The following are some general properties of parasitic plants.
1. Nutrients and water are transported via a physiological bridge, called the haustorium.
2. A parasite connects its vascular system (at least one of the tissues) to that of the host plant.
3. The parasite may totally discard its own photosynthesis.
4. Parasites may be mostly exposed at the surface of the host (epiparasite) or mostly hidden within the host organ (endoparasite). The endoparasitic portion is composed of thread-like haustoria permeating the host tissue with a sinker, a single structure that becomes embedded in the host tissue.
Parasites become established via germination. Seeds land on the host tissue, for stem parasites especially in bird droppings, and germinate after reading a chemical stimulus from the host. A modified lateral root becomes an haustorium; this root is chemotrophic, i.e., responding to a chemical gradient, and contacts the host epidermis. The root then attaches by pushing against the plant and forming a disc, called a hapteron, and secretes a polysaccharide adhesive. The root tip then mechanically penetrates the host, apparently without enzymatic digestion, and establishes a vascular connection by attaching vessels and positioning phloem next to leaky host phloem.
Mistletoes were formerly alleged to receive no host carbohydrates, but substantial heterotropic carbon gain has been measured in mistletoes, even without phloem connections. Direct xylem-to-xylem continuity between host and parasite is not easy to demonstrate. Mistletoes often exhibit high transpiration rates during the day, through stomates and cracks in the epidermis. Losing substantial water from the leaves and stems of the parasite results in a steep water potential gradient, favorable to drawing water into the mistletoe plant. Nitrogen is supplied to the parasite in the xylem stream, and the high transpiration rates, hence, high water demands appear instead to represent a nitrogen-gathering mechanism for the mistletoe.
Typical thick, fleshy root parasites generally lack any adaptations to restrict water loss from their achlorophyllous stems and leaves, because they tend to lack the waxy coating, cuticle.
Dodder and mistletoes are serious problems for plants. Dodder is weedy and can cover woody plants and damage certain economically important crop plants. Mistletoe can become so abundant on a tree that most of the foliage is of the parasite not the host. In general, experts generally state that parasitic plants rarely, perhaps never, kill the host plant, so that the host and parasite live unhappily together is some balance.
http://www.botgard.ucla.edu/html/botanytextbooks/lifeforms/parasiticplants/generalproperties.html
