Imágenes de página
PDF

light of day, and a second time committed to the waters, it has, by the aid of navigation, been conveyed to the scene of its next and most considerable change by fire; a change during which it becomes subservient to the most important wants and conveniences of Man. In this seventh stage of its long eventful history, it seems to the vulgar eye to undergo annihilation; its Elements are indeed released from the mineral combinations they have maintained for ages, but their apparent destruction is only the commencement of new successions of change and of activity. Set free from their long imprisonment, they return to their native Atmosphere, from which they were absorbed to take part in the primeval vegetation of the Earth. To-morrow, they may contribute to the substance of timber, in the Trees of our existing forests; and having for a while resumed their place in the living vegetable kingdom, may, ere long be applied a second time to the use and benefit of man. And when decay or fire shall once more consign them to the earth, or to the atmosphere, the same Elements will enter on some farther department, of their perpetual ministration, in the economy of the material world.

Fossil Coniferce.*

The Coniferae form a large and very important tribe among living plants, which are characterized, not only by peculiarities in their fructification (as Gymnospermous phanerogamicB,f) but also by certain remarkable arrangements in the structure of their wood, whereby the smallest fragment may be identified.

* See PI. 1. Figs. 1. 31. 32. 69.

t We owe to Mr. Brown, the important discovery, that Conifera? and Cycadeie are the only two families of plants that have their seeds originally naked, and not enclosed within an Ovary. (See Appendix to Captain King's Voyage to Australia.) They have for this reason been arranged in a distinct order, as Gymnospermous Phanerogamic. This peculiarity in the Ovulum is accompanied throughout both these families, by peculiarities in the internal structure of their stems, in which they differ from almost all dicotyledonous plants, and in some respects also from each other.

Recent microscopic examinations of fossil woods have led to the recognition of an internal structure, resembling that of existing Coniferae, in the trunks of large trees, both in the Carboniferous series,* and throughout the Secondary formations ;f and M. Ad. Brongniart has enumerated twenty species of fossil Coniferae in strata of the Tertiary series. Many of these last approach more closely to existing Genera than those in the Secondary strata, and some are referable to them.

It has been farther shown by Nicol, (Edin. New Phil. Journal, January, 1834) that some of the most ancient fossil Coniferae may be referred to the existing genus Pinus, and others to that of Araucaria; the latter of these comprehends some of the tallest among living trees, (See PI . 1, Fig. 1) and is best known in the Araucaria excelsa, or Norfolk Island Pine.

The recognition of these peculiar characters in the structure of the stem, is especially important to the Geological Botanist, because the stems of plants are often the only parts which are found preserved in a fossil state.

* The occurrence of large coniferous trees in strata of the great Coal formation, was first announced in Mr. Witham's Fossil Vegetables, 1831. It was here stated that the higher and more complex organizations of Coniferse exists in the Coal fields of Edinburgh and Newcastle, in strata which till lately have been supposed to contain only the simpler forms of vegetable structure.

t In the lower region of the Secondary strata, M. Ad. Brongniart has enumerated, among the fossil plants of the New red sand-stone of the Vosges, four species of Voltzia, a new genus of Conifers, having near affinities to the Araucaria and Cunninghamia. Branches, leaves, and cones of this genus are most abundant at Sultz les Bains, near Strasburgh.

Mr. Witham reckons eight species of Conifera? among the fossil woods of the Lias; and five species, of which four are allied to the existing genus Thuia, occur in the Oolite formation of Stonesfield. (See Ad. Brongniart's Prod. p. 200.) For figures of Cones from the Lias and Greensand near Lyme Regis, and the Inferior oolite of Northamptonshire, see Lindly and Hutton's Fossil Flora, Plates 89,135, 137.

Dr. Fitton has described and figured two very beautiful and perfect cones, one from Purbeck? and one from the Hastings sand. GeoL Trans. 2d. Series, Vol. iv. PI. 22, Figs.9, 10, p. 181 and 230.

These discoveries are highly important, as they afford examples among the earliest remains of vegetable life, of identity in minute details of internal organization, between the most ancient trees of the primeval forests of our globe, and some of the largest living Coniferae.*

The structure of Araucarias alone has been as yet iden

• The transverse section of any coniferous wood in addition to the radiating and concentric lines represented PI. 56a, Fig. 7, exhibits under the microscope a system of reticulations by which coniferse are distinguishable from other plants. The form of these reticulations magnified 400 times is given in PI. 56a, Figs. 2, 4, 6. These apertures are transverse sections of the same vessels, which are seen in a longitudinal section at PI. 56a; Fig. 8, cut from the centre towards the bark, and parallel to the medullary rays. These vessels exhibit a characteristic and beautiful structure, whereby a distinction is marked between the true Pines and Araucarias. In such a section the small and uniform longitudinal vessels, (PI. 56a, Fig. 8) which constitute the woody fibre, present at intervals a remarkable appearance of small, nearly circular figures disposed in vertical rows (See PI. 56a, Figs. 1, 3, 5.) These objects under the name of glands or discs, are differently arranged in different species; they are generally circular, but sometimes elliptical, and when near each other, become angular. Each of these discs has near its centre a smaller circular areola. PI. 56a, Fig. 1, represents their appearance in the Pinus strobus of North America.

