The effect of Ca2+ on root elongation has been reported to be both stimulatory and inhibitory (Burstrom 1969, Evans et al . 1990, Hasenstein and Evans 1986). In those initial studies , however, the whole root was treated with Ca2+. Because the site of action for Ca2+ in gravitropism is considered to be the root cap rather than the zone of elongation, we focused on the role of the Ca2+/cap interaction in root growth as well as in gravitropic responses. We found that Ca2+ at 10 or 20 mM applied to the cap end of pea and corn roots mediated elongation growth of roots for at least 3 to 4 h following treatment. Unilateral application of 1 to 20 mM Ca2+ to the root cap always induced unequivocal curvature of roots away from the Ca2+ source in Alaska pea and to a greater extent in the roots of the agravitropic mutant, ageotropum (Figs. 1 and 2). Roots of Merit and Silver Queen corn also always curved away from Ca2+ applied to the cap, although a somewhat higher concentration was required for the response than in the pea roots. [Several sentences were omitted here.] These results show a strong correlation between an increase of Ca2+ levels in the root cap and stimulation of root elongation. The results are in contrast to the previously proposed model that an increased level of Ca2+ in the root cap mediated inhibition of root growth (Hasenstein et al. 1988).