Sentence:  ... the legitimate unions between the two forms of the above nine species of Primula are much more fertile than the <WSD>illegitimate</WSD> unions; although in the latter case pollen was always taken from a distinct plant of the same form. 
Meaning of the word marked with <WSD> tags:  Involving the fertilization of pistils by stamens not of their own length, in heterogonously dimorphic and trimorphic flowers.