The changes of the diameter of the primary roots along their axes were investigated in corn plants in relation to the diameter and the density of lateral roots. The primary roots of the basal "shoot units", are formed earlier and those of the upper "shoot units" are formed successively later in the growth of the plant. In the basal "shoot units", the primary root diameter was relatively small and was almost uniform in any region of the axes. Whereas in the upper "shoot units", it was very large in the proximal regions, decreasing remarkably toward the root tips to the similar diameter as the basal "shoot units". Along with these changes in the diameter of the primary roots the average diameter of laterals formed on the primary roots also changed. That is, in the basal "shoot units", the average lateral root diameter was small and was almost uniform along the primary axes. However, in the upper "shoot units", it was large in the proximal regions of primary roots and decreased remarkably toward the root tips. The density of laterals was higher on the primary roots of the upper than on the basal "shoot units". Along a primary root axis toward the tip, it increased remarkably near the basal part and further was alomst uniform. Consequently, it was shown that the corn root system is composed of various kinds of primary roots, and the formation of the laterals is closely correlated with the diameter of the primary roots on which they are formed.