Any one who has wandered among old tree-trunks in search of insects, or been a careful observer in underground nooks and corners, must have seen the white tangles, often of beautiful shape, which constitute the forms of some fungi. They are frequently to be seen under old boards in frost-like designs of great delicacy, and many of these are supposed by some to have a certain relation to luminous woods. Around old tree-stumps, the decayed arms of the oak especially, long, cylindrical, flexible branches with a hard bark covering are often found. When freshly broken, the interior is pure white, later changing to a more or less deep brown tint. The white, flocculent extremities form the mycelium of the fungus known as _Rhizomorpha subterranea_, one of the most interesting of the luminous plants. Its mystic light is often seen in caves, where the rootlets have made their way, gleaming with a soft phosphorescence. In coal-mines this plant is quite common, and has been especially observed near Dresden. Ehrman speaks in enthusiastic terms of these “vegetable glow-worms,” as he calls them, which he observed gleaming on the walls and in the crevices of Swedish mines. In Bohemia the caves are not uncommonly illumined by this interesting cryptogam; and, according to Phipson, sufficient light has been emitted in English coal-mines from this source to enable miners to read ordinary print. In the mines of North Hesse, Germany, the conditions are particularly favorable for such displays, the gleams being described as resembling moonbeams stealing through the gloomy caverns. That this fungus is luminous when detached, is shown by the following from M. Tulasue, in the “Annals of Natural Science,” 1848. “On the evening of the day I received the specimens,” he writes, “the temperature being about 22° C., all the young branches brightened with an uniform phosphoric light the whole of their length. It was the same with the surface of some of the older branches, the greater number of which were still brilliant in some parts, and only on their surface. I split and lacerated many of these twigs, but their internal substance remained dull. The next evening, on the contrary, this substance, having been exposed to contact with the air, exhibited at its surface the same brightness as the bark of the branches. Prolonged friction of the luminous surfaces reduced the brightness, and dried them to a certain degree, but did not leave on the fingers any phosphorescent matter.” And again, “By preserving these _Rhizomorphæ_ in an adequate state of humidity, I have been able for many evenings to renew the examination of their phosphorescence; the commencement of desiccation, long before they really perish, deprives them of the faculty of giving light.”