Thallium as a Vital Stain for Yeast Mitochondria

BLACK, hexagonal crystals of thallium oxide, Tl 2 O 3 (identified by their shape and colour), appear in some cells of some strains of Saccharomyces cerevisiae when the culture is grown in nutrient medium containing 125 p.p.m. of thallium sulphate. The black, hexagonal plates of Tl 2 O 3 lie in a space between the cell wall and the plasma membrane. Usually only one large crystal is present, but as many as six small ones may appear; all are outside the protoplast. When a crystal is viewed from the edge, a bulge in the cell wall appears right over the crystal. The large crystals bend to conform to the curvature of the cell (Fig. 1).