In any case the Thread-plants cannot be considered as the progenitors of any of the higher vegetable cla.s.ses. Lichens, as well as fungi, are distinct from the higher plants in the composition of their soft bodies, consisting as it does of a dense felt-work of very long, variously interwoven, and peculiar threads or chains of cells-the so-called _hyphae_, on which account we distinguish them as a province under the name Thread-plants. From their peculiar nature they could not leave any important fossil remains, and consequently we can form only a very vague guess at their palaeontological development.

The first cla.s.s of Thread-plants, the _Fungi_, exhibit a very close relationship to the lowest Algae; the Algo-fungi, or Phycomycetes (the Saprolegniae and Peronosporae) in reality only differ from the bladder-wracks and Siphoneae (the Vaucheria and Caulerpa) mentioned previously by the want of leaf-green, or chlorophyll. But, on the other hand, all genuine Fungi have so many peculiarities, and deviate so much from other plants, especially in their mode of taking food, that they might be considered as an entirely distinct province of the vegetable kingdom.

Other plants live mostly upon inorganic food, upon simple combinations which they render more complicated. They produce protoplasm by the combination of water, carbonic acid, and ammonia. They take in carbonic acid and give out oxygen. But the Fungi, like animals, live upon organic food, consisting of complicated combinations of carbon, which they receive from other organisms and a.s.similate. They inhale oxygen and give out carbonic acid like animals. They also never form leaf-green, or chlorophyll, which is so characteristic of most other plants. In like manner they never produce starch. Hence many eminent botanists have repeatedly proposed to remove the Fungi completely out of the vegetable kingdom, and to regard them as a special and third kingdom, between that of animals and plants. By this means our kingdom of Protista would be considerably increased. The Fungi in this case would, in the first place, be allied to the so-called "slime moulds," or Myxomycetes (which, however, never form any hyphae). But as many Fungi propagate in a s.e.xual manner, and as most botanists, according to the prevalent opinion, look upon Fungi as genuine plants, we shall here leave them in the vegetable kingdom, and connect them with lichens, to which they are at all events most nearly related.

The phyletic origin of Fungi will probably long remain obscure. The close relationship already hinted at between the Phycomycetes and Siphoneae (especially between the Saprolegniae and Vaucheriae) suggests to us that they are derived from the latter. Fungi would then have to be considered as Algae, which by adaptation to a parasitical life have become very peculiarly transformed. Many facts, however, support the supposition that the lowest fungi have originated independently from archigonic Monera.

The second cla.s.s of Inophyta, the _Lichens_ (Lichenes), are very remarkable in relation to phylogeny; for the surprising discoveries of late years have taught us that every Lichen is really composed of two distinct plants-of a low form of Alga (Nostochaceae, Chroococcaceae), and of a parasitic form of Fungus (Ascomycetes), which lives as a parasite upon the former, and upon the nutritive substances prepared by it. The green cells, containing chlorophyll (gonidia), which are found in every lichen, belong to the Alga. But the colourless threads (hyphae) which, densely interwoven, form the princ.i.p.al ma.s.s of the body of Lichens, belong to the parasitic Fungus. But in all cases the two forms of plants-Fungus and Alga-which are always considered as members of two quite distinct provinces of the vegetable kingdom, are so firmly united, and so thoroughly interwoven, that nearly every one looks upon a Lichen as a single organism.

Most Lichens form small, more or less formless or irregularly indented, crust-like coverings to stones, bark of trees, etc. Their colour varies through all possible tints, from the purest white to yellow, red, green, brown, and the deepest black.

Many lichens are important in the economy of nature from the fact that they can settle in the driest and most barren localities, especially on naked rocks upon which no other plant can live. The hard black lava, which covers many square miles of ground in volcanic regions, and which for centuries frequently presents the most determined opposition to the life of every kind of vegetation, is always first occupied by Lichens.

It is the white or grey Lichens (Stereocaulon) which, in the most desolate and barren fields of lava, always begin to prepare the naked rocky ground for cultivation, and conquer it for subsequent higher vegetation. Their decaying bodies form the first mould in which mosses, ferns, and flowering plants can afterwards take firm root. Hardy Lichens are also less affected by the severity of climate than any other plants.

Hence the naked rocks, even in the highest mountains-for the most part covered by eternal snow, on which no plant could thrive-are encrusted by the dry bodies of Lichens.

Leaving now the Fungi, Lichens, and Algae, which are comprised under the name of Thallus plants, we enter upon the second sub-kingdom of the vegetable kingdom, that of the _Prothallus plants_ (Prothallophyta), which by some botanists are called phyllogonic Cryptogamia (in contradistinction to the Thallus plants, or thallogonic Cryptogamia).

