 Compare and contrast phloem and xylem.

 Identify the independent and dependent variables in this investigation.

 Identify any controlled variables in this experiment.

 Identify any possible errors that may have affected your results. Be sure to state whether it was a personal, systematic, or random error.

 State what this investigation tested, and describe your main results.

 Using your understanding of transpiration, explain your results.

 Explain the role of the cling wrap in this experiment.

 Explain the importance of using similarly sized celery stalks in this experiment.

Conclusion:

Summarise the findings of this experiment. Be sure to:

•  state whether the hypothesis was supported or refuted, and justify your choice

•  identify limitations in the experiment

•  identify potential ways to improve the experiment.

• suggest further study that could be complete on the hypothesis