It is to be observed, in conclusion, that the spore-cases of plants, in their indestructibility and richly carbonaceous character, only partake of qualities common to most suberous and epidermal matters, as I have explained in the publications already referred to. Such epidermal and cortical substances are extremely rich in carbon and hydrogen, in this resembling bituminous coal. They are also very little liable to decay, and they resist more than other vegetable matters aqueous infiltration—properties which have caused them to remain unchanged, and to continue free from mineral additions more than other vegetable tissues. These qualities are well seen in the bark of our American white birch. It is no wonder that materials of this kind should constitute considerable portions of such vegetable accumulations as the beds of coal, and that when present in large proportion they should afford richly bituminous beds. All this agrees with the fact, apparent on examination of the common coal, that the greater number of its purest layers consist of the flattened bark of Sigillariæ and similar trees, just as any single flattened trunk embedded in shale becomes a layer of pure coal. It also agrees with the fact that other layers of coal, and also the cannels and earthy bitumens, appear under the microscope to consist of finely comminuted particles, principally of epidermal tissues, not only from the fruits and spore-cases of plants, but also from their leaves and stems. These considerations impress us, just as much as the abundance of spore-cases, with the immense amount of the vegetable matter which has perished during the accumulation of coal, in comparison with that which has been preserved.
I am indebted to Dr. T. Sterry Hunt for the following very valuable information, which at once places in a clear and precise light the chemical relations of epidermal tissue and spores with coal. Dr. Hunt says: "The outer bark of the cork-tree, and the cuticle of many if not all other plants, consists of a highly carbonaceous matter, to which the name of suberin has been given. The spores of Lycopodium also approach to this substance in composition, as will be seen by the following, one of two analyses by Duconi,[CJ] along with which I give the theoretical composition of pure cellulose or woody fibre, according to Payen and Mitscherlich, and an analysis of the suberin of cork, from Quercus suber, from which the ash and 2·5 per cent of cellulose have been deducted.[CK]
[CJ] Liebig and Kopp, “Jahresbuch,” 1847-'48.
[CK] Gmelin, “Handbook,” xv., 145.
| Cellulose. | Cork. | Lycopodium. | |
| Carbon | 44·44 | 65·73 | 64·80 |
| Hydrogen | 6·17 | 8·33 | 8·73 |
| Nitrogen | .... | 1·50 | 6·18 |
| Oxygen | 49·39 | 24·44 | 20·29 |
| Total | 100·00 | 100·00 | 100·00 |
"This difference is not less striking when we reduce the above centesimal analyses to correspond with the formula of cellulose, C24H20O20, and represent cork and Lycopodium as containing twenty-four equivalents of carbon. For comparison I give the composition of specimens of peat, brown coal, lignite, and bituminous coal:[CL]
[CL] “Canadian Naturalist,” vi., 253.
| Cellulose | C24H20O20 |
| Cork | C24H182/10O67/10 |
| Lycopodium | C24H194/10NO56/10 |
| Peat (Vaux) | C24H144/10O10 |
| Brown coal (Schröther) | C24H143/10O106/10 |
| Lignite (Vaux) | C24H113/10O64/10 |
| Bituminous coal (Regnault) | C24H10O33/10 |
“It will be seen from this comparison that, in ultimate composition, cork and Lycopodium are nearer to lignite than to woody fibre, and may be converted into coal with far less loss of carbon and hydrogen than the latter. They in fact approach closer in composition to resins and fats than to wood, and, moreover, like those substances repel water, with which they are not easily moistened, and thus are able to resist those atmospheric influences which effect the decay of woody tissue.”