Positive Phototropism

• A plant part that moves to face a light source is said to be positively phototropic. This is common in plant leaves, stems and flower buds.
  
  

Negative Phototropism

• Some plant parts respond to light by turning away from it and are negatively phototropic. Root tips are an example of this since most plants require their roots to be underground. The roots grow away from light into the soil.
  
  

Mechanism

• The turning or movement of plants in response to light is a growth reaction. Cells on one side of the plant grow faster than cells on the other side, causing the plant to turn. Individual cell walls are also affected -- those closer to the light source shorten or elongate depending on whether the plant part is positively or negatively phototropic.
  
  

Light Requirements

• Some plants exhibit both positive and negative phototropic reactions to light depending on the intensity of the light source. If the light source is too strong, they grow away from the light; too weak and they grow towards the light.