How plants drink
evaporation and transpiration
Background
Evaporation and transpiration is what keeps your plant looking so good and standing upright. You come home from school on one of those hot summer days and notice that your favorite pot plant that sits on your window-sill is looking rather down and droopy (and it ain't because the rose bush outside dumped it for the daisy plant in the next flower bed)!
So you do what any concerned plant lover does and run and get a jug of clear water and lovingly pour it into the soil around the plant ('cos you know it needs water and the lack of water has got something to do with evaporation and transpiration!?).
Then with concerned glances during the rest of the afternoon you notice how your plant slowly picks itself up and by evening looks it's good old self again (and right in the view of the gorgeous rose bush just outside the window)!
Question: Why did you pour the water into the soil around your plant and not onto the plant?
Another question: How did the plant get the water right up into it's leaves?
These experiments will shed some light on both of these questions.
Let's do science...
The Experiment
You know from experience that water needs to get into the plant from the soil – right? So it must enter the plant through it's roots – that makes sense, not so? Put why would water want to go into a root? There have been enough hints that it has to do with evaporation and transpiration.
Firstly, check out the
definitions for evaporation and transpiration
in Good-Science-Fair-Project.com's online science dictionary.
Now onto the experiments.
The Aim
To show that water is released from plants during the process of transpiration and this is caused by evaporation.
To show that water moves up from the bottom of the plant into the top parts of the plant.
Equipment Needed
Experiment A
Two branches, or pieces of a branch with at least 20 large leaves on them (a stick with one leaf on it won't work too well)! The two branches need to be approximately the same size and preferably from the same plant.
Two clear, see-through plastic bags, large enough to fit and seal the entire branch inside.
Elastic bands.
A dark, cool cupboard.
A sunny day (difficult to find at the department store but freely available during summer and are known to pop up every now again during winter).
Experiment B
A drinking glass or vase.
Water.
Food coloring (I have found that blue works best).
A freshly cut white flower with a stem of at least 15cm.
A sunny spot – like a window-sill, not Hawaii (although if you are given a free holiday there to do your experiment, it will be fine)!
Method
Experiment A
Cut off the two branches from the plant (make sure they are dry and have not just been watered or that it has not just rained).
Immediately put one branch into one of the plastic bags and the other into the second bag. Seal the bags tightly using the elastic bands.
Place the one “branch in a bag” in a cool, dark cupboard.
Place the other in a sunny (preferably hot) spot – a window-sill is usually pretty good, or peg the packet onto the washing line.
Leave them both in their spots for an hour.
Experiment B
Fill the glass or vase with water and place a reasonable amount of food coloring into it.
Place the flower in it and stand it on the window-sill (or beach if you are on holiday).
Watch what happens to the color of the flower over the next couple of days.
Results
This is what you should see happening...
Experiment A
The “branch in a bag” that was left in the sun, will have a fair amount of moisture inside the bag and the leaves are wilted and dry, whereas the “branch in a bag” that was left in the cupboard will have hardly any or possibly no moisture in the bag and the leaves are still fresh.
The photo below shows the two bags with the branches in after the experiment was completed ...
Experiment B
The flower will start to turn the color of the food coloring.
The photo below shows my experiment. I used a white flower and blue food coloring ...
The Conclusion
In Experiment A the temperature and light from the sun was the only difference between the two “branches in bags”. The branches did not seem like they had any moisture on or in them, but the one that was left in the sun showed that it must have had moisture in it.
The plant heating up because of the sun on it resulted in evaporation and transpiration. In other words water was evaporated out of the plant via the leaves - when this happens it is called transpiration.
In Experiment B it was shown that the water must have been sucked up through the stem of the plant (due to evaporation and transpiration), as this was the only part in contact with the water. The color change in the flower showed that water must have been taken up and ended up in the flower.
When considering the conclusions of the two experiments together, it is seen that evaporation and transpiration resulted in water leaving through the top of the plant (specifically the leaves, however some also will leave through flowers and stems) and is drawn up from the part in contact with the water (generally the roots, but in the case of experiment 2, the stem).
So evaporation results in transpiration which results in more water being taken up by a plant.
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