• Plants response to external stimuli is through plant growth factors:
o They exert their influences by effecting growth which are produced by cells located throughout the plant and not a particular organ.
o Unlike animal hormones, some plant growth factors affect the tissues that release them rather than acting on a distant target
Indoleacetic Acid (IAA)

o IAA is produced in the tips of the shoots/roots
o IAA is initially transported evenly throughout all regions as it begins to move down the shoot

• Phototropism
o Light causes IAA to move away from it, and therefore move towards the shaded side of the shoot
o A greater concentration of IAA builds up on the shaded side compared to the light side causing the elongation of shoot cells (n.b. same number of cells, however now they are elongated)
o The increased growth on the shaded side causes the plant to grow towards the light
• Gravitropism
o Cells in the root tips produce IAA which is transported along the root
o The IAA is initially transport to all sides of the root
o Gravity causes the IAA molecules to move downwards to the bottom of the root
o IAA is a growth inhibitor when in roots causing the area which does not have IAA in it (the top side of the root) to elongated, causing the root to grow downwards.