Sentence:  Drawings and pictures are more than mere ornaments in scientific discourse. Blackboard <WSD>sketches</WSD>, 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:  A rapidly executed freehand drawing that is not intended as a finished work, often consisting of a multitude of overlapping lines.