The results of the first paper in this series (Pecket, 1957) indicated that lateral root initiation in isolated pea roots depends upon an apically moving factor or complex of factors provided by the mature tissue of the root. The present investigation analyses the effect of externally applied IAA on lateral root formation and uses the methods employed in the earlier work. Observations made on roots in which three regions have been distinguished show that increasing IAA concentration does not cause similar changes in all pans of the root, nor does it cause identical effects on primordium initiation and on the subsequent emergence of the initials as lateral roots. The latter fact explains why the emphasis in the present work has been laid on initiation rather than on emergence. The results of experiments are reported in which two variables are involved, IAA concentration and the presence or absence of the two terminal regions of the root. These results are considered to support the conclusions of the earlier paper. It is suggested that the stimulant moving from mature tissue is not IAA itself but that the production of the stimulant may be increased when IAA is supplied. A gradient of reaction to the stimulant appears to exist in the root, reaction being maximal in the mature tissue.