Guard cells of leaves on living plants, as indicated by 48 species, have a very pronounced ability to retain starch for prolonged periods in darkness. A number of exogenously supplied sugars act as precursors to guard-cell starch production when strips of epidermis are floated on sugar solutions in the dark. Mesophyll and bundle cells are associated with the uptake, transformation, and translocation of the test sugars. In some species exogenously supplied glucose-1-phosphate or sucrose can serve as a precursor to guard-cell starch without the aid of mesophyll or bundle cells.