bild

African mahogany stem. Polarised light micrograph of a section through the stem of an African mahogany (Khaya sp.) tree. The multicoloured cells are fibres, and fibre tracheids, which carry water. The horizontal files are medullary rays, which carry water, minerals and food to the outer and inner tissues. There are also sparse xylem vessels (dark brown). Magnification: x32, when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)