I've noticed that branches stemming from the trunk of a tree are capable of doing so at all 360 degrees about the axis of the trunk. However, branches stemming from the trunk's primary branches, or from any branch that is already horizontal, seem to have a strong tendency to do so horizontally - forming sort of a plane along their parent branch's axis. This is especially noticeable in some conifers like the eastern hemlock or white pine. I would like to understand what causes this to happen. Does a branch's phyllotaxis change based on its orientation or is the cause more external? Can anyone explain?