u 



KNOWLEDGE. 



[Febeuaey 1, 1894. 



Grand Croix silicified flowers, in which the pollen grains 

 contained in the anthers, as well as the details of the 

 structure of the seed, could be readily discerned. Such 

 materials enable us to trace the entire life-history of a 

 number of fossil species. Earely, indeed, are all the 

 characters of the original plant retained by the fossil ; a 

 very vivid idea of extinct vegetable life can, fortunately, be 

 obtained from the microscopic examination of specimens 

 Ln which the minute structure of the tissues has been 

 preserved. As a rule, it is only in true petrifactions that 

 this occurs. From casts, moulds or impressions which 

 constitute the great bulk of fossils the original substance 

 has, for the most part, been removed ; what remains is 

 generally the merest film of carbonaceous matter in which 

 no structure can be detected. 



In coal, again, the mineralizing process has usually gone 

 so far that the structure of the plants out of which it 

 was formed is almost entirely obliterated. By subjecting 

 exceedingly thin slices of coal to the microscope, Reinsh, 

 however, detected even in anthracite evident traces of 

 the tissues of the higher plants. From most kinds of 

 coal, Schulze also, by maceration in petroleum, soda and 

 other reagents, was able to separate organic fragments 

 showing all the structural elements of wood. Wigan 

 cannel coal and many other gas coals and lignites plainly 

 reveal their vegetable origin, and from Bradford and 

 Fordel collieries samples have been described consisting 

 entirely of the spores and spore-cases of ferns. But our 

 knowledge of the internal structure of extinct plants is 

 mainly derived from fossils, which have resulted from the 

 petrifying action of carbonate of lime or silica held in 

 solution in water. 



At the present day, springs and other natural waters 

 possessing this petrifying power are of frequent occurrence, 

 especially in limestone and volcanic districts. In the 

 vicinity of some of the geysers of the Yellowstone Park 

 of North America, for example, trees, still erect, in all 

 stages of petrifaction may be seen ; grasses, reeds and 

 ferns are converted into stone round the hot springs in the 

 island of St. Michael, and similar phenomena occur in 

 many other localities. Subjected to the action of a petri- 

 fying spring, the substance of a plant is gradually removed, 

 and as each particle dissolves it is replaced by a particle 

 of carbonate of lime or flint, as the case may be. Now 

 this transformation, which we can observe in operation, 

 has occurred very largely in past geological time. The 

 astonishing thing about this process is, that under favour- 

 able circumstances the exchange of material is eii'ected 

 without appreciably altering the structure. Polished 

 sections of completely silicified palm stems from Antigua 

 may be seen in many museums, in which, with the unaided 

 eye, all the details of the endogenous structure can be 

 easily recognized. Specimens of exogenous stems similarly 

 preserved are also met with, exhibiting the pith, annual 

 rings, and bark, almost as perfectly as a newly-felled trea 

 From the lower Eocene of Heme Bay, Carruthers obtained 

 a silicified fern allied to Osmunda, in the cells of which, 

 not only were the starch grains still visible, but traversing 

 the petrified tissues could be clearly traced the mycelium 

 of a parasitic fungus. More remarkable still, when thin 

 sections of petrified wood are examined with the microscope, 

 we find in many cases that its constituent cells not only 

 retain their distinctive shapes, but the markings on the 

 cell-walls are faithfully reproduced. Annular, spiral, 

 reticulated, scalariform, pitted, and border-pitted markings, 

 even the excessively tine cross strire, may be seen as 

 distinctly in the fossil as in sections of fresh plants. Nor 

 is it only thick-walled, lignified cells that are petrified ; 

 even the delicate, thin-walled, transparent cells of the 



cambium layer are often as plain as in a section of living 

 wood. This is the less surprising in view of what happens 

 in the Equisetaceie and other plants containing much 

 silica. The horsetails are practically siHcified while still 

 living. The Charas and Corallines might also be described 

 as living fossils, since they petrify as they grow. 



The oldest plants of which the structure is known are 

 AlgfB from the Silurian strata of Wales. Silicified woods 

 found by Dawson in the lower Devonian of Canada are by 

 him referred to Coniferre, but by Carruthers to the Algse. 

 Hugh Miller discovered wood resembling that of Coniferaj 

 in the lower Old Red Sandstone of Cromarty. Wood of 

 similar character occurs in the middle and upper Devonian, 

 but is much more abundant in the coal measures. Oldham 

 and Halifax are among the more noted English localities. 

 Specimens have also been obtained from Granton and 

 Craigleith quarries, from Coldstream, and Laggan Bay in 

 the island of Arran, where some years ago Wiinch dis- 

 covered a number of stems with well-preserved structure. 

 The volcanic deposits at Burntisland have also yielded 

 considerable quantities. The Fife and Arran plants are 

 referred to the genus Lepidophloios ; they have excited 

 considerable interest among British and foreign geologists. 

 St. Etienne and Autim in France, Chemnitz and Dresden 

 in Germany, are among the most noted localities on the 

 Continent. Of the secondary or Mesozoic formations the 

 oolite is richest in plant remains. Pinites occurs in this 

 formation in the island of Eigg, and Hugh Miller states 

 that cartloads of oolitic wood have been removed from 

 the shore at Helmsdale and burnt for lime. 



The Portland dirt-bed with its remarkable assemblage 

 of fossil cycads, also belongs to the oolitic series. Creta- 

 ceous rocks in Greenland contain abundant remains of 

 vegetation ; woody tissue even occurs occasionally in the 

 flint nodules of the chalk, and the Tertiary strata are 

 particularly rich in fossil plants. Even forests of petrified 

 trees are met with in some parts of the world, such as the 

 eastern coast of Australia, Tasmania, near Cairo, and in 

 several other places. In some of these the structure of 

 the wood has been preserved ; in others, as in the fossil 

 grove at Whiteinch, it has disappeared. 



Fossil wood thus occurs more or less plentifully in all 

 formations except the earliest. The chief interest attaches 

 to examples obtained from the older or paheozoic rocks. 

 Most of these show a remarkable uniformity of structure 

 and greatly resemble the wood of the araucaria or Chili 

 pine. Wood of this type is known as araucaroxylon. 

 From its coniferous character and abundance in Carboni- 

 ferous strata the older geologists were led to infer that 

 cone-bearing trees had been very plentiful during the coal 

 period. This is now known to be an error, for although 

 true conifers existed in palfeozic times, their remains 

 only occur sparingly in Carboniferous and Permian rocks. 

 Wood of the araucarian type is now known to have been 

 produced very largely by the Cordaitre, an extinct family 

 of plants at one time thought to be monocotyledons on 

 account of their grass-like leaves, but since proved to have • 

 the closest affinity with the Cycadaccie. 



As we should naturally expect, it is the harder and 

 tougher portions of vegetable tissue that most frequently 

 appear in petrifactions. The internal substance of a 

 fungus, seaweed or other thallogen, is entirely cellular. 

 If we slice the stalk of the common tangle it is found to 

 present a uniform appearance, and to admit of being cut 

 with equal ease in all directions. t)n cutting across the 

 root-stock of a fern, on the other hand, the knife encounters 

 a circle of thick fibres embedded in the softer substance. 

 These are the fibro-vascular bundles ; they run in a 

 vertical direction through the stem, uniting laterally at 



