Q.Lily With 5 Tepals?
A stand of flowers has appeared in my garden which I don’t remember planting. I’ve identified it as a lily, with 3 flower heads on a single stalk. But I only count 5 tepals (and four stamens). All on-line sources say there must be 6 tepals and 6 stamens. It looks very much like a lily to me. Is there a variety with this reduced number of tepals or is this impossible? If so, what is the flower that I have?

Certified GKH Gardening Expert
My research also shows 3 sepals and 3 petals for lilies. Your plant's foliage, buds and blooms do look like a lily. Some of my Stargazer lilies are growing twisted and look like they only have 5 tepals. Have you inspected under the petals for maybe a shorter or thinner sepal that doesn't show? It also could be a sport (mutation) of the sepals that are only a protective covering of the actual 3 petals.
Next year's blooms could look normal. Sports aren't always permanent.
https://www.gardeningknowhow.com/garden-how-to/info/what-is-plant-mutation.htm

Thanks for helpful reply,Susan.The answer to your question is mostly yes. On a few heads there is a small green leaf just below the flower head which could be a sepal. On one, this leaf is about 2 cm lower down, and could be just an ordinary leaf. I'll try to post a picture of the green sepal one.
I'm surprised that you suggest this effect may disappear in future. If it's a mutation, I thought it would be locked into the genes for all future bloomings of this particular plant. And I'm curious whether this is a rare or commonplace occurrence, as no one seems to mention it.