The plants would have roots in discs. They wouldn’t enter into the petroleum, (except, possibly just a little bit, in spikes) just spread out on top in an appropriate circumference based on plant size. Humans would be no good in that petroleum world. Worms and snakes would be better. Sharp ring-shaped, tornado-moving subterranean creatures would be ideal.

But maybe dissymmetry could be the answer once in a while.