botany - How do trees manage to grow equally in all directions?

I was walking down a road with these beautifully huge trees when this question occurred to me.

Large trees with many thick branches have to grow equally in all directions, or they would tip over. Is there some sort of mechanism to ensure this uniform growth? Or is it just a happy coincidence arising from uniform availability of sunlight on all sides? If one branch of a tree becomes too heavy due to many sub-branches, does this somehow trigger growth on the opposite side of the tree?

I have seen that potted plants in houses tend to grow towards sunlight. My mum often turns pots around by 180 degrees to ensure that plants don't bend in any one direction. I assume that this is because sunlight increases the rate of photosynthesis, leading to rapid growth of the meristem. For trees growing in open spaces, this wouldn't be a problem. But there are many large trees that grow in the shadow of buildings without bending away from the buildings, even though this is the only direction from which they would receive any sunlight. Is there any explanation for this?

Answer

Growth in plants is tightly controlled by auxins – plant hormones. Auxin itself usually has an inhibitory effect on growth [EDIT: see comments and Richard’s answer for correction]. As far as I know there is no active control to restore plant symmetry once it has gone awry (but I could be wrong!) but the inhibitory effect of auxin synthesised at the meristem and diffusing in all directions causes a symmetrical pattern of inhibition and activation, forming shoots at symmetrical distances around the shoot apical meristem – this is very visible in the symmetry of the romanesco broccoli:

Furthermore, there are several mechanisms involving auxin which shape the general growth of the plant. The most important ones are:

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  Apical dominance which causes the apex (the stem) of the plant to grow more strongly than other parts of the plant, ensuring a general centring of the growth.

  
  
  


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  Phototropism causes the plant to grow towards sunlight. Unlike you hypothesised, this isn’t simply due to more photosynthesis and hence faster growth at the front of the plant facing the light, it’s actively controlled.

  
  


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  Gravitropism is a very interesting effect which causes the plant to grow generally upwards. It’s interesting because the mechanism is actually using gravity: the auxin synthesised at the meristem diffuses downwards in the plant due to gravity, inhibiting the growth in lower regions (but note that in the root apical meristem the effect is somehow reversed).

  
  


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  Hydrotropism causes the plant to grow towards water.

  
  

All these effects combined cause the plant to grow in a generally upwards, laterally distributed fashion.