The curve of growth in length of the main axis of excised tomato roots, grown in a 6-day passage, shows a 'lag phase' at the beginning and a decrease in growth rate toward the end of the passage when growth of the apex is inhibited by the older tissues. The 'lag phase' is neither due to transfer of the excised tip to fresh medium nor a consequence of the developmental stage at which the tip is excised. The 'lag phase' may mean that the apical meristem requires substances supplied by the older tissues, but it is, at least in part, due to injurious effects of excision.From surgical experiments it is concluded that the growth of lateral roots is promoted by the presence of older tissues and inhibited by the main apex and younger parts of the main axis. A comparison is made between this latter effect and apical dominance in shoots.Growth of the main axis was increased by 2,4-dichloroanisole (DCA). Growth of the lateral roots was inhibited at concentrations of 3-indolylacetic acid (IAA) and DCA which did not affect growth of the main axis. This effect was not observed with 1-naphthaleneacetic acid (NAA). At concentrations which did not affect growth of the main axis, only NAA promoted the formation of lateral roots. The evidence is reviewed that the growth correlations in the root involve 'auxin-type' growth regulators but that different regulators may be involved in the different growth correlations.The author acknowledges the loan of apparatus from the U.S. Office of Naval Research.
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