I've wondered for some time why nepenthes produce a kind of "water level" mark on the inside of the pitchers.
The result is often the color of the pitcher is quite different below the "water level" mark.
The only explanation I've come up with is that the mark is made by the pitcher fluid being exposed to light when the pitcher first opens and because the fluid itself is often quite sticky it literally freezes/sets onto the pitcher as a fine layer. The thing I can't explain is how the "water level" mark gets so high up a pitcher, when often the fluid is at a much lower level when the pitcher finally opens.
So this leads me to make the assumption that the "water level" mark is then made whilst the pitcher is still inflating and even though the fluid level eventually is much lower, the mark of the fluid at its highest point is still left imprinted onto the pitcher wall.
I also discovered that certain species/hybrids are more susceptible to this happening than others, particularly in pitchers where the interior pitcher wall is lined with speckling.
Can anyone think of a better/ more scientific explanation?
Here's a few photos that hopefully demonstrate the point: