Summnary. Two-day-old lentil seedlings, (Lenv culiniaris Med.) were inctubated for a 48-houir period with and withouit gibberellin (GA) in the presence and absence of 5-fluorodeoxyuridine (FUDR). The number of cells per epicotyl did not increase c'uring this period. Growth of the epicotyl was thtus dtle to cell elongation alone.The elongating cells of this tissue synthesized DNA. GA promioted and FUDR inhibited cell elongation, DNA synthesis, and RNA synthesis in the tissue.FUDR promoted uptake of thymidine and thymidine incorporation into cellular DNA, presumably by inhibiting synthesis of ei'dcgenous thymidine. Presence of GA promoted thymidine incorporation into cellular DNA and uridine incorporation into cellular RNA. In either case, there was no effect on the uptake of the precursor into the tissue.Fractionation of thymidine-labeled nucleic acids on a MAK column showed that thymidine was exclusively incorporated into the DNA fraction. Presence of GA promoted thymidine incorporation into this fraction and also increased the anmount of ribosonial RNA.The data provide direct evidence for the for elongation of certain plant cells.Growth of plants proceeds by cell division and cell extension: the latter often proceeds mainly in 1 direction and is then called cell elongation. Since cell division is dependent on nuclear division it was obvious that it would be dependent on DNA synthesis. It was however found (12) that cell elongation may be inhibited by treating the tissue with FUDR3 and othe'r inhibitors of DNA synthesis. The inhibition by FUDR could be completely reversed with thymidine, but not with uridine. The tissues used were the lettuce hypocotyl and the lentil epicotyl, 2 tissues in which cell elongation is promoted by GA. The effect of the DNA inhibitors was evident both in the presence and absence of exogenous GA.If cell elongation in these tissues, both that taking place without supply of exogenous GA and that induced by added GA, depends on DNA synthesis it should be possible to detect synthesis of this compound in the elongating cells. Furthermore, GA may be expected to promote this synthesis; FUDR may be expected to inhibit both the DNA synthesis observed in the absence of GA and the increased DNA synthesis in the presence of GA; and thymidine should reverse this inhibition.
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