Turn

All the while a branch grows in length, it also keeps reorienting itself every step of the way. And although different trees have different strategies to steer towards the best direction, all of these strategies share a common goal – to find more light.

So how does the actual turning work? Well, in the case of turning to light, the growing tip of a branch is the captain of the ship as it senses the local light environment. For a branch to make a turn, the tip releases a different level of hormones on either side of the branch, to effectively signal the branch to grow more or less on opposite sides of the branch. This will make the growing side expand and make it bend the branch toward the stagnant side. And it’s even more straightorward for gravitropism, where gravity simply moves these hormones to the lower side of the branch, making the bottom grow faster and thereby bending the branch upward.

There are several environmental triggers that cause a branch to turn, and in the scientific world the tendency to grow towards a certain direction is called a tropism. I’ll be dropping the scientific names of each of the tropisms for anyone interested in finding out more on this fascinating subject.

Trees have a strong tendency to steer branches in a direction straight foreward, which is quite an impressive thing in the chaos of nature, but it’s kind of a given default in a computer simulation, so by default a branch will grow straight. Then to guide the branches through space, you can use the following selection of tropisms in the Turn panel.

• Up – Turn new growth upward, away from gravity. Use negative values to grow downward instead. Scientists call this negative gravitropism. Positive gravitropism, on the other side, is what makes roots grow down into the ground.
• Horizontal – Turn branch growth toward the horizontal plane when a branch is shaded. This is what scientists call plagiotropism.
• To Light – Turn new growth toward the brightest direction. This is the effect that makes a houseplant grow toward a window. On a tree, this effect will improve its distribution of branches. Scientists call this phototropism.
• Random – The branch is free to move in random, uncontrolled directions – unguided by light or gravity.
• Twist – This is the tendency for some trees to twist around the axis of growth. For example horse chestnut trees have very visible twisting along the length of their branches – you can clearly see the bark pattern swirling up along the trunk. Apart from the obvious visual quality , twisting also adds to the phyllotaxic rotation of the buds, and it improves the distribution of branches on species with opposite branching.

Now there are more tropisms involved in plant growth, but these few are by far the most important ones to simulate trees. There is however one tropism that I’d like to discuss called Heliotropism, which is what makes a sunflower track the sun during the day. It’s important to know that heliotropism is different from phototropism, because phototropism does not make a plant grow towards the sun. Phototropism only makes the branch turn to the blue color of the overall sky. That’s what makes a tree grow into a balanced structure instead of slanting towards the south (or north on the southern hemisphere). There are of course always exceptions to the rule, and in this case that exception is the Cook pine, which does lean towards the equator. Maybe its phototropism sensors are tuned to the wrong wavelenth of light, who knows? But the takeaway is that trees do not grow toward the sun, but instead are sensitive to the scattered blue light of the sky.