How Do Plants Manage to Turn to Face the Sun?

There are several differences between plants and animals. This is common knowledge and probably isn’t a surprise to anyone. While there are creatures that seem to almost be part plant and part animal, for the majority of the rest of the two kingdoms, there are some traits that define each.

For instance, most plants manufacture their own food, in a complex operation where the power of sunlight is combined with the carbon dioxide the plants breath and the water they drink in order to create various sugars. Animals also require sugars in order to survive, along with protein and other substances. However, animals can only get those sugars from plants or other animals that have consumed the plants, for only plants produce the sugars.

This is mentioned just to show how different plants and animals usually are and how interdependent they are. Plants rely on the carbon dioxide produced by animals in order to survive. If the level of atmospheric CO2 was to drop only a small amount, many plants would be devastated. Thus, the plants rely on animals and as has already been mentioned, animals rely on plants.

One of the most curious differences between plants and animals is that plants don’t have nerves like animals do. Even the most advanced plants lack nerves. This might not be a big thing but think about it for a moment.

If you were to turn toward the sun, you would be able to do so simply because of your nerves. We can actually feel the warmth of the sun so we can turn toward it even if we are blindfolded. Indeed, the act of turning toward it is triggered by nerves that cause certain muscles to contract while others relax. It is how animals move.

None of this should be surprising to anyone, but consider this; many plants, sunflowers, for example, turn toward the sun so that the flowers or leaves are pointed in the right direction to produce the maximum amount of sugar they can and in the case of the flower, for the seeds to develop and mature. Again, remember that plants don’t have nerves. They also don’t have muscles. So how are they able to turn toward the sun?

Believe it or not, this has been figured out. It has to do with both growth and photosynthesis (the production of sugar). The cells in the stem of a plant are all fairly uniform in maximum size since they are surrounded by a fairly rigid cell wall. However, each is capable of swelling up, becoming temporarily larger than normal. They can also shrink a little, becoming smaller than average for a short time.

This is necessary because photosynthesis requires more than sunlight, it also requires water. When a given cell is exposed to the maximum sunlight, it needs the maximum amount of water it can hold in order to produce as much sugar as it can. When the cell receives the least amount of sunlight, it requires the least amount of water in the cell, though the cells can still be swollen with excess water from when photosynthesis was peaking. At the same time, during peak sugar production, the cells actually shrink as dissolved sugars are passed on to other parts of the plant.

On the darkest side of the plant, the cells aren’t producing and transporting as much sugar as the cells on the sunniest part of the plant. On the sunny side, the cells initially swell, then shrink a little because sugar is being passed on to other parts of the plant. This means that because the cells are more swollen on the side facing away from the sun, the plant top is sort of pushed toward the sun.

What we observe as a consequence is that the sunflower apparently turns toward the sun as our world turns and the sun tracks through the sky.

A similar but different process causes the sunflower bloom to turn back toward the east during the night so that by morning, it is again ready to track the sun through the sky. It isn’t necessary here to go into how this happens. The point is that all of this is done without the benefit of nerve cells.

This naturally doesn’t explain why some plants, notably the so-called ‘sensitive-plant’ or Mimosa pudica seem to be able to pull back when something brushes against it, but that also happens without nerve cells. However, that would require a different article to explain.

Anyway, now you know how some plants are able to always seem to face the sun.

What do you think?

15 points

Written by Rex Trulove

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    • It is also humbling. Humans have a tendency of trying to define everything in terms of themselves. We move because of interactions between nerves and muscles. Plants have neither, so it shows that the ‘human way’ isn’t the only way. We tend to start getting into trouble when we start trying to think that what is true for us is true for everything. Knowing that can, but doesn’t always, lead to humility.

    • In a way, yes. However, it also isn’t necessarily a ‘byproduct’. The development of the mechanism was no doubt occurring at the same time plants developed a way to get water to gravity-defying heights and while the intricacies of photosynthesis were being worked out. It would be hard to say, definitively, that the development of one of these was a consequence of one of the others. They are all results of each other, which also explains why not all plants turn toward the sun.

      The whole system is very complex and interconnected. I’ve just over-simplified one portion of the system. I do find it all fascinating, though.

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