For everybody who doesn't know what the Index Kewensis is: this is a list
with all the published plant names since 1753, when Linnaeus published his
Species Plantarum. In this list are also included all the rejected names
(e.g. later published synonyms). The only other possibility left is that P.
morrii was an invalid publication (according to the ICBN (=International Code
for Botanical Nomenclature)), but I think this isn't likely.