​What defines a plant parasite?

 

The presence of a haustorium.  That's an organ induced by the parasite when it comes into contact with a potential host.  It is the bridge between the vascular systems of the parasite and host, through which the former extracts resources from its victim.

Haustoria produce by Pedicularis canadensis are really very small. Here is a picture from another hemiparasite, mullein foxglove (Dasistoma macrophylla) that we grew on sugar maple trees in the greenhouse.  The arrows are pointing to the haustoria on some of the hemiparasite’s root in the inset.