STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Grass stem. Coloured scanning electron micrograph (SEM) of a section through a grass stem (family Graminaceae). At upper centre is the vascular bundle, which consists of phloem (blue) and xylem tissue. Xylem is composed of annular tracheids (red), pitted vessels (brown), fibrous tracheids (purple) and an intercellular space (round, bottom). The xylem transports water and mineral nutrients from the roots throughout the plant, while the phloem carries carbohydrates and hormones around the plant. The vascular bundle is enclosed in a sheath (dark green) and surrounded by sclerenchyma (yellow thick-walled cells), which is supportive tissue, and parenchyma (yellow thin- walled cells), where the primary plant functions take place.
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