| Thallophytes (Thallus plants) | Algæ and algoid forms | Cyanophyceæ, blue-green algæ. | |
| Chlorophyceæ, green algæ. | |||
| Rhodophyceæ, red algæ. | |||
| Phæophyceæ, brown algæ. | |||
| Diatomaceæ, diatoms. | |||
| Coccospheres | Pelagic algæ(?). | ||
| Rhabdospheres | |||
| Charophyta, stoneworts. | |||
| Fungi and fungoid forms | Phycomycetes, algæ-fungi, water-molds. | ||
| Ascomycetes, ascus-fungi, mildews. | |||
| Basidiomycetes, basidium-fungi, mushrooms. | |||
| Æcidiomycetes, æcidium-fungi, “rusts.” | |||
| Schizomycetes, “fission-fungi,” bacteria. | |||
| Myxomycetes, “animal fungi,” slime-molds. | |||
| Lichens | Symbiont algæ and fungi. | ||
| Bryophytes (Moss plants) | Hepaticæ, liverworts. | ||
| Musci, mosses. | |||
| Pteridophytes (Fern plants) | Filicales | Filices, true ferns. | |
| Cycadofilices, cycad-ferns. | |||
| Equisetales | Equisetæ, scouring-rushes, horsetails. | ||
| Calamites. | |||
| Sphenophyllales. | |||
| Lycopodiales | Lycopodiaceæ, club-mosses. | ||
| Lepidodendra. | |||
| Sigillaria and stigmaria. | |||
| Spermatophytes (Seed plants) | Gymnospermæ (Naked seed) | Cordaiteæ, cordaites. | |
| Cycadales (cycads) | Bennettiteæ. | ||
| Cycadaceæ. | |||
| Coniferæ, evergreens. | |||
| Ginkgoaceæ, ginkgo. | |||
| Angiospermæ (Covered Seed) (Flowering plants) | Monocotyledoneæ, cereals, grasses, etc. (one-leafed seed). | ||
| Dicotyledoneæ, oaks, poplars, peas, etc. (two-leafed seed). | |||
The contribution of the Thallophytes (algæ, fungi, bacteria).—The Thallophytes embrace the simplest types of plants, and are probably the nearest present representatives of the ancestral forms. Some of them are minute one-celled organisms, as simple as an organism can well be conceived to be. The simple blue-green algæ of our fresh waters well represent this class. The most are, however, multicellular, and some (as the great seaweeds) rise to a degree of complexity and of a bodily segmentation resembling that of the higher plants. The various species are adapted to an extremely wide range of conditions; some live in hot springs at 170° Fahr., and some in Arctic seas at the freezing-point; some flourish in fresh water, some in brackish, some in salt water, and some even out of the water. This wide adaptation implies an ancient and plastic type. The fact that they flourish in waters so hot and sometimes also so sulphurous as to be fatal to most plants, suggests the possibility of their introduction during the very early volcanic stages of the earth, while conditions were yet uncongenial for other plants.
The geologic work of the thermal algæ is well shown in the beautiful travertine and sinter deposits of the Yellowstone Park (Figs. [215] and [218]). At the Mammoth Hot Springs the deposits are calcareous, while at most of the other hot springs silicious deposits are formed, in both cases partly, but not wholly, by the aid of algæ. The beautiful yellows, reds, browns, and greens of these springs are not mineral coloring, but living plants.[291] In the calcareous waters, the algæ are believed to cause the deposition of calcium carbonate from calcium bicarbonate by consuming the second equivalent of carbon dioxide that rendered the carbonate soluble.[292] In the silicious waters, the process of deposition is not understood. Similar deposits by the aid of algæ take place in the geyser regions of Iceland and of New Zealand, in the hot springs of Carlsbad, where they have been well studied by Cohn,[292] and in most other hot springs. The same, or very similar, forms of algæ abound in nearly all waters, fresh and salt, but the question whether they make calcareous and silicious deposits in notable quantity appears not to have received as yet the critical investigation its importance deserves, except in a few special cases. It is clear, however, that in the cool waters such deposits do not reach the conspicuous amounts that they attain in the thermal springs. In the shallow waters of the ocean, especially in the warmer regions, lime-secreting algæ are abundant and make large contributions to the lime deposits.
Among the higher algæ are the lime-secreting corallines or nullipores (Rhodophyceæ, red algæ), once regarded as animals, which contribute a notable part of the calcareous substance of coral reefs. They are important geologic agents in the temperate and tropical seas, and have been traced as far back in time as the early Paleozoic era.
The Challenger reports[293] describe two forms of minute calcareous spherical organisms, Rhabdospheres and Coccospheres, as very abundant in the surface-waters of the temperate and tropical seas, and as important in contributing to the calcareous deposits of the sea-bottoms. The affinities of these bodies are in doubt, but they are regarded by Murray as probably pelagic algæ.
The stoneworts (Characeæ), an aberrant group of algæ inhabiting fresh and brackish water, secrete notable quantities of calcium carbonate in and around their tissues, and the accumulation of these gives rise to marl or limestone. It has recently been urged that our so-called shell-marls are mainly due to Charæ,[294] the molluscan shells being incidental rather than essential constituents.
In very ancient and also in some of the later strata, there are limestones that do not carry any visible fossils, and their origin is, therefore, debatable. There are also not a few limestones that are made up of a fine-grained base through which are scattered molluscan shells, corals, etc., in a fine state of preservation. The condition of these fossils bears rather adversely on the view that shells, etc., have been powdered in sufficient numbers and to a sufficient degree to form the compact base. In all these cases the usual explanations leave something to be desired. It is worth considering whether low forms of plants may not be among the undemonstrated agents in forming these apparently unfossiliferous limestones or parts of limestones. The calcium carbonate deposited by the algæ is in minute and delicate form, and is usually crystalline while yet in the living tissues. It is, therefore, easily subject to comminution and to such further crystallization as would obscure the minute features that constitute the evidences of algal origin.
The more complex and conspicuous algæ, the seaweeds, have left impressions of their stems and fronds on the marine beds of most of the periods, but they are usually obscure. Seaweeds are perhaps the source of the vegetal matter in certain carbonaceous shales and limestones. As seaweeds extract bromine and iodine and certain metallic ingredients from the sea-water, some of the iodine and bromine springs issuing from ancient marine deposits, and certain ores, may owe their origin to ancient seaweeds.
Diatoms, minute plants of the Thallophyte group, secrete a delicate framework of silica which becomes a contribution to the silicious deposits. Diatoms have sometimes contributed the material for very considerable beds, such as those of the ooze-bogs now forming in the marshes of the geyser basins of the Yellowstone Park,[295] and the diatom oozes of the deep sea ([Fig. 353], [p. 425]).
Fungi, for obvious reasons, have left but scant traces of themselves.