DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Pumpkin stem. Light micrograph of a section through phloem sieve tubes in the stem of a pumpkin (Cucurbita pepo) plant. Each sieve tube element consists of cytoplasm along the cell wall and a central vacuole, containing slime-bodies (light green), and no nucleus. The top sieve tubes shows the transverse layer of cellulose, which is perforated by holes (red), known as the sieve plate, through which the slime flows forming a plug (callus). The contents flow up and down taking substances such sucrose (sugar) down to the roots and upwards to the shoot. To left of the phloem is a companion cell full of cytoplasm (dark green) and a nucleus. Magnification: x37 when printed 10 centimetres wide.
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