An optical brightener Calcoflour White M2R New has been used to stain cell walls of higher plants. It can be used either as a vital stain for intact plants or for hand sections and plastic-embedded thin sections. Walls are brilliantly fluorescent while most cytoplasmic components are normally unstained. The brightener binds strongly to cellulose, carboxylated polysaccharides, and callose. Staining for 20 sec to 2 min in a 0.01% solution of the brightener is preferred for most purposes.