TERTIARY PLANTS Introduction.—The great geological divisions into Primary, Secondary and Tertiary eras are based chiefly upon the observa tions of marine strata and marine organisms. The reason for this classification lies in the fact that oceans are more continuous than land, and sea organisms therefore present a more universal basis for classification than land organisms. But changes in land life do not always coincide with changes in marine life, and for this reason a different grouping of geological periods is better for the study of plants. The last great phase in the development of plant life is the corning in of flowering plants. Tertiary botany is chiefly concerned with the history of these; but for their origin we must go back to the Cretaceous.
With the advent of flowering plants the age of world-wide floras is left behind. From this time onwards plants are grouped into localized floras comparable with those of the present day.
Tertiary and Quaternary plant deposits are formed in various ways. Sometimes the portions of the plants are embedded in the silts of ancient lakes, as in central France, Switzerland, and the Rocky mountains ; sometimes in the silts of rivers, as in the beds formed by the old Rhine; or sometimes in the brackish water silts of estuaries, as in the beds of the Hampshire (England) coast. Again plant deposits may be the accumulations of swamps, e.g., the brown coal of Germany; or of successive layers of vege tation occupying the same spot, like the peat of northern Europe. Yet again plants may be buried by volcanic ejecta, as has hap pened in Scotland, Ireland and central France. Or they may have become entangled in resin, as has happened in the case of the Baltic amber. It is from such scattered information, overlapping now in one place, now in another, that the history of Tertiary plant life in Europe is pieced together. Whatever the mode of
origin, there are corresponding differences in the kind of matrix, in the parts of the plant preserved, and, generally, in the com ponents of the floras.
Leaves usually occur in fine clay or tufa, and most frequently as mere impressions showing more or less clearly the form and nervation ; but, in the case of coniferous and evergreen leaves, not infrequently part of the carbonized leaf itself is preserved covered by its tough cuticle. Delicate winged and pappus-bear ing fruits and seeds also occur in fine clays. Other fruits and seeds, also wood, are found embedded in coarse material such as sandy clays, sands and loam, and in peat. Usually if carbonaceous substance is present, it is impregnated with mineral matter. If, as is common, this is iron pyrites, the close intermixture of hard pyrites with rotten carbonaceous matter makes it difficult to examine the objects by sectioning. Sometimes the carbonaceous substance is entirely replaced by silica or calcite, when sectioning becomes possible.
Methods of Study.—Different organs being differently pre served, each part of the plant requires a special method of study. Leaf impressions require no preliminary treatment beyond ex posing any part concealed. When the leaf-blade is preserved, a fragment must be chipped off with a knife and, by treatment with nitric acid and chlorate of potash followed by ammonia, the cuticle can be isolated, mounted on a microscope slide, studied and photographed. Winged and pappus-bearing fruits and seeds must be studied in the same way. Other fruits and seeds are isolated by disintegrating the matrix. This may be done by boil ing the seed-bearing material in a strong soda solution, or warm ing in a to% solution of nitric acid over a water bath. The dis solved matrix is then strained off through sieves, leaving the fossils behind. These can be examined as free entities. In the investigation of pollen grains the material is boiled with a r o% solution of caustic potash. Wood, being large, is usually picked out of the matrix with a knife or other instrument, and, if possi ble, sectioned and studied microscopically.