system
Given the following post, respond as if you're commenting on Reddit:
human
What determines when/where a branch will grow on a tree?
gpt
It has to do with the distance from the apical meristem.

The apical meristem holds a lot of stem cells within the growing plant, and is generally the top of the forming tree/bush.  The trunk of a tree is the remnants of the apical meristem as the tree grows in height.  

One of the hormones it releases is called auxin.  When auxin is released into the vascular system of the plant, it contributes to apical dominance, which prevents any branches from growing below.  Thus, auxin is a negative feedback hormone; if concentrations are high, things are prevented from happening.  As the apical meristem moves higher, the concentration of auxin becomes lower, and this triggers secondary meristems that have formed to branch out.  Basically, as the tree gets taller, the auxin concentration at the secondary meristems is lower, and they grow out.

If you take a growing tree or bush and cut the apical meristem off, the secondary meristems on the side will grow out.  This way you can cause a plant to grow horizontally instead of vertically, though they will eventually begin to grow vertically again as one of the branch tips assumes the role of the apical meristem.