Tropism: a plant's growth response to a external stimuls
A phototrophism is the plants response to light
Auxins are produced in the tips of shoots in plants
for phototropisms- auxins will move to the side of the stem in the shade and causes cell elongation. this results in uneven growth and the shoot to bend towards the light.
Auxins stimulate the growth of the apicla bud. if the tip is removed then no more auxins are produced so the auxin conentration is lower resulting in the growth of side shoots to a plant
Just had a look at BBC bitesize- it's actually wrong for some of it:/
but hope that makes sense:) ask if you have any questions