SCIMAT / SCIENCE PHOTO LIBRARY SCIMAT / SCIENCE PHOTO LIBRARY
Garden rhubarb (Rheum rhabarbarum) pollen under a microscope (the right pollen grain is misformed). Only the leaf stalks of the plant are edible. They are used in pies for their tart flavor. The leaves, however, are toxic for the presence of oxalic acid and anthroquinone. Oxalic acid combines with calcium to form the less soluble salt, calcium oxalate, which is also found in kidney stones. Plant leaves, especially rhubarb, cabbage, spinach, and beet tops, contain oxalic acid. Growers are advised to prevent rhubarb from blooming (by pruning out the seed stalk) at the expense of edible stalk growing. Enhanced SEM. Image width: 98.5 micrometers. Magnification: 1015x if the image is printed 10 cm wide.
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