As the parasitic seedling gets closer to the host plant, it will experience the shading effect of the host plant's leaves blocking out sunlight. Whilst the parasitic plant does not have eyes, it does have specialized proteins called phytochromes that allow the plant to "see" different types of light, and to determine if it is in full sunlight or in shaded conditions. This helps the parasite to prepare for host contact and invasion.
Finally, the parasite will make contact with a host plant and experience "touch." Many plants respond to touch. For most plants, however, touch is associated with danger. Touch could indicate to a plant that insects are landing on its leaves and trying to eat them. In this case, plants use touch to stop growth and instead spend their valuable energy resources on defense. These plants make a range of toxic chemicals that will stop insects from feeding on them. Some plants also use touch to trigger a strengthening of the leaf cell walls, making it harder for the insects to eat them. For Cuscuta parasites, touch is a positive stimulus, telling the parasite that it has made contact with a potential host plant.

License information: CC BY 4.0
MPAA: G
Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00074