The results of experiments with isolated pea roots are presented which show the effects on growth of providing nicotinic acid and thiamin in the medium. Certain of the experiments involve two experimental variables, the presence and absence of vitamins in the medium and tip inocula of different lengths. Growth is analysed in terms of overall length increase, increment in number of cells, rates of division, and mature cell volume. It is shown that both vitamins affect division, and in the case of vitamin B 1 , through a primary effect which may be either on mature or meristematic tissue. Evidence has not been obtained that with the material of this investigation the vitamins have any effect on the growth of the cell. Further evidence is provided in favour of the hypothesis that division in the meristem depends on products formed in the meristem, and on a different set of metabolites synthesized in mature tissue.