Part 3

This polisher must be turned quite flat and smoothed by a plane, as the willow, from its softness, is peculiarly difficult to turn. It is also of consequence to remark that both sides should be turned, so that the _lap_, when dry, is quite parallel. This _lap_ is most conveniently adapted to the common face chuck of a lathe with a conical screw, so that either surface may be used. This is made evident, when we state that this polisher is always used moist, and, to keep both surfaces parallel, must be entirely plunged in water before using, as both surfaces must be equally moist, otherwise the dry surface will be concave and the moist one convex. The polishing substance used with this _lap_ is putty powder (oxide of tin), which ought to be well washed, to free it from grit. The calcareous fossils being finely ground, are speedily polished by this method. To polish softer substances, a piece of cloth may be spread over the wooden _lap_, and finely-levigated chalk used as a polishing medium.

In order to study fossil plants well, there must be an acquaintance with systematic botany, a knowledge of the microscopical structure of all the organs of plants, such as their roots, stems, barks, leaves, fronds, and fruit; of the markings which they exhibit on their different surfaces, and of the scars which some of them leave when they decay. It is only thus we can expect to determine accurately the living affinities of the fossil. Brongniart says, that before comparing a fossil vegetable with living plants, it is necessary to reconstruct as completely as possible the portion of the plant under examination, to determine the relations of these portions to the other organs of the same plant, and to complete the plant if possible, by seeing whether, in the fossils of the same locality, there may not be some which belong to the same plant. The connection of the different parts of the same plant is one of the most important problems in Palæophytology, and the neglect of it has led to many mistakes. In some instances the data have been sufficient to enable botanists to refer a fossil plant to a genus of the present day, so that we have fossil species of the genera Ulmus, Alnus, Pinus, etc. Sometimes the plant is shown to be allied to a living genus, but differing in some essential point, or wanting something to complete the identity, and it is then marked by the addition of the term _ites_, as Pinites, Thuites, Zamites, etc.

Before drawing conclusions as to the climate or physical condition of the globe at different geological epochs, the botanist must be well informed as to the vegetation of different countries, as to the soils and localities in which certain plants grow, whether on land or in the sea, or in lakes, in dry or marshy ground, in valleys or on mountains, or in estuaries, in hot, temperate, or cold regions. Great caution must be employed also in predicating from one species the conditions of another, inasmuch as different species of the same genus frequently exist in very different habitats, and under almost opposite conditions of moisture and temperature. It is only by a careful consideration of all these particulars that any probable inferences can be drawn as to the condition of the globe. Considering the physiognomy of vegetation at the present day, we find remarkable associations of forms. The Palms, although generally characteristic of very warm countries, are by no means confined to them; Chamærops humilis extending to Europe as far as lat. 43° to 44° N., and C. palmetto in North America to lat. 34° to 36° N., while C. Fortunei, from the north of China, is perfectly hardy in the south of England. Major Madden mentions the association of Palms and Bamboos with Conifers at considerable elevations on the Himalayas. (Edin. Bot. Soc. Trans. iv., p. 185.) Epiphytic Orchids, which usually characterise warm climates, have representatives at great elevations, as Oncidium nubigenum at 14,000 feet in the Andes, and Epidendrum frigidum at from 12,000 to 13,000 feet in the Columbia mountains. These facts point out the care necessary before drawing conclusions as to the climate which fossil plants may be supposed to indicate.

FOSSILIFEROUS ROCKS.

