bild

White bryony stem. Light micrograph of a transverse section through the stem of a white bryony (Bryonia alba) plant, showing the phloem sieve plates. The outer stem is covered with a thin epidermis (dark blue) which has stomata. Under the epidermis is a layer of flexible collenchyma (dark blue) for support. The cortex is composed of outer parenchyma (light blue) tissue and an inner of sclerenchyma fibres (red). The inner stem is filled with parenchyma (light blue) containing two rings of vascular bundles (circular). The bundles have an outer and inner phloem (dark blue) with large sieve plates and a centre of xylem (red). Filling the centre of the stem is a pith of parenchyma tissue. Magnification: x5 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR KEITH WHEELER/SCIENCE PHOTO LIBRARY)