An L-system is a kind of algorithm named after Aristid Lindenmayer, a Hungarian theoretical biologist and botanist. It uses a set of rules that mimic botany, which say when branches can split and how, with some randomness peppered in and a lot of trial and error for the painting style. Think: go straight for 2 steps, turn left. Go straight for 2 steps, branch, repeat. Normally they look very fractally and 80s, but to me they've always looked a bit like Joshua Trees.