Sentence:  Drawings and pictures are more than mere ornaments in scientific <WSD>discourse</WSD>. Blackboard sketches, geological maps, diagrams of molecular structure, astronomical photographs, MRI images, the many varieties of statistical charts and graphs: These pictorial devices are indispensable tools for presenting evidence, for explaining a theory, for telling a story. 
Meaning of the word marked with <WSD> tags:  Expression in words, either speech or writing.