sion on the contiguous fibre, whereby a small cavity is left between them. These markings or cavities very rarely exist on the sides of the fibres opposed to the pith or bark, but are very numerous on the sides parallel to the medullary rays. Wherever the markings occur, the saucer-shaped depression is thick at the circumference and for some distance towards the centre; but in the centre itself there is a spot so extremely thin and minute, that the light, which has to pass through it, becomes decomposed, and the spot looks either green or red, according to the adjustment of the focus.
"Having received from Professor Bailey a specimen of fossil wood which was found at Fredericsberg in Virginia, I perceived, on submitting it to the microscope, that it would easily break into minute fragments in the direction of the woody fibres, which, when carefully viewed, presented a most beautiful example of casts of woody tissue, with numerous spirals traversing the interior. At various points were arranged the ordinary coniferous dots, and to the outside there adhered small bodies of the same size, which projected beyond the outline of the fibre when seen obliquely, each bearing the precise representation of the coniferous disk. In other parts of the field of view were some of the same bodies detached from the sides of the fibres, which left no doubt that they were casts of the cavities existing in the original plant, and proved the correctness of the view above stated respecting the nature of these minute circular markings. Besides these siliceous bodies in the fragments of the fossil, there were others of such a shape as to leave no doubt that they were casts of the interspaces between the cells or woody fibres.
"There is very little doubt now, from the use of chemical tests, that fossil woods for the most part, or perhaps in all cases, still possess portions of the vegetable tissues, which are cemented together into a compact mass by silica, derived from the water to which the specimen had been subjected. It is difficult to account for the lodgement of silica in the tissues of plants; but it is possible that the molecules of silica, which exist as one of their organic constituents, form the first attractive points, to which others are added by the water, until the whole of the portion of the plant, the woody fibres, the vessels and cells, and the interspaces between these organs, is filled (in fact all places which in the recent plant are filled with sap and air), after the manner that the spicules of silica in a sponge form nuclei for the subsequent deposits of flinty matter, until the whole is converted into a shapeless mass like the original sponge.
It follows from these observations, as every fibre, cell and spiral
vessel is a closed sac or tube, that when any regetable tissue becomes fossilized, the silica occupying their interior and their interspaces is, in fact, in detached pieces, each being separated from the adjoining cell or vessel by the intervening walls of the tissue. If fossilization went no further, and there is reason to believe that in some cases it does not, the mass could easily be broken down by slight force, and each original fibre detached from its neighbour on account of the vegetable matter, after long maceration in the silicifying fluid, being almost decomposed. But frequently the process goes further; and as we know how readily vegetable membrane transmits liquids through its substance, it can be easily imagined how silica held in solution in the water would pervade it, and the intercellular spaces and the interior of the woody fibres would be cemented together into one mass of silica.
"The reason why some woods break down more easily than others after being fossilized, I have not yet been able to determine; but it is certain that coniferous woods are found to be the most frequent examples in which the tissue is not cemented, and I imagine that in those woods there is great power of resisting decomposition when immersed in water, or there exists little or no silica as an organized part of their skeleton, so that no points in the membrane for the commencement of deposits are offered; whereas, where silica does exist, the molecules form the first centres, and the whole become cemented together.
"It is thus, I am induced to believe, that silicification in the above instances proceeded so far as to fill the fibres, vessels and cells, and the spaces on their exterior; but as the vegetable membrane was interposed, the complete cohesion of the parts was prevented, and consequently they are now capable of being separated, and the frustules of silica when examined prove to be casts of the interior of the tissues and of the interspaces external to them, thus appearing to offer the most satisfactory evidence respecting the nature of the organs in question.”—p. 149.
IX. An Enumeration of the Plants of the Galapagos Archipelago, with Descriptions of those which are New. By Joseph Dalton Hooker, Esq., M.D., F.L.S., &c.
X. On the Vegetation of the Galapagos Archipelago as compared with that of some other Tropical Islands and of the Continent of America. By Joseph Dalton Hooker, Esq., M.D., F.R.S., F.L.S., &c.
The materials for constructing this Flora have been for the most part furnished by Charles Darwin, Esq., who drew the attention of the
author “to the striking peculiarities which mark the Flora of the Galapagos group, and to the fact that the plants composing it not only differ from those of any other country, but that each of these islands has some particular productions of its own, often representatives of the species which are found in the others of the group." The total number of species is 239, of which upwards of 100 are described as new. We scarcely see how the plants of these islands can be said to differ from those of any other country," since by far the greater number are also natives of North and South America, the West India Islands, many tropical countries, and some few even of Britain. This statement is indeed modified by Dr. Hooker in his remarks on the vegetation of the group, wherein the number of species differing from those of other countries is more properly stated to be one half the entire series; "a peculiarity shared by no other tract of land of equal size, excepting, perhaps, the Sandwich group." The author further states the result of his examination of the plants of the Galapagos to have shown "that the relationship of the Flora to that of the adjacent continent is a double one, the peculiar or new species being for the most part allied to plants of the cooler parts of America, or the uplands of the tropical latitudes, whilst the non-peculiar are the same as abound chiefly in the hot and damper regions, as the West Indian islands and the shores of the Gulf of Mexico; also that, as is the case with the Fauna, many of the species, and these the most remarkable, are confined to one islet of the group, and often represented in others by similar, but specifically very distinct congeners."
