On one of my recent trips to the grocery store I brought home African Violets for the kids. They were so pleased because they are SO pretty. Then along comes mama and she cuts off a leaf from each! The kids looked on in horror. What just happened? Science happened. It’s time for science kids!
You can learn about transpiration (we call it plant sweat) and watch what happens when you apply some “antiperspirant” to the leaf (we call this Vaseline) to plug up their stomata (we call these sweat pores).
- Plant with broad leaves
- Plastic cups (4 total)
- Plastic cup lids (2 total)
- Sunny window or grow light
- Preparation: 10 minutes
- Experiment: Days to Weeks
- Observations and Results: Continuous during the experiment
Day 1 – Set up
- Choose a plant that has broad leaves. Cut off 2 leaves. Try to keep as much of the stem as possible.
- To ONE of the leaves, spread Vaseline on the top and bottom of the leaf.
- Push the stem of the untreated (no Vaseline) leaf into a cup lid, the opposite way to how you put a straw into it. Try to have the hole flaps from the lid pointing up towards the leaf.
- Attach a cup to the lid to enclose the leaf.
- On the part of the lid with the stem poking through, tape up any gaps.
- Fill the other cup with water then sit the cup containing the leaf on top of the cup with water.
- Tape the cups together in a few spots for stability.
- Repeat steps 3 to 8 for the Vaseline treated leaf.
- Place both leaves in a nice sunny spot or under a grow light.
Day 2 and beyond
- Observations, observations. Keep watching your leaves!
- In a few hours you should start to see condensation forming on the inside of the cup for the untreated leaf.
- You should NOT see any condensation for the Vaseline-treated leaf.
- After a few days you may see water beginning to pool on the lid for ONLY the non-Vaseline treated leaf.
- Put the two leaf experiments side-by-side and get the kids to look for all the differences and similarities. Ask them what they think is happening.
Tips and Tricks
The next time you are at a fast food restaurant, save a couple of the plastic pop lids and bring them home. If you are not sure you have plastic cups at home that will fit these lids, bring the cups home too. Just give everything a good wash. I’m not sure plants like to drink soda pop or milkshakes.
We chose African Violets. Not for any particular reason except that our local grocery stores sells them and I know these leaf cuttings when immersed in water will survive.
It is so important to do both the untreated leaf and Vaseline treated leaf experiments. This is a good exercise for learning how to compare things and about controls and variables. When you make everything the same (the controls) except for only ONE thing (the variable), it helps you pinpoint what might be happening. Helping you make informed conclusions. A lesson in good experimental design. Our only variable is the addition (or absence) of Vaseline to the leaf. We kept everything else the same. See if the kids can figure out where the condensation forming on the inside of the cup is coming from. Can they guess that the Vaseline clogged up the little pores?
This experiment takes a relatively short time to set up, in fact we were able to do it while dinner was cooking. We found that by bedtime the condensation was beginning to form on the cup. After about a week, the water had started to pool on the cup lid. We have now been observing our leaves for almost 4 weeks and the ‘normal’ one is still transpiring.
Science Secrets Revealed
Plants suck water up from the dirt into their roots and all the way up to the leaves. For some plants, like trees, that is a LONG way! The warm sun causes some of the water in the leaves to evaporate. The water leaves the leaves (haha) as tiny water droplets. These water droplets exit the leaves through teeny tiny pores, called stomata. Most of these tiny ‘holes’ are on the underside of the leaf.
This process is called transpiration and is very important for the plant. First, just like us the plant ‘sweats’ to help cool itself on hot days but it also helps to pump the water (and minerals) up from the bottom to the top of plant. Imagine pumping water from the roots to the leaves against GRAVITY. How does it do this?
Plants contain a transportation or circulatory system, made up of xylem and phloem, just like we have veins and arteries. The xylem is a series of hollow tubes, like straws, sucking water up to the very top. Think of a long line at the grocery store, each time a customer leaves (our transpired water) the next customer in line can move forward (just like our water in the xylem tubes advances forward each time water evaporates from the leaf). Or you can think of sucking up a drink through a straw, each time the drink leaves the straw (and enters your mouth), you can suck up more drink! Scientists like to call this cohesion theory, I call it the “next-in-line” theory. And it allows the plants to successfully work against gravity.
Normally, stomata (the leaf pores) can open and close. These pores are surrounded by guard cells, and they get to decide when the doors to these pores can be opened or closed. But the Vaseline plugged up all the little stomata, making them permanently closed. The guard cells can’t open the pore doors and the water can’t exit (evaporate). The Vaseline became a serious road block to allowing normal transpiration!
Oh and next time you are looking at a tree that has been cut down. Look for the tree rings. Count the rings to find the age of the tree. You are actually counting the xylem! Each year, the tree makes new xylem! How cool is that!