This sub-kingdom comprises the two provinces of _Mosses_ and _Ferns_.

Here we meet with (except in a few of the lowest forms) the separation of the vegetable body into two different fundamental organs, axial-organs (stem and root) and leaves (or lateral organs). In this the Prothallus plants resemble the Flowering plants, and hence the two groups have recently often been cla.s.sed together as stemmed plants, or Cormophytes.

But, on the other hand, Mosses and Ferns resemble the Thallus plants, in the absence of the development of flowers and seeds, and even Linnaeus cla.s.sed them with these, as Cryptogamia, in contradistinction to the plants forming seeds; that is, flowering plants (Anthophyta or Phanerogamia).

Under the name of "Prothallus plants" we combine the closely-related Mosses and Ferns, because both exhibit a peculiar and characteristic "alternation of generation" in the course of their individual development. For every species exhibits two different generations, of which the one is usually called the _Prothallium_, or _Fore-growth_, the other is spoken of as the _Cormus_, or actual _Stem_ of the moss or fern.

The first and original generation, the Fore-growth, or Prothallus, also called Protonema, still remains in that lower stage of elaboration manifested throughout life by all Thallus plants; that is to say, stem and leaf-organs have as yet not differentiated, and the entire cell-ma.s.s of the Fore-growth corresponds to a simple thallus. The second and more perfect generation of mosses and ferns-the Stem, or Cormus-develops a much more highly elaborate body, which has differentiated into stalk and leaf (as in the case of flowering plants), except in the lowest mosses, where this generation also remains in the lower stage of the thallus.

With the exception of these latter forms the first generation of Mosses and Ferns (the thallus-shaped Fore-growth) always produces a second generation with stem and leaves; the latter in its turn produces the thallus of the first generation, and so on. Thus, in this case, as in the ordinary cases of alternation of generation in animals, the first generation is like the third, fifth, etc., the second like the fourth, sixth, etc. (Compare vol. i. p. 206.)

Of the two main cla.s.ses of Prothallus plants, the Mosses in general are at a much lower stage of development than the Ferns, and their lowest forms (especially in an anatomical respect) form the transition from the Thallus plants through the Algae to Ferns. The genealogical connection of Mosses and Ferns which is indicated by this fact can, however, be inferred only from the case of the most imperfect forms of the two cla.s.ses; for the more perfect and higher groups of mosses and ferns do not stand in any close relation to one another, and develop in completely opposite directions. In any case Mosses have arisen directly out of Thallus plants, and probably out of Green Algae.

Ferns, on the other hand, are probably derived from extinct unknown Mosses, which were very nearly related to the lowest liverworts of the present day. In the history of creation, Ferns are of greater importance than Mosses.

The branch of _Mosses_ (Muscinae, also called Musci, or Bryophyta) contains the lower and more imperfect plants of the group of Prothallophytes, which as yet do not possess vessels. Their bodies are mostly so tender and perishable that they are very ill-suited for being preserved in a recognizable state as fossils. Hence the fossil remains of all cla.s.ses of Mosses are rare and insignificant. It is probable that Mosses developed in very early times out of the Thallus plants, or, to be more precise, out of the Green Algae. It is probable that in the primordial period there existed aquatic forms of transition from the latter to Mosses, and in the primary period to those living on land. The Mosses of the present day-out of the gradually differentiating development of which comparative anatomy may draw some inferences as to their genealogy-are divided into two different cla.s.ses, namely: (1) Liverworts; (2) Leafy Mosses.

The first and oldest cla.s.s of Mosses, which is directly allied to the Green Algae, or Confervae, is formed by the _Liverworts_ (Hepaticae, or Thallobrya). The mosses belonging to them are, for the most part, small and insignificant in form, and are little known. Their lowest forms still possess, in both generations, a simple thallus like the Thallus plants; as for example, the Ricciae and Marchantiaceae. But the more highly developed liverworts, the Jungermanniaceae and those akin to them, gradually commence to differentiate stem and leaf, and their most highly-developed forms are closely allied to leaf-mosses. By this transitional series the liverworts show their direct derivation from the Thallophytes, and more especially from the Green Algae.

