Thanks Tracey and Della. Didn't realise that the RHS Lily Register mentioned the ploidy where known. I've realised it's incomplete, though, since I've found a paper that lists the ploidy of a few more cultivars:
http://www.liliumbreeding.nl/L...
I will have to see if I can get a lens for looking at the stomata. Very few of the plants I have or am hoping to get have their ploidy clearly stated, so I'm going on assumptions in most cases. Will be a bit of a challenge. As I result, I'm making a list where I can find out the ploidy (from different sources, such as papers, the lily register, these forums, etc.). It's in the early stages, but hopefully it will be useful.
