In this article, we will discuss a common misconception when considering the effect the mass of fruits has on their dispersal via wind or water.
In this final chapter on Plant Cycle, we will be discussing the following question:
Fruits dispersed by water or wind need to be light in order to float on water or be carried by the wind respectively. Do you agree?
When I posed this question to my students, most of them agreed with me.
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Here’re Their Explanations
#1: Fruits/seeds dispersed by water need to be light in order to float on water as heavier objects would sink.
For example, when we compare a block of metal and a block of styrofoam of the same size, the block of metal will sink in water as it is heavier than the block of styrofoam.
#2: Fruits/seeds dispersed by wind need to be light so that the wind can carry the seeds easily. This helps the seed to be dispersed further away from the parent plant.
Take a moment to ponder over their explanations. Do you agree?
Let’s Analyse Them Together!
Fruits dispersed by water need to be light in order to float.
This statement is incorrect.
The buoyancy of a fruit (i.e. whether it floats or not) is dependent on its density* (not in the PSLE Science syllabus).
To give your child an idea of what ‘Density’ is: Density is a measure of the mass of a substance per unit volume.
Look at how large and heavy ships can float on water. They do not float because they are light!
Your child should also take note that large objects may not necessarily sink in water. If you compare a large styrofoam block and a small metal block, the metal block would sink while the styrofoam block would float. The size of the object alone cannot determine if an object sinks or floats.
Hence, fruits do not need to be light in order to float on water.
Seeds need to be light so that wind can carry the seeds easily.
This statement is correct.
The amount of gravitational force acting on the seed (weight) depends on its mass.
The smaller the mass of an object, the lesser the gravitational force acting on the seed. This would allow the upward force of the wind acting on the seed to overcome the gravitational force acting on the seed more easily, allowing the seed to float more easily. This would enable the seed to float in the air for a longer period of time and be dispersed further away from the parent plant.
Hence, fruits need to be light in order to float in the air.
Of course, the questions that I have discussed above cannot fully cover the questions that have been tested in the PSLE Science syllabus. However, I hope these articles have provided you a much better understanding of some of the key concepts in ‘Plant Cycle‘ and how your child can apply them to the questions in his/her examinations! 🙂
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