A novel technique for determining the total and the meristematic number of cells in a root meristem is described; it involves macerating the tissue in a known volume of fluid, and subsequently determining the appropriate number of cells in an aliquot on a haemacytometer slide. Root tips are cultured in batches on sintered glass disks with different nutrient media and at four temperatures. Samples of four roots are taken from each culture at intervals of 12 hr., and with each sample the average total number, the average meristematic number of cells, and the average length of each root are determined. From these primary data rates of division and indices of extension are calculated. It is shown that no division occurs in the absence of sugar, and the rate of division is increased by providing inorganic salts with the sugar, and still further increased by supplying yeast extract. With each nutrient the rate of division is higher at 15 degrees than at 25 degrees C, but lower at 5 degrees C than at 15 degrees C. No extension occurs in the absence of sugar, and the greatest extension is given with sugar and inorganic salts. Yeast extract in the form and concentration used in these experiments tends to depress extension. The results of the main body of experiments are discussed along with others showing the effects of single mineral deficiencies. It is suggested tentatively that in these experiments the level in the cells of certain products of carbohydrate degradation determine the rate of cell division, and that in cell extension the process depends to some extent on a synthesis of protein which requires a supply of sugar and of certain inorganic salts.