The rocks of which the globe is composed are divided into two great classes--the Stratified or Aqueous, and the Unstratified or Igneous. The stratified rocks frequently contain fossil remains, and are then called fossiliferous; those with no such remains are designated non-fossiliferous or azoic. The igneous unstratified rocks, included under the names of Granitic and Trappean, show no appearance of animal or vegetable remains. Those trap rocks, however, which have been formed of loose volcanic ashes have often enclosed and preserved the remains of plants and animals; while even between the successive beds of old lava-like trap rocks organic remains are sometimes found. Thus, in Antrim, near the Giant's Causeway, deposits containing vegetable remains occur inter-stratified with basaltic rocks. These remains are of Miocene age, and have been referred to coniferous plants, beeches, oaks, plane trees, etc. Similar plants have been discovered in a similar position by the Duke of Argyll in the island of Mull. In trap rocks near Edinburgh, lignite with distinct structure has also been detected. Silicified wood and coal, imbedded in trap rocks, have been seen in Kerguelen's Land. The wood is found enclosed in basalt, whilst the coal crops out in ravines, in close contact with the overlying porphyritic and amygdaloidal greenstone. Hooker has also seen silicified wood, in connection with trap, in Macquarrie's Plains, in Tasmania. Several beds of trap-tuff or ash, formed into solid compact rock by infiltrated carbonate of lime, occur in the north-east of Arran, which contain numerous stems, branches, and fruits of carboniferous plants. These represent the remains of successive forests which grew on this locality, and were one after the other destroyed by the ash-showers poured forth from a neighbouring volcano during its intermittent periods of activity.

Fossil remains are extremely rare in certain rocks, which, from the changes they have undergone, have been denominated Metamorphic. These include Gneiss and Mica-slate, which are stratified rocks subsequently altered by heat and other causes, and so completely metamorphosed that the traces of organisms have been nearly obliterated. Nevertheless, recognisable traces of plant and animal remains have been found in what were recently thought to be azoic rocks. The absence of organic remains in rocks is therefore not sufficient to enable us to state that these rocks were formed before animals or vegetables existed.

The stratified rocks which contain fossils have been divided into three great groups--the Palæozoic, the Secondary, and the Tertiary, or into Palæozoic and Neozoic groups. The formations included under these are exhibited in the following table, taken from Lyell's Manual of Geology:--

1. Recent. } Post Tertiary. } Recent. 2. Post Pliocene. } }

3. Newer Pliocene. } Pliocene. } 4. Older Pliocene. } } } 5. Upper Miocene. } Miocene. } Tertiary } 6. Lower Miocene. } } or } } Cainozoic. } 7. Upper Eocene. } } } 8. Middle Eocene. } Eocene. } } 9. Lower Eocene. } } } } Neozoic. 10. Maestricht Beds. } } } 11. White Chalk. } } } 12 Chloritic Series. } } Secondary } 13. Gault } Cretaceous. } or } 14. Neocomian. } } Mesozoic. } 15. Wealden. } } }

16. Purbeck Beds. } } } 17. Portland Stone. } } } 18. Kimmeridge Clay. } } } 19. Coral Rag. } Jurassic. } } 20. Oxford Clay. } } Secondary } 21. Great or Bath Oolite. } } or } Neozoic. 22. Inferior Oolite. } } Mesozoic. } 23. Lias. } } } } } 24. Upper Trias. } } } 25. Middle Trias. } Triassic. } } 26. Lower Trias. } } }

27. Permian. Permian. } 28. Coal Measures. } } 29. Carboniferous } Carboniferous. } limestone. } } } 30. Upper } { Devonian or } 31. Middle } Devonian. { Old Red } } 32. Lower } { Sandstone. } Primary } } or } Palæozoic. 33. Upper } } Palæozoic. } 34. Lower } Silurian. Silurian. } } 35. Upper } } 36. Lower } Cambrian. Cambrian. } } 37. Upper } } 38. Lower } Laurentian. Laurentian. }

NATURAL ORDERS TO WHICH FOSSIL PLANTS BELONG.

The plants found in different strata are either terrestrial or aquatic, and the latter exhibit species allied to the salt and fresh water vegetables of the present day. Their state of preservation depends much on their structure. Cellular plants have probably in a great measure been destroyed, and hence their rarity; while those having a woody structure have been preserved. The following is the number of fossil genera and species, as compiled from Unger's work on Palæophytology--(Unger, Genera et Species Plantarum Fossilium, 1850).

DICOTYLEDONES. Genera. Species.

