It is generally believed that parasites evolved from "normal" plants. As haustoria developed and became more specialized, structural parts and physiological processes normally associated with support and nutrient assimilation were reduced until they were no longer capable of supporting the plant, which became dependent on its parasitic habit. For instance, within the mistletoe family there is a complete gradient from terrestrial root-parasites, in which the roots of the treemistletoe parasitize the roots of host plants, to epiphytic shoot parasites, which are completely dependent on their host. This gradient suggests that the epiphytic forms evolved from terrestrial species through increasing specialization of the haustorial organs and reduction of roots and leaves.
The visible effects of a parasite on its host ranges from spectacular malformations, such as "witches' brooms" (a proliferation of short branches by the host at the site of parasite attachment), to no discernable effect. The physiological effects are also variable. While all parasites weaken their host to some degree, in some cases the effect is hardly discernable. This makes sense because it would not be advantageous for the parasite to so weaken its host that the host dies. Nevertheless, this extreme is also evident, as described above for late blight of potato.
See also: Angiosperm Parasites, Fungal Parasites
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