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.
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’s 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.
Turn Random – The branch is free to move in random, uncontrolled directions – unguided by light or gravity.