The   triple  response  of  stems
   mediated by ethylene involves:
<P>
<BR><LI> slowing of stem elongation
<BR><LI> thickening of the stem (which makes it stronger)
<BR><LI> curvature that causes the stem to start growing horizontally.
<P>
   The alternation of vertical and horizontal growth, mediated by absence
   or presence of ethylene, allows a plant to grow past obstacles.