I believe one (if not more) is a Somona hybrid, Euphorbia milii. See link below for a few. There are many named, so you'd have to check on colors & so forth.

Here's a site that explains something about it.

Nice plants!

~Rosemarie

Here is a link that might be useful: Euphorbia milii Somona hybrids

Thanks, Rosemarie.

After all that reading, I am more confused. I think I may just end up labeling them all Euphorbia millii sp., unless there is some less confusing way to tell. Thanks for the links, though...I saw some nice ones with light lavender-colored flowers.

Kate

Just bumping this up, in hope that Frank sees it, and sheds some more light on the subject.

K

Yes, I think they're the Thai Giant Crown of Thorns hybrids. Looks like you must be treating them well, Kate, as they appear happy as clams!!

Denise in Omaha

They look like giant Thai crown of thorns. I'm not even going to venture a name for the cultivar, as I myself as a Thai person have hard time keeping up with numerous Thai names that associated with these cultivars, some of which look similar and hard to tell apart.
You can take a look at some cultivars at my friends website:

Here is a link that might be useful: Thai plants