sound like wood under the beat of the hammer; and large areas, when thin slices are placed under the microscope, are found in every portion to retain the woody-fibrous structure. III. THE BOTANICAL CHARACTERS of the flora of the coal period form of themselves an interesting subject of study, and suggest some very important considerations as to the history and purpose of the formation. These will be best understood by a reference to the structure and habits of plants in general. Those of the coal, it will thence be seen, belong exclusively to one or two families,—as ferns, palms, and coniferæ,—which seem to have grown in every soil, and to have been adapted to every climate. The most general divisions of existing plants are into the _vasculares_ and _cellulares_. The former kinds all bear flowers, possess a system of spiral vessels, and are termed phonogamous. The latter, on the other hand, are flowerless, have no spiral vessels, and are denominated cryptogamous. Another extensive subdivision of plants proceeds upon their anatomical structure, and the laws which regulate their mode of growth. Thus one class, it has been observed, increase in bulk by additional increments to the outside of all the parts which compose the plant, as the roots, stems, and branches; another, by additions to the inside of all these members: and for this reason the former are called exogenous, and the latter endogenous. In the one case the new or youngest growth is always exterior to the old; and if thus left unprotected, it will be readily admitted that the growth of all such plants would be greatly and constantly endangered by atmospheric as well as innumerable other causes. The remedy provided by nature against this, is a covering of the substance called bark, which is folded round the entire exterior, stem and branches, of the whole exogens, and within which the newly-formed tissue is all safely deposited. No plant, on the other hand, whose growth is from within, needs any such protection, and accordingly none of them—as all the grasses, corns, canes, and fungi—are possessed of bark, or any analogous membrane. The bark is an ephemeral substance, which lasts only for a year, and has annually to be renewed.