seeds viable or not.

This is something that I'm not always sure about with some new plants/seeds.
opinions seem to vary, even in text-books.

I was told that you couldn't grow Stipa gigantea from seeds, but I've collected a load from my plant this year, almost by accident. I've never checked for seeds before, then when I was reading a book about grasses, they were talking about grass seeds having a 'tail' on them. when I looked at the seed heads on this grass, I saw these 'tails' , pulled them , and out came the seeds.
I've also seen the seeds offered for sale. So does this mean that it is viable ,? or not?

Also last autumn I recieved seeds for the purple majesty millet, sowed them, raised some great plants, these were a deep purple, then read that allegedly,they didn't come true from seed?

And what prompted me to write this is reading the query about Purple pennesitum, saying that the seeds for this weren't viable, but further down, someone said that they'd raised some plants from seed, so again ,is it or isn't it? I get really confused.(very easily done...)

I've saved what I think are the seeds from my purple pennesitum, and will try them next year.

When you look through the catalogues you see pennesteum seeds for sale, so is it just some varieties are sterile, and not others,

Oh I can't wait to see the answers you get....excellent question Sueone!
I too grew Purple majesty from seed I collected last year...grew true for me.

I don't specifically know about your species of Stipa but the tail on the seeds is called an awn.
Sometimes the condition of the awn can indicate seed maturity. Some Stipas twist, curl, develop a bend, or additional hairs may "fluff" out.
If the seed came off with little to no resistance then it is pretty mature and can be collected before it drops. For some species of grasses, germination is pretty low naturally. but i would try this one since you are seeing seeds offered and not just plants.