Grateful acknowledgement is made to the following sources: 

• Figure 1a: Paul Fellows/The Open University
• Figure 2: Paul Fellows/The Open University
• Figure 3a and 3b: Heather Davies
• Figure 4a: Heather Davies
• Figure 4b: The Open University

Appendix 2:

• Human cheek cell: Kerry Murphy/The Open University
• Skeletal muscle: Kerry Murphy/The Open University
• Blood cells: Kerry Murphy/The Open University
• Onion root: Kerry Murphy/The Open University
• Nymphaea: Kerry Murphy/The Open University

Every effort has been made to contact copyright holders. If any have been inadvertently overlooked the publishers will be pleased to make the necessary arrangements at the first opportunity.