Are the stomas open?

[Orchid Whisperer]

When Naoki says nail polish, remember that it is clear nail polish.

You can do roughly the same thing if you have clear acetate film and acetone. Wet a small area on the leaf with acetone, apply the acetate film to the wet spot, let it dry then peel off.

Remember that any efforts to look at stomata will destroy some leaf tissue. Probably don't want to so this too many times.