bild

Marrow stem. Light micrograph of a section through the stem of a marrow (Curcurbita sp.), showing the sieve plates in the phloem. A single collateral vascular bundle can be seen. This has an outer and inner phloem (blue) with large sieve tubes some of which show sieve plates (dotted blue) and companion cells, with an inner xylem (pale orange) with large vessels and small woody parenchyma. The cambium, made up of small cells in-between the phloem and xylem, has broken down. Surrounding the bundle is the ground tissue parenchyma (violet). At upper right is the epidermis (blue) and underneath is a band of supporting fibres (pale orange). Magnification: x37, when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)