This paper explores the role of diagrams and text in middle school students' understanding and visualisation of human body systems. We develop a common framework based on structure and function to assess students' responses across diagram and verbal modes. Visualisation is defined in terms of understanding transformations on structure and relating these with function. We found in Indian students a high dependence on the verbal mode. Students expressed structure as well as function concepts significantly better through text rather than diagrams. Prior knowledge strongly influenced visualisation (as assessed through transformations) and comprehension of text and diagrams including the ability to move flexibly between text and diagrams. We suggest better use of line drawings for integrating structure and function: practices that should equip the learner to "dual-code" text with diagrams, thereby leading to enhanced understanding and expression. (Contains 4 tables and 8 figures.)