"one surface to grow at a different rate from the other" ie two rates, in a
curious relationship to each other. Rate of growth is best expressed in a
mathematical formula.

I was afraid you were going to say that. Do you think that is what is causing
the twist, that the different sides of the leaf are growing at different rates?
Incidentally, It usually doesn't seem to make any noticeable difference whether
the plant is diploid or polyploid. However, I have one sophro hybrid which is
said to be 4.5n and has leaves like boards. The twist is there, but less
pronounced.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)