There are no phenomena associated with fungi that are of greater interest than those which relate to luminosity. The fact that fungi under some conditions are luminous has long been known, since schoolboys in our juvenile days were in the habit of secreting fragments of rotten wood penetrated by mycelium, in order to exhibit their luminous properties in the dark, and thus astonish their more ignorant or incredulous fellows Rumphius noted its appearance in Amboyna, and Fries, in his Observations, gives the name of _Thelephora phosphorea_ to a species of _Corticium_ now known as _Corticium cæruleum_, on account of its phosphorescence under certain conditions. The same species is the _Auricularia phosphorea_ of Sowerby, but he makes no note of its phosphorescence. Luminosity in fungi "has been observed in various parts of the world, and where the species has been fully developed it has been generally a species of _Agaricus_ which has yielded the phenomenon."[A] One of the best-known species is the _Agaricus olearius_ of the South of Europe, which was examined by Tulasne with especial view to its luminosity.[B] In his introductory remarks, he says that four species only of Agaricus that are luminous appear at present to be known. One of them, _A. olearius_, D. C., is indigenous to Central Europe; another, _A. igneus_, Rumph., comes from Amboyna; the third, _A. noctileucus_, Lév., has been discovered at Manilla by Gaudichaud, in 1836; the last, _A. Gardneri_, Berk., is produced in the Brazilian province of Goyaz, upon dead leaves. As to the _Dematium violaceum_, Pers., the _Himantia candida_, Pers., cited once by Link, and the _Thelephora cærulea_, D. C. _(Corticium cæruleum_, Fr.), Tulasne is of opinion that their phosphorescent properties are still problematical; at least no recent observation confirms them. The phosphorescence of _A. olearius_, D. C., appears to have been first made known by De Candolle, but it seems that he was in error in stating that these phosphorescent properties manifest themselves only at the time of its decomposition. Fries, describing the _Cladosporium umbrinum_, which lives upon the Agaric of the olive-tree, expressed the opinion that the Agaric only owes its phosphorescence to the presence of the mould. This, however, Tulasne denies, for he writes, "I have had the opportunity of observing that the Agaric of the olive is really phosphorescent of itself, and that it is not indebted to any foreign production for the light it emits." Like Delile, he considers that the fungus is only phosphorescent up to the time when it ceases to grow; thus the light which it projects, one might say, is a manifestation of its vegetation.