What Happens When You Put Vaseline on the Underside of a Leaf? A Hydrangea Transpiration Experiment
I’m Ken Kuwako, Science Trainer. Every day is an experiment.
If plants could sweat, where do you think it would come from? As it turns out, plants release a surprising amount of water into the air every single day, completely out of our sight. This phenomenon is called “transpiration.” In this post, I’ll show you an experiment that lets you see it in action, using hydrangea leaves.
Catching Transpiration with Hydrangea Leaves
Here’s the experiment setup using hydrangea leaves to observe transpiration. A fellow teacher had it all ready to go, so I snapped a few photos.
Comparing Four Branches: A “Controlled Experiment”
We prepared four types of branches and tracked how much water each one lost: one left untouched, one with its leaves removed, one with Vaseline on the top of the leaves, and one with Vaseline on the underside. To keep water from simply evaporating off the surface, we poured a thin layer of oil on top of the water.
Changing just one thing while keeping everything else the same is called a “controlled experiment.” Here, the only differences are whether the branch has leaves and which side of the leaves got the Vaseline. That means any difference in water loss can be traced back to its source.
Leaves are covered in tiny pores called “stomata.” Most of the water that leaves a plant during transpiration escapes through these little openings. Vaseline is a sticky, oily substance, so it works like a lid that plugs the stomata.
In many plants, stomata are more common on the underside of the leaf. So we can predict that the water loss will differ depending on whether the Vaseline goes on the top or the bottom.
Why Put Oil on the Water?
If you leave the water uncovered, some of it will evaporate straight from the surface. Then you couldn’t tell whether the missing water was drawn up by the plant or simply evaporated into the air. By covering the surface with oil, we make sure the only way water can disappear is through the plant: it sucks the water up and releases it. It’s a small trick, but it’s a big part of what makes the experiment trustworthy.
Here’s how to read your results. The branch with its leaves removed gives you a baseline, showing how much water is lost through things other than the leaves (like the stem) and through the experimental setup itself. The difference between that and the untouched branch gives you a rough idea of how much water the leaves transpire.
And if the branch with Vaseline on the underside loses less water than the one with Vaseline on top, that’s evidence that stomata are more common on the underside. What result do you expect? Make a prediction first, then check it against what actually happens!
Stomata: Doors a Plant Wants Open, but Not Too Open
Stomata are exits for water, but they’re also entrances for the carbon dioxide that plants need for photosynthesis. The catch is that every time a plant opens its stomata to take in carbon dioxide, water slips out too.
Plants open and close their stomata depending on how bright it is and how dry the air is, constantly balancing “I want that carbon dioxide, but I don’t want to lose my water.” Plants can’t move around, yet they’re quietly making all kinds of fine adjustments just to stay alive.
Transpiration isn’t just wasting water, either. It’s thought to do some important jobs. As water leaves through the leaves, it creates a pulling force that draws water up from the roots. In other words, transpiration acts like a pump that lifts water up through the plant. On top of that, when water evaporates, it takes heat with it (this is called “latent heat of vaporization”), which keeps the leaves from getting too hot.
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