DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Dendrobium orchid root. Light micrograph of a section through an aerial root from a Dendrobium sp. orchid. The outer tissue (velamen radicum, not shown) is composed of hexagonal cells. These are dead cells with lignified walls that contain pores that take up water. The next layer, the exodermis, is a ring of cells with walls of suberin that contain passage cells for water flow. The next layer is the cortex of parenchyma cells (deep blue). The central core has an outer ring (endodermis) with thick walls (grey), then an inner ring (pericycle, blue), and next a ring of vascular bundles with alternate rings of xylem (red) and phloem (blue), surrounded by fibres (red). Magnification: x37 when printed 10 centimetres wide.
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