As regards the subterraneous flora, Dr. Schneider's investigations resulted in showing that the plants which thrive in the dark regions under ground are those which possess no chlorophyl and are sensitive to light. Those which vegetate most luxuriantly there are the _fungi_, and among them especially the _pyrenomycetes_, which are frequent in the waters of mines. Their general aspect is shown in a 480 times magnified form in Fig. 2. They resemble fine threads of delicate structure, and where found are always discovered in great abundance. Most conspicuous by their shape and considerable size are the _rhizomorphæ_, Fig. 3a, and they are remarkable, not only for their brilliant phosphorescence, but also for the peculiar fact that they are only found in places where light does not enter. These _rhizomorphæ_, though this is not easily recognizable from their external appearance, also belong to the fungi and are often seen in strings of the length of over a meter and the thickness of a quill, spreading out in peculiar branches and hanging down from moist beams in dark places. Sometimes they grow like seaweed in the water of the mines, and in this case they give much embarrassment to the miners, because they are apt to obstruct the channels constructed for leading off the superfluous water. In the mines of Freiberg these _rhizomorphæ_ exist in great abundance, and Humboldt already mentions specimens of the length of 4½ feet. Miners in Germany call them _zwirn_ (thread). The student of natural sciences, when encountering these peculiar forms of vegetation, will ask in how far they are the product of their surrounding circumstances (i.e., of the absence of light or the presence of moisture), and in order to find a reply to this question experiments have been made to grow these _rhizomorphæ_ under different conditions of existence. These experiments have shown that from several species of _rhizomorphæ_ other ordinary fungi can be developed, and that the subterraneous specimens therefore may be considered a degeneration and variation of the fungi found above the surface of the ground. [Illustration: FIG. 4. a. Agaricus myurus of Hoffmann, a subterranean fungus. b. Himantia villosa, a species of rhizomorpha found in the Upper Harz Mountains.]