The Mosses, which are generally the only ones known to the uninitiated-and which, in fact, form the princ.i.p.al portion of the whole branch-belong to the second cla.s.s, or _Leafy Mosses_ (Musci frondosi, called Musci in a narrow sense, also Phyllobrya). Among them are most of those pretty little plants which, united in dense groups, form the bright glossy carpet of moss in our woods, or which, in company with liverworts and lichens, cover the bark of trees. As reservoirs, carefully storing up moisture, they are of the greatest importance in the economy of nature. Wherever man mercilessly cuts down and destroys forests, there, as a consequence, disappear the leafy mosses which covered the bark of the trees, or, protected by their shade, clothed the ground, and filled the s.p.a.ces between the larger plants. Together with the leafy mosses disappear the useful reservoirs which stored up rain and dew for times of drought. Thus arises a disastrous dryness of the ground, which prevents the growth of any rich vegetation. In the greater part of Southern Europe-in Greece, Italy, Sicily, and Spain-mosses have been destroyed by the inconsiderate extirpation of forests, and the ground has thereby been robbed of its most useful stores of moisture; once flourishing and rich tracts of land have been changed into dry and barren wastes. Unfortunately in Germany, also, this rude barbarism is beginning to prevail more and more. It is probable that the small frondose mosses have played this exceedingly important part in nature for a very long time, possibly from the beginning of the primary period.

But as their tender bodies are as little suited as those of all other mosses for being preserved in a fossil state, palaeontology can give us no information about this.

We learn from the science of petrifactions much more than we do in the case of Mosses of the importance which the second branch of Prothallus plants-that is, Ferns-have had in the history of the vegetable world.

Ferns, or more strictly speaking, the "plants of the fern tribe"

(Filicineae, or Pterideae, also called Pteridophyta, or Vascular Cryptogams), formed during an extremely long period, namely, during the whole primary or palaeolithic period, the princ.i.p.al portion of the vegetable world, so that we may without hesitation call it the _era of Fern Forests_. From the beginning of the Devonian period, in which organisms living on land appeared for the first time, namely, during the deposits of the Devonian, Carboniferous, and Permian strata, plants like Ferns predominated so much over all others, that we are justified in giving this name to that period. In the stratifications just mentioned, but above all, in the immense layers of coal of the Carboniferous or coal period, we find such numerous and occasionally well preserved remains of Ferns, that we can form a tolerable vivid picture of the very peculiar land flora of the palaeolithic period. In the year 1855 the total number of the then known palaeolithic species of plants amounted to about a thousand, and among these there were no less than 872 Ferns.

Among the remaining 128 species were 77 Gymnosperms (pines and palm-ferns), 40 Thallus plants (mostly Algae), and about 20 not accurately definable Cormophyta (stem-plants).

As already remarked, Ferns probably developed out of the lower liverworts in the beginning of the primary period. In their organization Ferns rise considerably above Mosses, and in their more highly developed forms even approach the flowering plants. In Mosses, as in Thallus plants, the entire body is composed of almost equi-formal cells, little if at all differentiated; but in the tissues of Ferns we find those peculiarly differentiated strings of cells which are called the vessels of plants, and which are universally met with in flowering plants. Hence Ferns are sometimes united as "vascular Cryptogams" with Phanerogams, and the group so formed is contrasted as that of the "vascular plants" with "cellular plants,"-that is, with "cellular cryptogams" (Mosses and Thallus plants). This very important process in the organization of plants-the formation of vessels-first occurred, therefore, in the Devonian period, consequently in the beginning of the second and smaller half of the organic history of the earth.

The branch of Ferns, or Filicinae, is divided into five distinct cla.s.ses: (1) Frondose Ferns, or Pteridae; (2) Reed Ferns, or Calamariae; (3) Aquatic Ferns, or Rhizocarpeae; (4) Snakes Tongues, or Ophioglossae; and (5) Scale Ferns, or Lepidophyta. By far the most important of these five cla.s.ses, and also the richest in forms, were first the Frondose Ferns, and then the Scale-ferns, which formed the princ.i.p.al portion of the palaeolithic forests. The Reed Ferns, on the other hand, had at that time already somewhat diminished in number; and of the Aquatic Ferns, we do not even know with certainty whether they then existed. It is difficult for us to form any idea of the very peculiar character of those gloomy palaeolithic fern forests, in which the whole of the gay abundance of flowers of our present flora was entirely wanting, and which were not enlivened by any birds. Of the flowering plants there then existed only the two lowest cla.s.ses, the pines and palm ferns, with naked seeds, whose simple and insignificant blossoms scarcely deserve the name of flowers.

