growth hormones

• Plants respond to some stimuli using growth hormones - these are chemicals that speed up or slow down plant growth. Growth hormones are produced in the growing regions of the plant (e.g. shoot tips and root tips) and they move to where they're needed in the other parts of the plant.
• A growth hormone called gibberellin stimulates seed germination,

  stem elongation, side shoot formation and flowering
• auxins 


  stimulate the growth of shoots by cell elongation
• Auxins are produced in the tips of shoots in flowering plants and diffuse backwards to stimulate the cell just behind the tips to elongate- this is where cell walls become loose and stretchy, so the cells get longer. If the tip of a shoot is removed, no auxin will be available and the shoot stops growing. Auxins stimulate growth in shoots but high concentrations inhibit growth in roots.
• phototropism- auxins move to the shaded part of the roots and shoots causing cells to elongate so that the roots and shoots bend
• geotropism- auxins move to the underside of shoots and roots causing the cells there to elongate and causing them to bend
• Auxins and gibberellins sometimes work together to affect plant growth.

  They're often synergistic - this means that they work together to have a really big effect.
• Auxins and gibberellins are sometimes antagonistic - this means they oppose each other's actions e.g Gibberellins stimulate the growth of side shoots but auxins inhibit the growth of side shoots.