On 17/08/2020 20:58, jeff Layman wrote:

I'm not sure where that might leave us with Chris' "Rhododendrum"!

Linnaeus used Rhododendron, so that's the correct spelling. We follow
the choice of the original author between the Greek -dendron and the
Latin(ised) -dendrum. Same for -stemon and -stemum, and -stylon and
-stylum, and -phyllon and -phyllum.

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SRH