Here is a study of a tree with a gravitropism. 

https://en.wikipedia.org/wiki/Gravitropism

it helped me understand how Loop and accumulator is working. the function has been delivered recently and it is a world in itself to discover.

here is the overall logic

• for each segment (base segment)
  • from its top extremity, create 3 new segments
    • they have between 25 and 35 deg from base segment
    • they are evenly distributed around the base segment
  • add a tropism -> modify angle of the segments by a -k * ratio of angle from vertical
• loop on the segments

in my picture the base segment has a 70deg angle from the vertical. We can see that the tree bend toward the vertical little by little. 

with loop as feature, there was no need to add code or script, which made the logic quite intuitive.

big thanks to DS R&D and to @GV ​​​​​​​for his support

3dxml is in the zip

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