The apical meristem at the tip of a shoot is s major site of auxin synthesis. As auxin moves
downward, diffusing from cell-to-cell, it stimulates growth of the stem by making cells elongate.
o As the blue curve in Figure 24.3b shows, auxin promotes cell
elongation in stems only within a certain concentration range.
Above a certain concentration (0.9g of auxin/litre of solution, in
this case), it usually inhibits cell elongation in stems, probably
inducing the production of ethylene, a hormone that generally
counters the effects of auxin. o The red curve on the graph shows the effect of auxin on root
On the one hand, an auxin concentration too low to stimulate
shoot cells will cause root cells to elongate.
On the other hand, an auxin concentration high enough to make
stem cells elongate is in the concentration range that inhibits
root cell elongation.
Higher concentrations benefit the plant by promoting stem
growth while inhibiting root growth nearer to the sites of