Imágenes de página
PDF

This family of Plants seems destined, like the Cocoa nut Palm, to be among the first vegetable Colonists of new land* just emerging from the ocean; they are found together almost universally by navigators on the rising Coral islands of tropical seas. We have just been considering the history of the fossil stems of Cycadeee in the Isle of Portland, from which we learn that Plants of that now extra-European family were natives of Britain, during the period of the Oolite formation. The unique and beautiful fossil fruit represented in our figures (Plate 63, Figs. 2, 3, 4,) affords probable evidence of the existence of another tropical family nearly alliedto the Pandaneae at the commencement of the great Oolitic series in the Secondary formations.*

In structure this fossil Fruit approaches nearer to Pandanus than to any other living plant, and viewing the peculiarities of the fruit of Pandaheae,f in connexion with the office assigned in the Economy of nature, to this family of sea-side plants, viz. to take the first possession of new-formed land, just emerging from the water, we see in the disposition of light buoyant fibres within the interior of these fruits, an arrangement peculiarly adapted to the office of vegetable colonization.* The sea -side locality of the Pandanese, causes many of their fruits to fall into the water, wherein they are drifted by the winds and waves, until they find a resting place upon some distant shore. A single drupe of Pandanus, thus charged with seeds, transports the elements of vegetation to the rising volcanic and coral islands of the modern Pacific. The seed thus stranded upon new-formed land, produces a plant which has peculiar provision for its support on a surface destitute of soil, by long and large aerial roots protruded above the ground around the lower part of its trunk- (See PI. 68. Fig. 1.) These roots on reaching the ground are calculated to prop up the plant as buttresses surrounding the basis of the stem, so that it can maintain its erect position, and flourish in barren sand on newly elevated reefs, where little soil has yet accumulated.

* This fossil was found by the late Mr. Page, of Bishport near Bristol, in the lower region of the Inferior Oolite formation on the E. of Charmouth, Dorset, and is now in the Oxford Museum. The size of this Fruit is that of a large orange, its surface is occupied by a stellated covering or Epicarpium, composed of hexagonal Tubercles, forming the summits of cells, which occupy the entire circumference of the fruit. (Figs. 2, a. 3, a. 4, a. 8, a.)

Within each cell is contained a single seed, resembling a small grain of Rice more or less compressed, and usually hexagonal, Figs. 5, 6, 7, 8, 10. Where the Epicarpium. is removed, the points of the seeds are seen, thickly studded over the surface of the fruit, (Fig. 2, 3, e.) The Bases of the cells (Fig. 3 and 10 c.) are separated from the receptacle, by a congeries of footstalks (d) formed of a dense mass of fibres, resembling the fibres beneath the base of the seeds of the modern Pandanus (Fig. 13, 14, 15, d.) As this position of the seeds upon foot-stalks composed of long rigid fibres, at a distance from the receptacle, is a character that exists in no other family than the Pandanese, we are hereby enabled to connect our fossil fruit with this remarkable tribe of plants, as a new genus, Podocarya. I owe the suggestion of this name, and much of my information on this subject, to the kindness of my friend, Mr. Robert Brown.

+ The large spherical fruit of Pandanus, hanging on its parent tree is represented at pI. (53, Fig. 1. Fig. 11 is the summit of one of the many Drupes into which this fruit is usually divided. Each cell when not barren contains a single pblong slender seed; the cells in each drupe vary from two to fourteen in number, and many of them are abortive, (Fig. 13.) The seeds within each drupe of Pandanus are enclosed in a hard nut, of which sections are given at Figs. 14, 15. These nuts are wanting in the Podocarya, whose seeds are smaller than those of Pandaneje, and not collected into drupes, but dispersed uniformly in single cells over the entire circumference of the fruit. (See PI. 63, Figs. 3,8,10.) The collection of the seeds into drupes surrounded by a hard nut, in the fruit of Pandanus, forms the essential difference between this genus, and our new genus, Podocarya.

In the fruit of Pandanus, PI. 63, Figs. 11, 16, 17, the summit of each cell is covered with a hard cap or tubercle, irregularly hexagonal, and crowned at its apex with the remains of a withered stigma. We have a similar covering of hexagonal tubercles over the cells of Podocarya (PI. 63, Figs. 2, a. 8, a. 10, a.) The remains of a stigma appear also in the centre of these hexagons above the apex of each seed. (Figs. 8, a. 10, a.)

* There is a similar provision for transporting to distant regions of the ocean, the seeds of the other family of sea.side plants which accompanies the Pandanus, in the buoyant mass of fibrous covering that surrounds the fruit of the Cocoa-nut,

We have as yet discovered no remains of the leaves, or trunk of Pandaneae in a fossil state, but the presence of our unique fruit in the Inferior Oolite formation near Charmouth, carries us back to a point of time, when we know from other evidence that England was in the state of new-born land, emerging from the seas of a tepid climate; and shows that combinations of vegetable structure such as exist in the modern Pandaneae, adapted in a peculiar manner to the office of vegetable colonization, prevailed also at the time when the Oolite rocks were in process of formation.