In some Conifers, the discs are in single rows; in others, in double as well as single rows, e. g. in Pinus strobus, PI. 56a, Fig. 1.

Throughout the entire genus of the living Pines, when double rows of discs occur in one vessel, the discs of both rows are placed side by side, and never alternate, and the number of the rows of discs is never more than two.

In the Araucarias the groups of discs are arranged in single, double, triple and sometimes quadruple rows, see PI. 56, Fig, 3. 5. They are much smaller than those in the true Pines, scarcely half their size, and in the double rows they always alternate with each other, and are sometimes circular, but mostly polygonal. Mr. Nicol has counted a row of not less than fifty discs in a length the twentieth part of an inch, the diameter of each disc not exceeding the thousandth part of an inch; but even the smallest of these are of enormous size, when compared with the fibres of the partitions bounding the vessels in which they occur.

lifted in trees from the Carboniferous series of Britain.* That of ordinary Pines occurs in wood from the Coal formation of Nova Scotia and New Holland.

The same ordinary structure of Pines predominates in the fossil wood of the Lias at Whitby; trunks of Araucarias also are found there in the same Lias; and branches, with the leaves still adhering to them, in the Lias at Lyme Regis.f

Professor Lindley justly remarks that it is an important fact, that at the period of the deposite of the Lias, the vegetation was similar to that of the Southern Hemisphere, not alone in the single fact of the presence of Cycadeae, but that the Pines were also of the nature of species now found only to the south of the Equator. Of the four recent species of Araucaria at present known, one is found on the east coast of New Holland, another in Norfolk Island, a third in Brazil, and the fourth in Chili. (Foss. Flora, vol. ii. p. 21.)

Whatever result may follow from future investigations, our present information shows that the largest and most perfect fossil Coniferae, which have been as yet sufficiently examined from the Coal formation and the Lias, are referable either to the genus Pinus, or Araucaria,J and that both these modifications of the existing Family of Coniferae date their commencement from that very ancient period, when the Carboniferous strata of the Transition formation were deposited.

* A trunk of Araucarias forty-seven feet long was found in Cragleith Quarry near Edinburgh, 1830. (See Witham's Fossil Vegetables, 1833, PI. 5.) Another, three feet in diameter, and more than twenty-four feet long, was discovered in the same quarries in 1833. (See Nicol on Fossil Conifers, Edin. New Phil. Journal, Jan., 1834.) The longitudinal sections of this Tree exhibit, like the recent Araucaria excelsa, small polygonal discs, arranged in double, and triple quadruple rows within the longitudinal vessels; so also does a similar section from the Coal-field of New-Holland.

t See Lindley and Button's Fossil Flora, PI. 88. A fossil cone referable to Coniferse, and possibly to the genus Araucaria, from the Lias of Lyme Begis, is represented at Plate 89 of the same work.

t Mr. Nicol states that in fossil woods from the Whitby Lias, when concentric layers are distinctly marked on their transverse section, (PI. 56a, Fig. 2, a, a) the longitudinal sections have also the structure of Pinus (PI. 56a, Fig. 1.;) but when the transverse section exhibits no distinct annual layers, (PI. 56a, Fig. 4.) or has them but slightly indicated,. (PI. 56a. Fig. 6. a) the longitudinal section has the characters of Araucaria. (PI. 56a. Fig. 3, 5.) So also those Coniferte of the great Coal formation of Edinburgh and Newcastle, which exhibit the structure of Araucaria in their longitudinal section, have no distinct concentric layers; whilst in the fossil Conifers from the New Holland and Nova Scotia Coalfield, both longitudinal and transverse sections agree with those of the recent tribe of Pinus.

Fragments of trunks of Coniferous wood, and occasionally leaves and cones occur through all stages of the Oolite formation, from the Lias to the Portland stone. On the upper surface of the Portland stone, we find the remains of an ancient forest, in which are preserved large prostrate silicified stumps of Coniferae, having their roots still fixed in the black vegetable mould in which they grew. Fragments of coniferous wood are also frequent throughout the Wealden and Greensand formations, and occur occasionally in Chalk.*

It appears that the Coniferae are common to fossiliferous strata of all periods; they are least abundant in the Transition series, more numerous in the Secondary, and most frequent in the Tertiary series. Hence we learn that there has been no time since the commencement of terrestrial vegetation on the surface of our Globe, in which large Coniferous trees did not exist; but our present evidence is insufficient, to ascertain with accuracy the proportions they bore to the

Mr. Witham also observes that the Conifera e of the Coal formation, and mountain limestone group, have few and slight appearances of the concentric lines, by which the annual layers of the wood are separated, which is also frequently the case with the Trees of our present tropical regions, and from this circumstance conjectures that, at the epochs of these formations, the changes of season, as to temperature at least were not abrupt.

* There is in the Oxford Museum a fragment of silicified coniferous wood, perforated by Teredines, found by Rev. Dr. Faussett, in a chalk flint at Lower Hardrcs, near Canterbury.

« AnteriorContinuar »