PIE *plōtus etymology
I'm sorry for the back and forth about this one. I was working off of the AHD of IE Roots (http://web.archive.org/web/20080627204039/http://www.bartleby.com/61/roots/IE415.html) section I.6. which lists the the lengthened o-grade of *pleu- as *plō(u)- (or *plō(w)-, as the case may be). It seems very convenient of them just to list the u/w as optional, but do you think that this is a fair explanation, particularly if we list it as *plō(w}- + -*tus?
The problem is that there is no established sound change that would turn -ōw- into -ō-. -ōw- was allowed as a diphthong in Proto-Indo-European, just compare *gʷṓws (“cow”). So the change of -ōw- to -ō- must be accounted for somehow. Furthermore, the Germanic form does not necessarily need to reflect *plōtus. The -ō- could also originate from -eh₂- or -eh₃-. In fact, M Philippa's etymological dictionary of Dutch (which I've found to be very reliable over time) mentions a root *pleh₃- as a possible source of the Germanic words.
Ok, thanks for the input. Hopefully others will try to figure that out then. I wonder then why the AHD of IE Roots would allow that etymology in then? I had always understood it to be very reliable. Also, thank you for helping me in these early days of my Wiktionary editing.
Kroonen gives -ōw- to -ō- as regular before a hiatus. I can go through and list some examples if you want.
Confusingly, he lists the Pre-Germanic form of *flōduz as *ploh₃tús, but states it is derived from the root of *flōaną, which he gives as *plōw-, as necessary for related *flawją (“ship”) and *flaumaz (“stream”). Maybe it's a middle ground of *pleh₃-u-?