Why does hop always grow clockwise?

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There are quite a few plants like this. It's coded into their DNA. Some vines spiral in the right-handed sense, others left, e.g., morning glory and honeysuckle go the opposite way. There can also be a screw-handedness in various other things like the way the leaves are arranged around the stalk or the organization of petals into flowers.

Similarly, humans have their liver on the right and heart on the left, our brain hemispheres are asymmetric, and so on. The asymmetry also exists on the molecular level in all forms of life. For example, the molecule that gives spearmint its characteristic smell has a mirror-image version that exists in caraway seeds and has a different smell. Ultimately all of these asymmetries probably originated by chance at an early stage in the evolution of life on earth. They're coded in DNA, which is itself asymmetric.

In terms of evolution, I think there are likely to be different reasons in different animals and plants why it's advantageous to have an asymmetry rather than symmetry. Then if asymmetry is advantageous, it's probably random which of the two occurs. I googled for info on vines, but mainly found only info on animals. So the following is just my speculation, but I would think that in the case of a climbing vine, it would be advantageous for it to have a definite screw-sense programmed into it, because it makes it more likely that it will actually form a spiral, thereby staying firmly attached to the thing it's climbing up. If it didn't have a handedness, then it could kind of randomly start going up one way, then turn the other, and the result might be that it would sort of zigzag back and forth without ever securely wrapping around.

posted over 7 years ago

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