ELI5 How does plant seeds know to root downward and to sprout upwards?

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Plants have the ability of sensing gravity and orienting themselves accordingly. This is called gravitropism. In the roots, there are some small pouches in the cells which produce and store starch and basically pull down the roots through some specialized signaling mechanisms. Roots have positive gravitropism (they grow towards higher gravity) and stems have the opposite. Additionally, both roots and stems orient themselves according to the amount of light. This is called phototropism. Stems are attracted by the light (phototropism), while roots are repelled by it.


Have they tried to grow something in space yet? What happens to plants in zero gravity?


This is an area of very active research. There are several factors and mechanisms involved and different plants can rely on different mechanisms. One of the mechanisms is based on measuring the gravity. This is done by producing two hormones, a root growth hormone which is denser then the sap and a trunk growth hormone which is lighter then the sap. Over time the root growth hormones will sink to the bottom of the plant and the trunk growth hormone will rise to the top of the plant. Another mechanism is to have the roots grow where the humidity is highest and the trunk grow where the humidity is lower. Similarly it can be based on oxygen levels. When the plant grows above the soil then light levels can be used in order to grow in the direction of the light. We are still doing research on this with various different plants in various different growing conditions. For example we are trying to grow plants upside down or in space to see how they grow in order to find out which mechanisms governs this.


Imagine you are blind, you can’t see anything. Now, imagine you are holding a bag full of rocks in your hand. As you can’t see, you can’t tell what is up or down according to your vision, since you have none. However, you are holding the bag of rocks right? Although you can’t see, you can feel the rocks pulling down inside the bag because of their weight. This means you know what direction is “down” as well as “up”. This is exactly what plants do, only difference their bag is inside their cells and their rocks are starch molecules.


I think a lot about sunflowers. We know a ton about plants and why they do what they do, but sunflowers flowers only bloom facing east and no one knows why.