Thalamifloræ. 24 84 Calycifloræ 56 182 Corollifloræ 23 60 Monochlamydeæ Angiospermæ 48 221 ------------- Gymnospermæ 56 363

MONOCOTYLEDONES. Petaloideæ 38 130 Glumiferæ 5 12

ACOTYLEDONES. Thallogenæ 31 203 Acrogenæ 121 969 Doubtful 35 197 ---- ---- 437 2421

These plants are arranged in the different strata as follows:--

{Cambrian, Silurian, and Devonian 73 Palæozoic {Carboniferous 683 {Permian 97

{Triassic 115 Mesozoic {Jurassic 294 {Cretaceous 183

{Eocene 414 Cainozoic {Miocene 496 {Pliocene 35

Recent Post-Pliocene 31 ---- Fossil Species. 2421

During the twenty years that have elapsed since this enumeration was made, the number of fossil species has been very greatly increased. The proportion exhibited in this table is likewise greatly altered from the enormous additions made to the Tertiary Flora by Unger, Ettingshausen, and Heer, and from the important contributions by Principal Dawson to the Devonian Flora.

Among the fossil Thalamifloral Dicotyledons, Unger mentions species belonging to the orders--

Magnoliaceæ. Anonaceæ. Nymphæaceæ. Capparidaceæ. Malvaceæ. Byttneriaceæ. Tiliaceæ. Aurantiaceæ. Malpighiaceæ. Aceraceæ. Sapindaceæ. Cedrelaceæ. Zygophyllaceæ. Xanthoxylaceæ. Coriariaceæ.

Among Calycifloral Dicotyledons--

Celastraceæ. Rhamnaceæ. Anacardiaceæ. Amyridaceæ. Leguminosæ. Rosaceæ. Calycanthaceæ. Combretaceæ. Melastomaceæ. Myrtaceæ. Halorageaceæ. Cucurbitaceæ. Cornaceæ. Loranthaceæ. Rubiaceæ.

Among Corollifloral Dicotyledons--

Ericaceæ. Styracaceæ. Ebenaceæ. Aquifoliaceæ. Sapotaceæ. Oleaceæ. Apocynaceæ. Gentianaceæ.

Among Monochlamydeous Angiosperms--

Nyctaginaceæ. Lauraceæ. Proteaceæ. Aquilariaceæ. Samydaceæ. Santalaceæ. Euphorbiaceæ. Urticaceæ. Artocarpaceæ. Ceratophyllaceæ. Salicaceæ. Myricaceæ. Betulaceæ. Altingiaceæ. Platanaceæ. Corylaceæ. Juglandaceæ. Rafflesiaceæ.

Among Monochlamydeous Gymnosperms--

Coniferæ. Taxaceæ. Gnetaceæ. Cycadaceæ.

Among Petaloid Monocotyledons--

Smilaceæ. Orchidaceæ. Zingiberaceæ. Musaceæ. Liliaceæ. Palmæ. Pandanaceæ. Araceæ. Typhaceæ. Naiadaceæ. Restiaceæ.

Among Glumiferous Monocotyledons--

Cyperaceæ. Gramineæ.

Among Acrogenous Acotyledons--

Filices. Marsileaceæ. Lycopodiaceæ. Equisetaceæ. Musci. Hepaticæ.

Among Thallogenous Acotyledons--

Lichenes. Characeæ. Algæ. Fungi.

PERIODS OF VEGETATION AMONG FOSSIL PLANTS.

On taking a general survey of the known fossil plants, Brongniart thought that he could trace three periods of vegetation, characterised by the predominance of certain marked forms of plants. In the ancient period there is a predominance of Acrogenous Cryptogamic plants; this is succeeded by a period in which there is a preponderance of Gymnospermous Dicotyledons; while a third period is marked by the predominance of Angiospermous Dicotyledons. There is thus--1. The reign of Acrogens, which includes the plants of the Devonian, Carboniferous, and Permian periods. During these periods there seems to be a predominance of Ferns, and a great development of arborescent Lycopodiaceæ, such as Lepidodendron and Sigillaria, and with them are associated some Gynmosperms, allied to Araucaria, and some anomalous plants, as Noeggerathia. 2. The reign of Gymnosperms, comprehending the Triassic and Jurassic periods. Here we meet with numerous Coniferæ and Cycadaceæ, while Ferns are less abundant. 3. The reign of Angiosperms, embracing the Cretaceous and the Tertiary periods. This is characterised by the predominance of Angiospermous Dicotyledons, a class of plants which constitute more than three-fourths of the present vegetable productions of the globe, and which appear to have acquired a predominance from the commencement of the Tertiary formations. These plants appear sparingly even at the beginning of the chalk formation in Europe, but are more abundant in this formation as developed in North America.

