How do planted seeds know which was is up?
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There are multiple techniques, each plant tend to use more then one of them and it varies between plants which is doing it. Most plants have hormones with different density. The lighter hormones will end up in the top of the plant because the plants do not have a heart to circulate and mix all tho bodily liquids. The heavier hormones end up at the bottom. So by looking at the ratio between these hormones the cells in the plant can tell if they are towards the top or towards the bottom and then change their behavior accordingly.
Another technique we see is to sense the water that is found deeper in the ground. The wetter the soil the deeper it is. This allows roots to grow deeper into the ground even on a slope. The plants also react to the light, the more light the higher it is. This allows it to grow towards the light even on a slope or when partially obscured by other plants. Water and light can also make existing limbs change behavior so a branch might turn into a root and a root can turn into a branch. So it does not matter which direction a seed will grow as the limbs will become correct either way.
This is an area of biology which is actively being researched. We are doing things like planting various plants upside down and see how they react. There is also a planter in space aboard the ISS where all the effects of gravity can be eliminated which allow us to introduce one element at a time to observe how different plants behave. The result of this can help us build more efficient greenhouses, genetically engineer plants to grow in a more favorable way, or to better understand things like soil compaction to better manage the soil on our fields to suit the plants optimal growing environment.
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