Abstract: The 5 mm long root tips of pea seedlings that had germinated 3 days at 25 ℃were each cut into twenty 0.25 mm sections. The RNA and DNA contents of the sections were determined by the method of Ogur and Rosen, and their nitrogen con-tents of digested protein, by Conway's method of microdiffusion. The RNA and DNA contents determined were maximum at distances between0.5 mm and 1.5 mm from the tip of the root, whereas the relative content of nucleicacid in the tissue was maximum at the meristem of the root. The relative content of nucleic acid decreased continuously during the growth of the tissue. The elongation growth of the root ceased at the time when the relative content of nucleic acid dropped to a constant level. Possibly, a high content of nucleic acid was essential for maintaining the tissue's capacity for growth. Thus, when the nucleic acid content haddropped to its final constant level, tissue growth stopped. Moreover, under all experimental conditions, the changes in protein content at different distances from the tip of the root were similar to the changes in nucleic acid content.