FLORA OF THE PRIMARY OR PALÆOZOIC PERIOD.

REIGN OF ACROGENS.

In the present day, acrogenous plants are represented by cellular and vascular Cryptogams. In considering fossil plants our attention is specially directed to the latter. In the recent Floras, vascular Acrogens are represented by such plants as Ferns, Lycopods, and Equisetums. Some of them have an arborescent habit, but the greater number are shrubby and herbaceous. Many of them have creeping rhizomes, which are either subterranean, or run along the surface of the ground. One of these arborescent forms is seen in Tree-ferns (Fig. 9). Another form with a rhizome is seen in Fig. 10. The trunks of ferns are marked by scars, which indicate the parts where the bases of the fronds were attached, and where the vascular tissue passes out from the interior (Fig. 11, _a_ and _b_). A transverse section of the stem (Fig. 12) shows a continuous cylinder of scalariform vessels (Fig. 13), enclosing a large mass of cellular tissue frequently penetrated by small scalariform bundles. The cylinder is pierced by meshes, from the inner sides of which rise the vascular bundles going to the leaves, while some of the free bundles of the axis pass through the mesh, carrying with them a portion of the cellular tissue into the petiole. The fructification consists of spore-cases (sporangia), often with an elastic ring round them, containing spores in their interior (Fig. 14).

[Sidenote: Fig. 9. Tree-fern, with a slender cylindrical trunk and a crown of drooping fronds. It is a vascular acrogen.]

[Sidenote: Fig. 10. _Asplenium_; a species of Spleenwort. A. Rhizome, _r_, covered with the bases (stalks or stipes) of the fronds; _f_, fronds in bud, rolled up in a circinate manner (this is very rarely seen in fossil ferns); _g_, fronds bearing fructification on their backs. B. Portion of a frond separated to show the linear sori or clusters of sporangia (spore-cases).]

Among Acrogens of the present day there are also plants belonging to the natural order Lycopodiaceæ or Club-mosses (Fig. 15), having creeping stems, which give rise to leafy branches. The leaves are small, sessile, and moss-like, and the fructification consists of two kinds of cellular bodies, small spores or microspores (Fig. 16), and large spores or macrospores (Fig. 17). They consist of cellular and vascular tissues, the latter occurring in the form of woody, annular, and scalariform vessels, which occupy the axis or central part of the stem. They differ from ferns in the distribution of their vascular bundles. The order is represented also by such plants as Selaginella, Psilotum, Phylloglossum, and Isoetes. In the plant called Isoetes (Quillwort) there is a peculiar short stem which does not increase in height. It produces additions laterally, so that the stem increases in thickness. The leaves continue to multiply, and bear fructification at their bases. They have both large and small spores.

[Sidenote: Fig. 11, _a_. Bifurcating (forked or dichotomous) trunk (caudex) of a Tree-fern (_Alsophila Perrottetiana_), showing the scars (cicatrices) left by the fallen fronds. These scars exhibit the arrangement of the vascular bundles. Fig. 11, _b_. Rhizome of _Lastrea Filix-mas_ (male fern), showing scars of the leaves, _c_, with markings of the vascular bundles.]

[Sidenote: Fig. 12. Transverse section of the stem (caudex) of a Tree-fern (_Cyathea_), showing the arrangement of the cellular and vascular tissue. The cellular tissue of the centre, _m_; that of the circumference, _p_; vascular cylinder, _f v_, consisting of dark-coloured pleurenchyma or ligneous tubes, _f_, and paler vessels, _v_, chiefly scalariform and closed spiral, and pierced by the meshes for the leaf-bundles at _m_; the outer cortical portion connected with the bases of the leaves, _e_.]

[Sidenote: Fig. 13. Scalariform vessels taken from a Tree-fern. They are marked with bars like the steps of a ladder, hence their name. The membrane occasionally disappears, so that the walls are made up of fibres only at some parts.

Fig. 14. Sporangia of a Fern, supported on stalks, _p_, each of which ends in an elastic cellular ring, _s_, partially surrounding the spore-case, and opening it when mature.

