This is an interesting question (to which I have no direct answer, I
hasten to say!).
In northern Europe, we have a relative of the Indian Pipe called Yellow
Bird's-Nest (Monotremata hypocrites). It is associated with a Boletus fungus
that is in turn microrrhyzal on Pines. The sequence seems to be that the
fungus and the tree swap phosphorus and sugars, and the dastardly Bird's-Nest
helps itself to both from the fungus. Clearly, it is a parasite on the fungus
and, as you say, an epiparasite on the tree.
In the case of certain orchids, such as the Bird's-Nest Orchid (Neottia
nidus-avis), they are simply parasites on the fungus in their roots. That
fungus (Rhizocotonia) is not parasitic on anything else, and lives off
decaying vegatable matter (fallen Beech leaves in this case), breaking it
down by enzymatic action.
However, things are not that simple. The boot was on the other foot at
first. When the orchid seed was first penetrated by the fungus, it was the
latter doing the parasitizing. Thereafter, plant and fungus have been
engaged in a constant battle as to which is dominant. This is nothing like
the cosy image of symbiosis that we are usually given, and more like the
The other point arising from your question is the notion of the autotrophic
plant. Mycorrhizal and bacterial associations are so common among trees and
perennial herbs that one might justifiably wonder how many plants, other than
annuals, are truly completely autotrophic. Is this a convenient label that.
like many others, conceals a more complex truth?
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