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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?

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    $\begingroup$ Lots of questions are nested here - tree growth and its regulation, reaction wood, heliotropism - might be good to focus it $\endgroup$
    – gremau
    Commented Apr 18, 2012 at 20:40
  • $\begingroup$ I was asking the same question when I had observed my tropical plant to balance on the very margin of the shelter as it was becoming more and more tilted against the wall. No magic, one day it just fell over. $\endgroup$
    – Probably
    Commented May 20, 2016 at 14:28

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There are some other good answers which provide part of the picture, but I think there is a fundamental organising principle which has been missed. Konrad has touched on it in his answer.

The reason trees, and most plants, tend to grow equally in all directions is that they have iteratively generated branching and radial symmetry which is controlled in a feedback loop of the growth promoting hormone auxin and auxin-sensitive auxin transporters. This is an elegant biological algorithm which explains all branching growth.

The things Konrad identifies (phototropism, gravitropism, etc.) serve as orientation cues which help the plant determine which axes to grow along, but fundamentally the process is about auxin gradients. There are exceptions, as others have pointed out in their answers, and they usually result from severe imbalances in the orientation cues.

I'll try to explain the growth process clearly (and it gives me an opportunity to try my hand at diagramming again ^_^)...

Auxin is a plant hormone (actually a class of hormones, but mostly when people say auxin, they mean indole-3-acetic acid) which promotes cell elongation and division. The basic principle which allows auxin to act in the organising way it does is that auxin is produced inside cells, and proteins which export auxin from a cell develop on the side of the cell which has the highest auxin concentration (see figure below).

cells export auxin more on the side which has the highest auxin concentration

So auxin gets transported up the concentration gradient of auxin! Thus if you get an area of high auxin concentration developing somehow, more auxin is then transported towards that area. An area of high auxin concentration relative to the surrounding tissue is called an auxin maximum (plural 'maxima').

For most of the life of the plant, auxin is produced pretty much equally in most cells. However, at the very early stages of embryo development, it gets produced preferentially along the embryonic axis (see figure below, part 1). That creates a meristem - a group of cells where cell division is taking place - at the auxin maximum at each end of the embryo. Since this particular meristem is at the apex of the plant, it is called the apical meristem, and it is usually the strongest one in the plant.

auxin patterning of plant growth

So by having a meristem at each end, the embryo then elongates as cell division is only taking place at those points. This leads to part 2 of the image above, where the two meristems get so far apart that the auxin gradient is so weak as to no longer have its organising effect (area in the red square). When that happens, the auxin produced in cells in that area concentrates in a chaotic way for a short time until another center of transport is created. This happens, as the first one did, when a particular area of the tissue has a slightly higher concentration of auxin, and so auxin in the surrounding tissue is transported towards it. This leads to part 3 of the figure, in which two new meristems are created on the sides of the plant (called lateral meristems).

Lateral meristems are where branches occur on plants. If you then imagine this process continuing to iterate over and over, you will see that the branches, as they elongate, will develop meristems at the tips and along the sides. The main stem will also continue elongating, and develop more lateral stems. The root will begin to branch, and those branches will branch, etc. If you can understand how this elegant system works, you understand how plants grow, and why they grow in repeating units as opposed to in a body plan like animals.

It also explains why, if you cut off the tip of a stem, it promotes branching. By removing the apical meristem, you get rid of the auxin gradient and enable the creating of multiple smaller meristems which each develop into branches.

So far I've explained regular branching, but the same system causes the radial symmetry which makes trees (usually) grow in all directions equally...

enter image description here

Imagine taking a cross section through a stem and looking down all the way through it (as depicted crudely above). Just as auxin gradients act to coordinate growth along the length of the plant, they also coordinate it radially, as the maxima will tend to space themselves out as far from one another as possible. That leads to branches growing in all directions equally (on average).

I welcome comments on this answer, as I think its so important to understanding plant growth that I'd like to hone my answer to make it as good as possible.

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  • $\begingroup$ While I am intrigued by this explanation, it has things backwards. As stated, the model implies that apical meristems are consumers of auxin, when it is well known that apical meristems are the primary producers of auxin (ref. Thimann-Skoog). So, apply a '-' sign to the transport vector so everything goes in the opposite direction as indicated and maybe the story still works. $\endgroup$
    – user24965
    Commented Oct 22, 2016 at 18:44
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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:

Romanesco

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

  • 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.

  • 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.

  • 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).

  • Hydrotropism causes the plant to grow towards water.

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

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    $\begingroup$ This makes sense. The gravity thing is really interesting! Isn't there also a hormone which induces growth? I can't remember what it's called. Maybe that's more dominant in the root. Again, just hypothesizing. $\endgroup$
    – Amu
    Commented Apr 19, 2012 at 11:33
  • $\begingroup$ Giberellic acid is another plant hormone that can induces plant cell growth or elongation, among many other functions. $\endgroup$
    – gremau
    Commented Apr 19, 2012 at 12:29
  • $\begingroup$ @Amu There are, yes. I’ve forgotten the exact names though. If I remember correctly different types of auxins have different influence anyway. $\endgroup$
    – Konrad Rudolph
    Commented Apr 19, 2012 at 12:35
  • $\begingroup$ Konrad you have the role of auxin backwards - it promotes growth rather than preventing it. I'll try to answer with the stuff you've missed about restoring plant symmetry. $\endgroup$
    – Rik Smith-Unna
    Commented Jun 8, 2012 at 23:33
  • $\begingroup$ @Richard I’m pretty sure there are auxins for both, and their effect differs depending on other factors (which is why I wrote “usually”). Now, I’m sure you know much more about plants than I do (admittedly, that’s not hard). But the specific function I was thinking of is indeed inhibitory. I can’t remember the name of the particular auxin I’m thinking of, I have it somewhere on my other computer. By the way, just saw that you’re about to start a PhD in Cambridge, congratulations & welcome! $\endgroup$
    – Konrad Rudolph
    Commented Jun 9, 2012 at 11:26
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They don't always. For example, this apple tree grows just outside my window:

Tilted apple tree

So far, it hasn't fallen over yet. The reason it grows that way is because all the light is coming from the right side of the picture: the tree leans roughly to southeast, while the building is to southwest of it and casts a shadow on the center of the yard for much of the day. Also, to the left you can see the branches of other, taller trees that block pretty much all scattered skylight from that direction.

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    $\begingroup$ I like the tree. :) $\endgroup$
    – J. Musser
    Commented Apr 19, 2012 at 1:25
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    $\begingroup$ that also happens in areas with strong wind $\endgroup$
    – nico
    Commented Apr 19, 2012 at 9:38
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    $\begingroup$ +1 for the tree! :) I guess the roots would also spread towards the southeast, so that the center of mass of the tree remains above the roots. $\endgroup$
    – Amu
    Commented Apr 19, 2012 at 11:36
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As shown in the answer by @IlmariKaronen, that is not always the case. most trees grown next to the shady sides of buildings tend to lean away from the building. Some trees can stand more shade than others, and these will send branches into the shade of buildings. They never grow as dense or robust in the shade. The reason for the plants growing toward the light is Phototropism. When light hits the plant stem, the auxins that determine the length of the stem are destroyed, decreasing the amount of growth on that side of the stem. When this happens, the stems bend toward the light source from the imbalance of auxins in the stem. The reason that the shady side is weaker is that all trees rely on photosynthesis for their energy. Of course, the side without direct sunlight will not perform as much photosynthesis, and so has that much less energy to grow with. Also, many large trees are imbalanced, sometimes all of the weight on one side. The roots hold them up.

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