The spores of the truffles are in the soil, and the mycelium seems to maintain some symbiotic relation with the roots of the young trees, which results in the increase in the production of the fruit bodies.
"Studies of American Fungi. Mushrooms, Edible, Poisonous, etc."
George Francis Atkinson
The light of one animal at least, and I believe many others also, cannot be due to any sort of symbiotic organism.
"The Nature of Animal Light"
E. Newton Harvey
This would point to the symbiotic character of galls and their guests.
"Disease in Plants"
H. Marshall Ward