In this hub we continue the series of examples and experiments dedicated to teach plant biology to schildren between the ages of 7 and 12 years old. Now we discuss some of the main factors that affect plant development, i.e. light and water.
Activity 3: What Do Plants Need for Growth?
Many children have already observed that many species plants growth at different times of the year under different environmental conditions of temperature and humidity. Some of your students may already know that some crops need a lot of water and grow best on very wet environments, like rice. On the opposite side, there are plants that prefer arid and hot environments like cacti and desert plants. Discuss with your students examples of species/crops that they know that grow at different times of the year or in different regions and relate them with the expected climate conditions for those periods or regions. You will probably get many examples. However, from the discussion, it will be possible for students to realize that although plants grow at different times under different conditions it is possible to point out two factors that most likely affect their growth, i.e. water and light. Both these factors change throughout the day, from place to place and plants have to adapt to those changes. With this activity, we expect students to understand two of the main factors that affect and are essential for plant growth, water and light, and how we can study their effects on plant growth.
Start by putting several scenarios to your students and ask them what will be their predictions. For example:
- what will happen if we have two sets of seeds exposed to light but we water one of them only?
- Or, what will happen to two sets of seeds that we water but one of them is kept in the dark?
It is very likely that your students, judging from what they have already learned from the previous activities made or from their background, will get the right answers. So, from a discussion on plant growth, two questions were formulated and hypothesis/predictions were made about the expected outcomes. Thus, all we need is confirmation!
From the previous discussion, students understood that water and light affect plant growth in some way. Introduce to students the concepts of variables. That is, plant growth is what we want to observe or measure and what we chose to represent it is called the dependent variable(s). Both light and water, known to affect plant growth (what we want to measure), are called independent variables. At this point, students must realize that the answer of the questions put before will only be valid if we control the independent variables of which its effects we want to observe.
Let us star by studying the effect of water on seed germination, as suggested, by measuring the time that it takes for a plant seed to germinate. Therefore this will be our dependent variable, the variable that we measure. Usually, and if time is not on your side in terms of class scheduling, use bean, broad bean or soybean, as they will not take more than 2 weeks to germinate.
- Use the same protocol of activity 2 used to observe seed germination and divide seeds in two sets A and B, both submitted to normal light conditions at your classroom/home. Thus, light will be an independent variable that we will keep constant and equally given to both sets of seeds.
- Start by having both sets of seeds A and B dry in the germinators; that is with no water. Set A will be watered at regular time intervals (e.g. a minute volume of water, no more than 2 ml, daily, just enough to be visible that the paper stays moist) and set B will not be watered during the time of this experiment. So, here we will measure the time that it takes for seeds to germinate, that is our dependent variable and we will control(decide) the amount or water that we give to seeds. Water is thus the independent variable that we control in order to observe its effects on seed germination (what we measure).
- Instruct students to note all changes that they observe on both sets of seeds and keep track of the time at which those observations are made. Present results on tables in order to have a timeline of events. If appropriate determine the average germination time of seeds.
- If suitable, you may add other sets of seeds of different species or varieties and show students that different species take different periods of time to germinate reflecting their physiological specificities. This way, they can compare the germination time of the different species/varieties and determine which ones are faster.
- Also, if you have time, you can let seedlings develop and let students monitor and observe plant growth by measuring leaf or total plant size by measuring their length. This can be quite instructive as students can observe that plants can grow at different rates and this can be advantageous for economic or agricultural purposes. They can also do this at home by taking plants with them and then discuss results at school.
- Discuss with students the results obtained and the final conclusions to be made. Talk about the importance of defining and distinguishing the variables needed for best performing the experiments planned to test the hypothesis made.
In this experiment we want to know what will happen to two sets of plant seeds regularly watered but one of them is kept in the dark? So, this time light is independent variable that we control and give differently to plants, and its effect we want to observe on plant growth, while water is the other independent variable that we keep constant and equally distributed to all plants studied. With this experiment students should conclude that although light is not essential for seed germination, unlike water, it will affect plant growth. The total length of the plant or of one of its leaves will be the dependent variable most easily measured that can be used to study plant growth.
- As before, use the same protocol used before for seed germination and start by dividing seeds in two sets A and B both equally treated in terms of water. Set A is put under normal light conditions in the classroom/home and set B is put inside a box thus with no access to light. To help children to achieve best the goals of this activity use the same plant species that you studied in the previous experiment.
- Instruct students to water both sets of plants regularly but being careful enough not to expose too much to light the plants of set B when they have to open the box in order to water them. Remind students that we want to keep those plants in the dark as much as possible during the time of experiment so that we can study the effect of light on plant growth properly.
- Instruct students to monitor and register all the changes that they observe in both sets of plants, namely leaf or plant size expressed as total length. Also, determine the average (if possible) germination time for the two sets of plants. As before, put results in a table in order to best compare the two sets of plants and construct a timeline for the changes that children observed on plant growth.
- As with the previous experiment, if you still have time, use different plant species or varieties and have children determine the different responses, in terms of growth, that the different plant species/varieties show.
- In the same way and depending on the time that you still have left on your school year plan, set a second box, named C for example, with a hole in it through which a small amount of light can pass. In this case, we will observe that the plants kept inside will grow towards the hole, the light source, in what is called phototropism, or phototropic response. Plants search for light.
A cool experiment showing phototropism
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