Auxins are produced at the apices of shoots and roots, but move to the region of cell elongation to bring about their effect. Thus, they function as hormones or 'chemical messengers'.
Phototropism
When a shoot receives light from all sides, the auxins produced at the shoot apex pass down and exert a stimulating effect on the region of cell elongation, causing the shoot to grow evenly and vertically. When a shoot receives light from one side only, it bends and grows towards the light. If the shoot tip is covered, however, the shoot continues to grow vertically. This shows that somehow one sided lighting causes auxins to gather on the illuminated side and more to gather on the shaded side. Thus makes the cells on the shaded side grow and enlarge faster , causing the shoot to bend towards the light.
Geotropism
If a young plant is placed horizontally, more auxins seems to collect on the lower sides of her shoot and root.
In the shoot, the high auxin concentration on the lower side stimulates growth and tends to make this side grow rapidly, so that the shoot bends and grows vertically
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