The growth of pollen tubes towards the ovule is an example of:

Chemotropism is the growth or movement of a plant or plant part in response to a chemical stimulus. The growth of the pollen tube down the style towards the ovule is guided by chemical signals (like sugars or calcium ions) secreted by the ovule.

• Phototropism is response to light.
• Geotropism is response to gravity.
• Thigmotropism is response to touch.