While a branch is growing in length, it is searching for the best direction to grow in. There are several outside influences that cause a branch to turn to a certain direction. The scientific term for this is a tropism, but that’s just a fancy word for the tendency to grow in a certain direction. I’ll be dropping the scientific names here for anyone interested in finding out more on this fascinating subject.
The strongest tropism of all is called autotropism and it steers the branch in a straight direction. Although this is quite impressive in nature, it is kind of a given in a computer simulation, so there is no parameter for this. You can find the other tropisms in the Turn panel.
Up – Turn new growth up and away from gravity. Use negative values to grow down instead. Scientists call this negative gravitropism.
Up in Shade – Turn shaded growth up and away from gravity. Use negative values to grow downward instead.
To Horizon – 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.