Sentence:  You know, Darwin studied their fertilisation, and showed that the whole structure of an ordinary orchid <WSD>flower</WSD> was contrived in order that moths might carry the pollen from plant to plant. 
Meaning of the word marked with <WSD> tags:  A reproductive structure in angiosperms (flowering plants), often conspicuously colourful and typically including sepals, petals, and either or both stamens and/or a pistil.