Mitotic frequency determinations and microphotometric Feulgen dye measurements in root tips.

Treating a Feulgen stained (10 min. acetic-alcohol fixation, 8 min. hydrolysis) onion root tip with 5% aqueous pectinase for 6 hours causes it to disintegrate on shaking in 1 ml. water into a suspension of stained cells, either individual or in small groups. Vicia faba and pea root tips require 15 minutes hydrolysis and 12 hours pectinase treatment. Absence of cell destruction allows absolute cell-number determination by Brown and Rickless' counting chamber technic. By centrif uging the cells down, resuspending them in 2 drops of a Karo syrup-phosphate buffer mixture (2 parts Karo to 1 part 0.5M, pH7 buffer) and mounting a small drop of the now concentrated suspension, a semipermanent slide containing a concentration of well-spread, randomized cells suitable for rapid mitotic frequency determination is obtained. Scoring about 1,000 cells as to nuclear stage gives a representative, workable statistical sample and takes only 20–30 minutes if interphases are recorded on a mechanical tally. Permanent slides c...