The geographical distribution of plants is one of the most interesting branches of botanical science; and to this subject these two papers are a very valuable contribution. Dr. Hooker enters at considerable length into the consideration of the mode by which each district of the earth, whether local or general, originally became possessed of its own peculiar vegetation, and the means whereby the seeds of certain plants were primarily transported and deposited in the localities on which they have subsequently conferred some of the most striking features. These means of transport, as more peculiarly respecting the Galapageian plants, he classes under the several heads of "oceanic and aërial currents, the passage of birds, and man." The conveyance of the majority of the littoral species, as well as of several of the non-littorals, is most probably due to the first-named of these agents; while such species as have small seeds, or seeds furnished with wings or other appendages, may be looked upon as well adapted for conveyance by the winds: and the agency of birds and of man to
the same end is well known.
Dr. Hooker describes the course of the principal oceanic currents by which many of the Galapageian plants have probably been conveyed to their present localities; and concludes this part of his essay by showing the adaptation of the seeds of several plants for transportation, arranged under their natural orders, as follows:
"Menispermeæ.-Cissampelos presents a hard inner coat of the
"Cruciferæ.-Senebiera didyma, the only Galapageian species,
"Malvacea.-Indurated pericarp of many. The floral envelopes
Sapindacea.-Crustaceous testa and exalbuminous seed of Cardiospermum.
Zygophylleæ. - Tribulus cistoides offers singular advantages for transportation in its woody seed-vessels, their spines beset with reversed prickles, and exalbuminous seeds. "Xanthoxylea.-Osseous testa of Xanthoxylum.
"Simarubeæ. - Castela has a crustaceous endocarp and scanty albumen.
"Leguminosa. - Generally firm testa, exalbuminous seeds, and great power of some to retain vitality.
"Rubiacea.-The densely corneous albumen of many may afford a sufficient protection to the seed.
Umbelliferæ.—Helosciadium laciniatum is one of the few species enjoying a wide range, for which I can offer no explanation. "Composite.-Exalbuminous seed. Pappus of Baccharis and adhesive pubescence of Siegesbeckia orientalis.
"Lobeliacea and Scrophularinæ. — Very minute seeds of Scoparia dulcis and Lobelia Xalapensis.
Rhizophora, Avicennia and Scævola.-These all have a predilection for salt water, and constitutional power in the embryo of resisting its destructive effects. Scævola has a hard putamen and scanty carnose albumen; the other species are exalbuminous.
"Apocynes.-Vallesia I believe to be a salt-marsh or sea-side plant; it has a scanty albumen.
"Convolvulaceæ.-These have a scanty mucilaginous albumen.
Two of them, Ipomea maritima and Calystegia Soldanella, are sea-side species, with particularly wide ranges.
"Solaneæ. - Small seeds and adhesive glands of Nicotiana glutinosa; indurated osseous testa of Dictyocalyx, Solanum and Lycopersicum.
"Verbenacea.-Exalbuminous embryo and osseous endocarp of Clerodendron and Lantana.
"Labiata, Cordiacea and Boraginea.-Nucumentaceous pericarps and very scanty albumen. Cordia and Boragineæ are exal
"Acanthacea.-Exalbuminous hooked seeds.
"Plumbagineæ and Plantagineæ. — Viscid glands on calyx, and hooked prickles of some Pisoniæ.
"Euphorbiacea and Urticeæ. -Non-peculiar species of these may have been introduced through the agency of man into Charles Island.
Hypoxidea and Commelineæ offer no apparent facilities for the extraordinary range of the two species that represent these orders.
Cyperacea. These have some facilities for adhesion to foreign substances, and the firm nature of the pericarp, further covered by the coalescing scales of the perianth, are protections. "Graminea. The ciliated glumes of Poa ciliaris and the awns of Setaria Rottleri are the only very evident aids to migration which I can adduce. The resistance of the seed to the action of salt water must be very slight indeed.
Cryptogamia. The excessive minuteness of the sporules in this great class, together with the sporadic appearance of these where they are most minute, and the sudden development of others in suitable situations, leave little doubt that their diffusion by the winds is a never-ceasing though invisible ope