DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Parasitic plant stem. Light micrograph of a section through the stem of a stinging nettle (Urtica dioica) that has been entwined by the stem of a greater dodder (Cuscuta europaea) parasitic plant. Here the parasite's haustoria have penetrated the stinging nettle's tissues. The vascular tissue in each haustorium penetrates the host's vascular bundles where it 'sucks' sugars from the phloem (light brown, oval) tissues, and water and minerals from the xylem (pink with white circles), which it passes back to the stem of the host. The parasite also produces on its epidermis large adhesive cells that stick to the host's stem. The parasitic dodder has little or no chlorophyll, so cannot make its own food, and the base of the stem lacks roots so cannot absorb water. Therefore these substances must be obtained from the host plant. Magnification: x4 when printed 10 centimetres wide.
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