Printed in Great Britain ACTION OF COLCHICINE AND HEAVY WATER ON THE POLYMERIZATION OF MICRO-

The effects of colchicine and heavy water on cell division in wheat roots have been examined by means of mitotic index measurements correlated with an electron-microscope examination of the tissue. Heavy water alone caused an arrest of mitosis both in normal tissue and in roots which had been brought into partial synchrony of division by means of 5-aminouracil. Colchicine alone caused an accumulation of cells at c-metaphase in the synchronized tissue. Colchicine dissolved in heavy water showed variable effects; at high heavy water concentrations (greater then 70 %) mitosis was arrested; when the heavy water concentration was less than 70%, the mitotic index was approximately equal to that obtained by treatment of the tissue with colchicine dissolved in ordinary water. These actions of colchicine and heavy water on the mitosis of the cells have been related to the formation and organization of the microtubules in the dividing cells. Heavy-water treatment was characterized by an increase in the number of microtubules found in the preprophase band and in the mitotic spindle. The microtubules of the preprophase band in cells treated with heavy water were arranged in a highly disoriented manner. After colchicine treatment the numbers of microtubules at the preprophase band and in the mitotic spindle were depleted. The preprophase band consisted of a few microtubules situated close to the wall. In the mitotic spindle the microtubules were either not present or visible only in the vicinity of profiles of endopla8mic reticulum or the chromosomes. When the cells were exposed to a solution of colchi-cine in 70 % heavy water microtubules were absent from the mitotic spindle and the pre-prophase band consisted of a few microtubules which were sometimes found scattered both along the wall and dispersed further into the cytoplasm