From this brief description it will be seen that all plants and trees arrange themselves under two great classes, namely, the soft and spongy, or the hard and fibrous-woody structure. The remains of such as have been detected in the carboniferous rocks belong almost exclusively to the former class, the cryptogamiæ and endogenæ, while of the three hundred and upward of fossil species which have been described and figured, not more than ten, and some of these still of doubtful characters, can be regarded as of exogenous and true woody growth. Ferns, mosses, palms, and gigantic succulent plants, now all allied to those of tropical climates, constitute the vast preponderancy of the fossil flora of the age in question. Are we to infer from this that the other families and tribes which at present so abundantly cover the earth were not then in existence? The botanist can now refer to his catalogue of eighty to a hundred thousand species of existing plants, growing in the different regions of the globe, and of widely distinguished habits and forms; and were few or none of these in being then? We possess not, as yet, sufficient data for the solution of this very interesting problem, although in the progress of geological discovery, every year is adding to the list, and giving us a more extended acquaintance with the vegetable products of the coal period. An important experiment recently made by Professor Lindley would seem to favor the probability that a far more numerically abundant flora had then existed. One hundred and seventy plants were thrown into a vessel containing fresh water, and among them were species belonging to all the natural orders of which the flora of the coal-measures consists, and also to other natural orders which it might have been expected would be found associated with them. In the course of two years, one hundred and twenty-one species had disappeared, being entirely decomposed, and of the fifty which remained, the most perfect specimens were those of coniferous plants, ferns, palms, lycopodiaceæ, and the like—the families, all of them, most allied to those preserved in the coal-measures.