Sentence:  It [the term "disk"] means a <WSD>supernumerary</WSD> organ, different from the stamens or petals, and originating at the base of one or other of them. Nothing can well be more variable in its nature than this disk; in the Mignonette it is, as you see, a one-sided, hairy lobe; ... 
Meaning of the word marked with <WSD> tags:  Of an organ or structure: additional to what is normally present.