found on iron, lead, etc., certainly not living at the expense of these metals, the distinction of the aerial nutrition of Lichens from the parasitical of Fungi evidently does not hold good in all cases. The presence of green spores is very constant, but their absence in forms like Alrothallus makes Lichens of this kind undistinguishable from Fungi. Lichens, as a rule, are aerial plants; yet some forms are always immersed in water, as in most Algae. The early stages of many Lichens resemble so closely certain Algae that botanists cannot separate them. The Lichens are considered by most naturalists as standing between the Algae and Fungi. According to Haeckel ("Natural History of Creation," p. 416), "each Lichen is composed essentially of two different plants, of a low form of Alga (Nostoc, Protococcus) (Fig. 111) and of a parasitic Fungus (Ascomycetes) (Fig. 110), which is parasitic on the first, and lives off the assimilated material which this furnishes. The green chlorophyll-holding cells (gonidia), which one finds in every Lichen, belong to the Alga. The colorless threads (hyphi), on the contrary, which, thickly woven, form the principal mass of the body of the Lichen, belong to the parasitic Fungus. But always are both plant-forms—Fungus and Alga, which are considered as belonging to different classes—so firmly bound with one another, and so intimately grown together, that every one regards the Lichen as a single organism."
Notwithstanding the differences in size, color, form, reproduction, and habitat seen in this brief survey, the structure of Algae, Fungi, and Lichens has always appeared to be the same, cellular. When we compare plants apparently so distinct as mushrooms, mildews, encrusting matter of rocks, greenish layers of ponds, sea-weed, etc., the closest examination rarely reveals more than a combination of cells, no Alga, Fungus, or Lichen offering us the distinction of stem, leaves, vessels, or flowers observed in the
