from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Having the filaments of a flower united into two groups: diadelphous stamens.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Having its stamens fused together at least partly by the filaments so that they form two separate bundles, or a bundle and a lone separate stamen.
from The Century Dictionary and Cyclopedia
- In botany, having stamens united in two sets by their filaments, the sets being equal or unequal; grouped together in two sets: as, diadelphous stamens. In papilionaceous flowers, out of ten stamens nine are often united, while one (the posterior one) is free. Also diadelphian.
"Nature," recalling your generalisation about the diadelphous structure, and now explaining the exception of Coronilla.
It seems that Darwin did not know that the staminal tube in the diadelphous Leguminosae serves as a nectar-holder, and this is surprising, as Sprengel was aware of the fact.)