We all love to teach germination. Get some seeds, plant them in some soil, and wait patiently for them to germinate. A simple, hands-on activity that really illustrates a very important biological process. However, teachers often slide right by the mechanism that jump starts seed germination. Osmotic Turgescence (OT) happens when the seed absorbs moisture from the surrounding soil. Once absorbed this water starts a chain reaction that causes the seed to swell in size and make the room needed for the seed to sprout.
What is so important about Osmotic Turgescence?
As mentioned above, this biological process is the trigger to start all plant growth. Further, there are tremendous forces involved when these seeds soak up water. This swelling pushes soil away from the seed so that the root and stem can emerge and begin to develop as a plant and establish itself in the soil. If soil conditions are not favorable or the necessary steps in planting are not followed, then seed germination rates will decrease and the crop or planting will suffer.
Why is Osmotic Turgescence so important?
OT relies on many planting techniques to be performed correctly so that germination can begin and be sustained until the plant can take over by beginning normal biological functions like photosynthesis and respiration. All seeds, and subsequently their roots, need sufficient poor space. Seeds need those pore spaces to collect moisture and retain it until the seed can absorb it and start growing. They need pore space for expansion as the seeds swell and enlarge. Seeds need enough moisture to initiate OT, but not so much that they begin to rot before the stem and roots emerge. So soil moisture is critical to seed development. OT won't occur without the correct soil temperature. This temperature varies based on plant species, so planting seeds at the proper temperature also helps initiate OT and then helps sustain plant growth from that point forward.
How can you demonstrate the importance of Osmotic Turgescence to students?
There are several different methods to illustrate OT, so read over these and see which one fits within the time frame you want to use to cover this topic. I prefer lima bean seeds for most basic activities. They are big, easy to handle, germinate quite easily, cheap, and you can pick them up at the grocery store and not wait for the gardener supply house to deliver them to your school. Plus grocery stores are open later than farm and garden stores.
Version #1 - Don't hand out any planting material, water, or pots until after these steps. Divide the seeds among your students. You can even take a few minutes and select a standard sized seed from the bag of dry seeds so that you have a good deal of uniformity. Measure several of the lengths and widths of seeds so that you have an approximate width and thickness of the seeds used. As a group, weight all the seeds. Record all this data. At this point you can get creative. You can wet a paper towel, line up the seeds in a row, and then roll it them up like a fruit roll up. Or bring in old cd cases, add a seed and wet paper towel. Or use clear Solo cups, place seeds around the side and a wet paper towel in the middle. Place them in a warm, but not hot area of the room. Check on them daily. Weight and measure as you see them starting to germinate.
Version #2 - Similar to #1, but each group gets a mason jar and a measured amount of potting soil that will fill the jar with damped and packed firmly. Dampen the potting soil add in around 20 or 30 beans. Mix well and pack firmly but not to excess into the jar. Find a warm spot and observe. The pressure exerted by the OT of 20-30 seeds should crack the mason jar as they all expand. Great visual, costly lab. A bit dangerous and messy too. You might just do that one at the front.
Version #3 - Instead of soil and mason jars use plaster and muffin cups. Mix a few seeds in each muffin cup with the wet plaster. Silicone cups are preferred because they will hold shape. Remove the plaster cupcakes when firm. They too should crack under the pressure of OT.
As you can see it doesn't always have to be a deeply involved and highly managed lab. Sometimes you just need to a different way to illustrate an important concept. The great part is that you already have some seeds started for the rest of your discussion on germination and parts of the seed. Hit us up on Pinterest and Twitter with a few pictures of how it turns out.
Good luck and have a great week at school!