A plant that is obdiplostemonous may be defined as “having two sets of stamens, with the outer series opposite the petals and the inner series alternating with them.” When it takes 19 words to define a term, you might think it’s describing something that’s extraordinarily obscure, but you’ve probably got examples in your garden.
It’s one of a large number of descriptive terms in botany that appear in formal descriptions of families, genera or species, such as this:
Mostly herbs with compound, exstipulate leaves; flowers regular; sepals 5, imbricate, persistent; petals 5, imbricate or contorted, free or slightly united; stamens 10, obdiplostemonous, united below; carpels 5 with free styles; placentation axile; fruit a capsule.
Taxonomy of Angiosperms, by A V S S Sambamurty, 2005. If you haven't worked it out yet, the description is of the Oxalidaceae, the family of the wood sorrels.
An obdiplostemonous flower has twice as many stamens as petals; the stamens are arranged in two rings, with those in the outer ring placed opposite the petals and those in the inner one in the gaps between the outer ones. About 20 plant families have flowers like this; they include such common sorts as heathers and geraniums, as well as oxalis, the sorrels and many others.
If you break the word down into its component bits you will see that they add up to precisely what it describes; it’s formed from ob-, opposite, plus diplo- (Greek diploos, doubled or twofold), plus stemon, the Greek word for a thread (the Latin equivalent is our stamen), plus the adjectival ending -ous. Hence “relating to two oppositely placed sets of stamens”.
The state of being obdiplostemonous is obdiplostemony, a word I defy you to drop into your next dinner party without stopping the conversation dead.