How do trees decide when and where their branches grow?
In: Biology
And how do they balance their branches?. Eg. Counter balance with branches on the other end.. Thickness of branches.. Etc.
To complete and go more in depth on what was already answered, it is a mix of influences from genetics and environmental conditions (as for everything in biology actually). A stem or a branch usually end with a bud (called apical bud) which contain a special group of cells called a meristem. Meristems are where cell multiplication happens in a plant and so are what allow a plant to grow. During the growth season, this apical meristem will produce new cells at the base of the bud and those cells will then elongate, which will make the branch grow longer. As others said, this process is controlled by hormones (the principal one being called the auxin), and those hormones are strongly sensitive to the environmental conditions. For example, light destroy the auxin, which will make the shadowed side of a stem grow more than the other side and the stem will “go” toward the light source.
Every now and then during this period, the meristem will produce a leaf and on the top of the junction between the leaf and the branch, a little piece of meristem will separate to create a new bud (called axillary bud). This bud will stay dormant until the next growing season (where there is different seasons) and then it will start to be active the same way as the apical bud (it actually is an apical bud at this point) and develop into a new branch. The rhythm and the places the leaves and the axillary buds will be produce is mainly controlled by genetics.
So to summarize, environmental conditions like light (but also wind, gravity, animal grazing,…) control the shape of the branches and where they go, and genetics controls when and where a new branch is produced.
DNA/genes. Their genetic code says to “be like a tree” and that means growing roots to absorb water and nutrients from the ground and to hold firmly in place, expanding the thickness of the trunk, and to reach up towards sunlight and produce a lot of leaves to absorb sunlight and carbon. Since it needs lots of leaves, it needs many branches. It’s pretty tough for DNA to code perfect symmetrical shapes and growth (like a perfect sphere or circle where the branches are *too* perfectly spaced) so it says to sort of grow for a while and then split a new branch “sometimes.”
This results in near-fractal-like structures, which is where the pattern of branch-smaller-branch repeats, but not at perfect intervals. That’s why most trees of the same type look similar but aren’t exactly the same. The branches split and new ones form at frequent, but imperfect intervals. This psuedo-randomness is actually quite pleasing to look at, like music: music has repeating patterns (like a chorus) but with small variations to prevent boredom. These patterns likely help attract all different kinds of life that the trees also need to thrive by spreading their seeds and nuts and fruits around and producing waste to enrich the soil.
Also, by somewhat randomizing the growth patterns, leaves can fill in more spaces and probably absorb the most sunlight this way by creating a more full canopy. Imagine sticking toothpicks into an apple, straight out from the center, and then extending those toothpick lines out for another foot. If tree branches grew straight out at perfectly spaced intervals, their branches would eventually spread away from each other and leaves wouldn’t cover as much area to soak up the sunlight. It also makes for less interesting places for animals to make their homes.
So they grow in a psuedo-random pattern, which is a lot like fractals with small variations. If you don’t know what fractals are then please check out a good YouTube video on them, it is beyond the scope of this question.
Trees communicate using different hormones. These hormones are made by leaves and branches in response to different conditions such as different types of light, temperature, humidity, stress and gravity. These hormones then affect how the other branches grow depending on their genetic encoding. And this determines the shape of the tree. The hormones may also be transfered to different trees either through the air or the roots and therefore shape the entire forest. This is an area of very active research where we study how different plants grow in different conditions and also check what different mutations does to the growth patterns. On of the most exiting new innovation in this work is that the ISS recently got a small greenhouse allowing us to grow plants without the effects of gravity. Not only will this allow us better understand the effect of gravity on plant growth but by removing the effect of gravity we can better see other effects that might be hidden from us.
The full answer is pretty complicated, but the short version is that plants have hormones just like animals. They have growth hormones and the ability to sense light and the direction of gravity. Most plants try to grow towards light and against gravity. Many plants also grow more in the summer because they sense that the days are longer. There have been experiments done with plants kept in similar environments but with different durations of light, and it was found that you can cause certain plants to bloom depending on how long the lights are on.
Duh, they use their eyes and ears?
Also an added question to this, how/why do trees grow around powerlines? In my city we have powerlines and the tops of the trees have grown in a semi-circle around the lines. It doesn’t look like they’ve been cut, nor have I ever seen them do work to the trees. They just seem to, grow.. around the lines..
Plants are not actually inanimate objects, they just move very slowly and don’t have the kind of brain that can make particularly complicated decisions.
Plants are able to communicate with each other. There’s a variety of acacia tree that can produce poison (tannins), but it’s expensive to make, so it doesn’t normally create much of it. But if an enemy starts to prey on one, the tree will release a smell that makes the rest of the acacia trees around know that shit just got real and they’ll all arm themselves with tannin for the entire area, a pretty good way to get animals to eat something else. It’s even more interesting because a human can break a branch off the tree without causing the “we’re being eaten” alert to go out like it does when an animal starts munching its leaves. The poison isn’t just a blind response to any and all damage, but only to things that seem like an animal eating the tree. And communicating saves the trees from all having to produce the poison constantly which would cost them energy.
There are other plants that have teamed up with parasitic wasps. When a caterpillar (the wasp’s food) starts to eat the plant, the plant releases a special smell that shouts “wasp dinner time” and the wasp comes over and ganks the caterpillars for it.
Even mushrooms are in on the intelligence and communication, Leafcutter ants actually communicate with their mushroom farm partners. If the ants bring back a poisonous plant the fungus can’t tolerate, the fungus tells the ants to toss that crap away and the ants will get rid of the poisonous plant for them. Win-win for everybody.
There’s a plant called a *touch-me-not* that curls up the leaves when bothered. People guessed it was just an automatic thoughtless reaction. An experimenter dropped 56 of them from a certain height and they all curled up. After a few more drops, less of them bothered curling up. After a while doing these safe drops, they all stopped curling up for the drop completely. But they still would curl up if poked with a stick or something other than dropping them, so they’d learned that specific thing (dropping) could be ignored. The plants remembered this info for a full month from when taught (some insects like bees forget info after only a few days)
As for deciding where branches would be placed best, plants use different tools to decide the right conditions than we would. Plants have as many as 11 different photoreceptors (light sensors), while the human eye has only 4 kinds of them, so plants can actually analyze sunlight in more complexity than we can. It’s like our astronomers use different telescopes to look at things in space in ultraviolet or infrared or other kinds of light we can’t see with the naked eye, plants do the same thing in a way. It might be useful for moving their growing branches to the optimal spot for their green food-factories to get the ideal amount of the right colors of light they need to eat (even something like light that’s the wrong color can mean worse eating for a plant. Humans care less about color because we don’t eat it with photosynthesis. The reason brown algae and a lot of aquatic plants are brown instead of green is because they use a brown dye to eat better underwater because light bends different down there, so being pure green like a tree would actually produce less food)
The mechanism for plant limbs to go to their best spots is also interesting. There are climbing vines that as they spread, they basically rub their fingertips along the walls and feel different features, then they do things like coil around supports, find certain plants they like to stay tied to and wrap around those plants, move towards the best light sources, etc. Researchers cut the fingertips off of those vines, and after that the vines still grew, but they did it in a much more stupid way, just moving in basically straight lines and not finding the good climb holds and not placing themselves well. It was like the “brains” in charge of the movement activity were in the plant’s fingertips and not the the rest of the vine’s body. I don’t know if ordinary tree branches have the same branch-intelligence in their fingertips, it wouldn’t surprise me if they did, they’d have millions of years to work on it.
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ELI5 version:
Trial and error. Tree needs light and water. Tree grows towards light and water.
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