Fig. 15. _Lycopodium clavatum_, a common Club-moss. The leafy branch, _l_, ends in a stalk bearing two spikes of fructification, _f_.

Fig. 16. A kidney-shaped 2-valved case, containing small spores (microspores) of Lycopodium.

Fig. 17. Two-valved case, containing large spores (macrospores) of Selaginella.]

[Sidenote: Fig. 18. Fructification of _Equisetum maximum_, Great Water Horse-tail, showing the stalk surrounded by membranous sheaths, _s s_, which are fringed by numerous processes called teeth. The fructification, _f_, at the extremity, is in the form of a cone bearing polygonal scales, under which are spore-cases containing spores with filaments.]

Another important order of vascular Acrogens is the Equisetaceæ or Horse-tails (Fig. 18). These are Cryptogams, having rhizomes, bearing hollow, striated branches, which secrete in their epidermis a considerable amount of silex. These branches are jointed and have membranous sheaths at the articulations, which are whorls of leaves reduced to a very rudimentary condition. The fructification consists of cone-like bodies (Fig. 18, _f_) bearing peltate polygonal scales, under which are spore-cases (Fig. 19), enclosing spores with four hygrometric club-shaped filaments called elaters (Figs. 20 and 21). At the present day some of these plants in tropical regions have stems of 15 or 16 feet high.

[Sidenote: Fig. 19. Polygonal scale, _s_, of a species of Horse-tail (_Equisetum_), bearing membranous sacs, _t_, which open on their inner surface to discharge spores.

Fig. 20. Spore of Equisetum, surrounded by two filaments with club-shaped extremities. The filaments are represented as coiled round the spore.

Fig. 21. Spore of Equisetum, with the filaments (elaters) expanded.]

Among vascular Acrogens is included the natural order Marsileaceæ or Rhizocarpeæ, the Pepperworts (Fig. 22). The order consists of aquatic plants, with creeping stems, bearing leaves, which are either linear, or divided into three or more wedge-shaped portions not unlike clover. The fructification is at the base of the leaf-stalks, and consists of sacs (sporocarps) containing spores of two kinds, microspores and macrospores. The order contains Marsilea, Pilularia, Azolla, and Salvinia.

For a fuller account of Acrogenous plants, see Balfour's Class Book of Botany, p. 954.

These orders are represented in the Palæozoic flora. Many of the fossil species assume a large size, and show a greater degree of development than is seen in their recent congeners. The most important coal plants belong to the Ferns, Lycopods, and Horse-tails. The examination of the structure and conformation of the plants of the present flora assists much in the determination of the fossil carboniferous flora.

[Sidenote: Fig. 22. _Marsilea Fabri_, a species of Pepperwort or Rhizocarp, with a creeping stem, quadrifoliate stalked leaves on one side, and roots on the other. The fructification, _s_, is at the base of the leaves, and consists of sporangia, called sporocarps.]

In the lower Palæozoic strata the plants which have been detected are few. In the Silurian and Cambrian systems, we meet with the remains of ancient marine plants, as well as a few terrestrial species. Even in the still older Laurentian rocks, if the remarkable structure known as Eozoon canadense be considered, as it generally is, an animal, the existence of contemporary plants may be inferred, inasmuch as without vegetable life animals could not obtain food. In the Lower Silurian or Grauwacke, near Girvan, Hugh Miller found a species resembling Zostera in form and appearance. In the Lower Old Red Sandstone of Scotland he detected Fucoids, a Lepidodendron, and Lignite with a distinct Coniferous structure resembling that of Araucaria,[1] besides a remarkable pinnate frond. In the middle Old Red of Forfarshire, as seen in the Arbroath pavement, he found a fern with reniform pinnæ and a Lepidodendron. In the Upper Old Red, near Dunse, a Calamite and the well-known Irish fern Cyclopteris Hibernica occur.[2] This fern, Palæopteris Hibernica of Schimper (Plate I. Figs. 1 to 4), along with Sigillaria dichotoma, is very abundant in beds of the same age in the south of Ireland, from which the specimens described by Edward Forbes were obtained. The fructification has recently been discovered. This shows that the fern belongs to the Hymenophylleæ, and is consequently nearly related to the equally famous Killarney fern, Trichomanes radicans.