This fruit also adds a new link to the chain of evidence which makes known to us the Flora of the Secondary periods of geology, and therein discloses fresh proofs of Order, and Harmony, and of Adaptation of peculiar means to peculiar ends; extending backwards from the actual condition of our Planet through the manifold stages of change, which its ancient surface has undergone.*

SECTION IV.

VEGETABLES IN STRATA OF THE TERTIARY SERIES.f

It has been stated that the vegetation of the Tertiary period presents the general character of that of our existing Continents within the Temperate Zone. In Strata of this Series, Dicotyledonous Plants assume nearly the same proportions as at present, and are four or five times more numerous than the Monocotyledonous; and the greater number of fossil Plants, although of extinct species, have much resemblance to living Genera.

* Fruits of another genus of Pandanese, to which Mr. Ad. Brongniart has given the name of Pandanocarpum, (Prodrome, p. 138,) occur together with fruits of Cocoa-nut, at an early period of the Tertiary formations, among the numerous fossil fruits that arc found in the London clay of the Isle of Shep

pey

t See PL 1, Figs. 66 to 72.

I

This third great change in the vegetable kingdom is considered to supply another argument in favour of the opinion, that the temperature of the Atmosphere, has gone on continually diminishing from the first commencement of life upon our globe.

The number of species of plants in the various divisions of the Tertiary strata, is as yet imperfectly known. In 1828, M. Ad. Brongniart considered the number then discovered, but not all described, to be 166. Many of these belonging to Genera at that time not determined. The most striking difference between the vegetables of this and of the preceding periods is the abundance in the Tetiary series, of existing forms of Dicotyledonous Plants and large trees, e. g. Poplars, Willows, Elms, Chestnuts, Sycamores, and many other Genera whose living species are familiar to us.

Some of the most remarkable accumulations of this vegetation are those, which form extensive beds of Lignite and Brown-coal.* In some parts of Germany this Brown-coal occurs in strata of more than thirty feet in thickness, chiefly composed of trees which have been drifted, apparently by fresh-water, from their place of growth, and spread forth in beds, usually alternating with sand and clay, at the bottom of then existing lakes or estuaries.

The Lignite, or beds of imperfect and stinking Coal near Poole in Dorset, Bovey in Devon, and Soissons in France, have been referred to the first, or Eocene period of the Tertiary formations. To the same period probably belongs the Surturbrand of Iceland, (see Henderson's Iceland, vol. ii. p. 114.) and the well-known examples of Brown-coal on the Rhine near Cologne and Bonn, and of the Miesner mountain, and Habichtswald near Cassel. These formations occasionally contain the remains of Palms, and Professor Lindley has lately recognised, among some speci

* See an admirable article on Lignites by Alexandre Brongniart in the 26th vol. of the Dictionnaire des Sciences Naturclles.

[graphic]
[graphic]

* At Putzberg near Bonn, six or seven beds of Brown-coal alternate with beds of sandy clay and plastic clay. The trees in the Brown-coal are not all parallel to the planes of the strata, but cross one another in all directions, like the drifted trees now accumulated in the alluvial plains, and Delta of the Mississippi; (see Lyell's Geology, 3d, edit. vol. i. p. 272.) some of them are occasionally forced even into a vertical position. In one vertical tree at Putzberg, which was three yards in diameter, M. Nflggerath counted 792 concentric rings. In these rings we have a chronometer, which registers the lapse of nearly eight centuries, in that early portion of the Tertiary period which gave birth to the forests, that supplied materials for the formation of the Brown-coal.

The fact mentioned by Faujas that neither roots, branches, or leaves are found attached to the trunks of trees in the Lignite at Bruhl and Liblar near Cologne, seems to show that these trees did not grow on the spot, and that their more perishable parts have been lost during their transport from a distance.

In the Brown-coal Formation near Bonn, and also with the Surturbrand of Iceland, are found Beds that divide into Laminae as thin as paper (Papier Kohle) and arc composed entirely of a congeries of many kinds of leaves. Henderson mentions the leaves of two species of Poplar, resembling the Ptremula and P. balsamifera, and a Pine, resembling the Pinus abies as occurring in the Surturbrand of Iceland.

Although we have followed Brongniart in referring the deposites here enumerated to the first or Eocene period of the Tertiary series, it is not improbable that some of them may be the products of a latter era, in the Miocene or Pliocene periods. Future observations on the Species of their animal and vegetable remains will decide the exact place of each, in the grand Series of the Tertiary formations.

« AnteriorContinuar »