PETRIFIED WOOD.
In 1836 Stokes described certain stems in which the tissues had been partially mineralised. In describing a specimen of beech from a Roman aqueduct at Eibsen in Lippe Bückeburg], he says:—
“The wood is, for the most part, in the state of very old dry wood, but there are several insulated portions, in which the place of the wood has been taken by carbonate of lime. These portions, as seen on the surface of the horizontal section, are irregularly circular, varying in size, but generally a little less or more than ⅛ inch in diameter, and they run through the whole thickness of the specimen in separate, perpendicular columns. The vessels of the wood are distinctly visible in the carbonate of lime, and are more perfect in their form and size in those portions of the specimen than in that which remains unchanged[114].”
Fig. 14.
- Araucarioxylon Withami (L. and H.). Radiating lines of crystallisation in secondary wood, as seen in transverse section.
- Lepidodendron sp. Concentric lines of crystallisation, and scalariform tracheids, as seen in longitudinal section.
This partial petrifaction of the structure in patches is often met with in fossil stems, and may be seriously misleading to those unfamiliar with the appearance presented by the crystallisation of silica from scattered centres in a mass of vegetable tissue. A good example of this is afforded by the gigantic stems discovered in 1829 in the Craigleith Quarry near Edinburgh[115]. Of those two large stems found in the Sandstone rock, the longest, originally 11 meters long and 3·3–3·9 meters in girth, is now set up in the grounds of the British Museum, and a large polished section (1 m. × 87 cm.) is exhibited in the Fossil-plant Gallery. The other stem is in the Botanic Garden, Edinburgh. Transverse sections of the wood of the London specimen show scattered circular patches (fig. 14 A) in the mineralised wood in which the tracheids are very clearly preserved; while in the other portion the preservation is much less perfect. The patch of tissue in fig. 14 A shows a portion of the wood of the Craigleith tree [Araucarioxylon Withami (L. and H.)] in which the mineral matter, consisting of dolomite with a little silica here and there, has crystallised in such a manner as to produce what is practically a cone-in-cone structure on a small scale, which has partially obliterated the structural features. This minute cone-in-cone structure is not uncommon in petrified tissues; it is precisely similar in appearance to that described by Cole[116] in certain minerals. The crystallisation has been set up along lines radiating from different centres, and the particles of the tissue have been pushed as it were along these lines.
Fig. 15. Transverse section of the central cylinder of a Carboniferous Lepidodendroid stem in the collection of Mr Kidston. From Dalmeny, Scotland. s. Silica filling up the central portion of the pith. p. Remains of the pith tissue. x1. Primary xylem. x2. Secondary xylem. c. Innermost cortex.
PRESERVATION OF TISSUES.