Sentence:  Drawings and pictures are more than mere ornaments in scientific discourse. 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 <WSD>explaining</WSD> a theory, for telling a story. 
Meaning of the word marked with <WSD> tags:  To make plain, manifest, or intelligible; to clear of obscurity; to illustrate the meaning of.