It is amazing how plants seem to know so much about their surroundings. They don’t have the same senses as animals have, but they can gather information about the world somehow. How do they know which way is up, for example?
Imagine a very steep hill. It’s kind of rocky and the plant are having quite a hard time. Something like this:
Do you see how the trees are growing perfectly straight up? Not vertical in relation to the rocks, but as part of a straight line from the middle of the earth to the sky. Incredible, isn’t it?
To do this, all plants have an inherent “sense” of gravity. The sensory organs for this are located in the basic cells of a growing plant. At the top of a shoot and at the bottom of a root, a cluster of dividing cells can be found. These cells are responsible for the direction of the growth, and thus make the tree grow straight.
In the “growth cluster”, you can find cells that contain little starch granules. These granules are affected by gravity, like all loose things on earth, so they will always be at the lowest point of the cell. That is how these cells know which way is down.
Through a chain of signals, using the plant hormone auxin, the rest of the dividing cells then react to the direction of gravity. In the root cells, they will grow in the direction of the gravity, in the top shoot cells, they will grow in exactly the opposite way.
Some years ago, astronaut André Kuipers tested this theory in space. He took some seedlings of arugula with him on his space expedition and watched them grow. Indeed, the little plants started growing in all possible directions. Without the gravity to guide them, their direction of growth was completely uncontrolled.
You can also try it out yourself. Not in space, of course. But in your own back yard. If you have a small growing shoot, place it a little tilted, or even completely horizontal. In a little while, you will see your plant adjusting itself and growing in the right direction again. Just wait and see!
I think it’s a pretty ingenious construction, really! If you want to find out more, the whole process is called gravitropism (or geotropism). There is quite some nice information available online, so you should be able to find some interesting things using google. Or just give it a try with a real-life backyard-experiment.
Is it just me, or are plants really very amazing?