Distribution patterns of lateral roots (LRs) were analysed in intact (control) and decapitated adventitious roots (ARs) of Allium cepa L. Removal of the root tip caused: (1) The cessation of elongation of the AR. (2) An increase in the number of LRs per cm which developed on the AR. (3) Changes in the distribution of LRs along the mother root One aspect of the distribution modified by the decapitation was the suppression of an apical zone without LRs which was observed immediately behind the apex in intact roots. In decapitated ones by contrast accumulation of LRs was enhanced in this zone. Moreover, decapitation appeared responsible for successive LRs opposite the same xylem pole developing nearer each other than in intact roots. (4) Changes in the frequency and location of clumps of LRs. In decapitated roots 21.6%, of the total number of LRs were clumped. In intact ARs this figure was only 13.5%. Moreover, intact roots showed a tendency to LR clumping preferentially in their basal zone In contrast, clumps of LRs were noted indistinctly in both basal and distal zones of decapitated ARs. I his fact implies that removal of the root tip enhances the natural tendency toward clumping of LRs, chiefly in distal zones of the AR. Other aspects of the pattern remain unchanged after removing the root apex. Hence in decapitated as well as intact ARs there were two neighbouring xylem poles opposite which large number of LRs were produced, whereas the remaining ones were considerably less prolific. A further aspect which was not altered by the decapitation was the significantly high frequency of formation of neighbour LRs opposite adjacent xylem poles.