Simple Things* #10

The sun throws light from the sky while the ground gives food to all green things. But how do they know to put their long and thin feet into the ground and their green hands in the air? Green living things have a sense to know what is up and what is down. In one kind of special tiny water bag they make tiny stones – these stones are made from the stuff we use for food made in a hot box that we eat in the morning.

The stones go down within the tiny water bags – and the water bags feel where the stones are. This way the green living thing can know inside of its hands and its feet where it is up and down. Even if you turn the whole green thing around, the stones will go down and the green thing knows again where is up and where is down.

Can you guess what we are describing?

Solution

The answer is: Gravitropism!

Plants need to know their ups and downs – up in the sky is the sun giving them energy and down in the ground are the nutrients they take up with their roots. If they would just randomly grow, chances are their leaves are oriented to the side and the roots just crawl across the top soil and miss out on the water that’s often found deep below.

Luckily for plants, they evolved a sense for gravity: gravitropism. The sense is quite elegant and simple. Specialised cells, the statocytes in leaves, root tips and the stem, contain statoliths. These specialised amyloplasts contain very dense packets of starch and they sink downward in the cytosol like rocks in a jar of water.

Cells are capable of sensing the position of the statoliths and can react accordingly. Roots try to follow the statoliths downward (they show positive gravitropism) and shoots try to move in the opposite direction (they show negative gravitropism). Overall, this mechanism allows plants to orient themselves in the dark, like the dark underground.

*In which we use Randall Munroe’s ‘simple writer‘ to explain plant-and-pipette topics. Can you guess what they are?

Monroe’s ‘simple writer’ limits language use to only the 10 hundred most common words in the English language. So the word ‘chloroplast’ is out. But so is ‘duck’, ‘cuddle’, and ‘explosion’.

We’ve tried to define a plant and pipette related word using only these common words. Can you tell what we’re talking about? The solution is shown at the bottom.

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