The phylogeny of Ferns, and of the Gymnosperms which have developed out of them, has been made especially clear by the excellent investigations which Edward Strasburger published in 1872, on "The Coniferae and Gnetaceae," as also "On Azolla." This thoughtful naturalist and Charles Martins, of Montpellier, are among the few botanists who have thoroughly understood the fundamental value of the Theory of Descent, and the mechanical-causal connection between ontogeny and phylogeny. The majority of botanists do not even yet know the important difference between h.o.m.ology and a.n.a.logy, between the morphological and physiological comparison of parts-which has long since been recognized in zoology-but Strasburger has employed this distinction and the principle of evolution in his "Comparative Anatomy of the Gymnosperms,"

in order to sketch the outlines of the blood relationship of this important group of plants.

The cla.s.s among Ferns which has developed most directly out of the Liverworts is the cla.s.s of real Ferns, in the narrow sense of the word, the _Frondose Ferns_ (Filices, or Phyllopterides, also called Pteridae).

In the present flora of the temperate zones this cla.s.s forms only a subordinate part, for it is in most cases represented only by low forms without trunks. But in the torrid zones, especially in the moist, steaming forests of tropical regions, this cla.s.s presents us with the lofty palm-like _fern trees_. These beautiful tree-ferns of the present day, which form the chief ornament of our hot-houses, can however give us but a faint idea of the stately and splendid frondose ferns of the primary period, whose mighty trunks, densely crowded together, then formed entire forests. These trunks, acc.u.mulated in super-inc.u.mbent ma.s.ses, are found in the coal seams of the Carboniferous period, and between them, in an excellent state of preservation, are found the impressions of the elegant fan-shaped leaves, crowning the top of the trunk in an umbrella-like bush. The varied outlines and the feather-like forms of these fronds, the elegant shape of the branching veins or bunches of vessels in their tender foliage, can still be as distinctly recognized in the impressions of the palaeolithic fronds as in the fronds of ferns of the present day. In many cases even the cl.u.s.ters of fruit, which are distributed on the lower surface of the fronds, are distinctly preserved. After the Carboniferous period, the predominance of frondose ferns diminished, and towards the end of the secondary period they played almost as subordinate a part as they do at the present time.

The Calamariae, Ophioglossae, and Rhizocarpeae seem to have developed as three diverging branches out of the Frondose Ferns, or Pteridae. The Calamariae, or Calamophyta, have remained at the lowest level among these three cla.s.ses. The Calamariae comprise three different orders, of which only one now exists, namely, the Horse-tails (Equisetaceae). The two other orders, the Giant Reeds (Calamiteae), and the Star-leaf Reeds (Asterophylliteae), are long since extinct. All Calamariae are characterized by a hollow and jointed stalk, stem, or trunk, upon which the branches and leaves (in cases where they exist) are set so as to encircle the jointed stem in whorls. The hollow joints of the stalk are separated from one another by part.i.tion walls. In Horse-tails and Calamiteae the surface is traversed by longitudinal ribs running parallel, as in the case of a fluted column, and the outer skin contains so much silicious earth in the living forms, that it is used for cleansing and polishing. In the Asterophylliteae, the star-shaped whorls of leaves were more strongly developed than in the two other orders.

There exist, at present, of the Calamariae only the insignificant Horse-tails (Equisetum), which grow in marshes and on moors; but during the whole of the primary and secondary periods they were represented by great trees of the genus Equiset.i.tes. There existed, at the same time, the closely related order of the Giant Reeds (Calamites), whose strong trunks grew to a height of about fifty feet. The order of the Asterophyllites, on the other hand, contained smaller and prettier plants, of a very peculiar form, and belongs exclusively to the primary period.

Among all Ferns, the history of the third cla.s.s, that of the _Root_, or _Aquatic Ferns_ (Rhizocarpeae, or Hydropteridae), is least known to us. In their structure these ferns, which live in fresh water, are on the one hand allied to the frond ferns, and on the other to the scaly ferns, but they are more closely related to the latter. Among them are the but little known moss ferns (Salvinia), clover ferns (Marsilea), and pill ferns (Pilularia) of our fresh waters; further, the large Azolla which floats in tropical ponds. Most of the aquatic ferns are of a delicate nature, and hence ill-suited for being petrified. This is probably the reason of their fossil remains being so scarce, and of the oldest of those known to us having been found in the Jura system. It is probable, however, that the cla.s.s is much older, and that it was already developed during the palaeolithic period out of other ferns by adaptation to an aquatic life.

The fourth cla.s.s of ferns is formed by the _Tongue Ferns_ (Ophioglossae, or Glossopterides). These ferns, to which belongs the Botrychium, as well as the Ophioglossum (adder's-tongue) of our native genera, were formerly considered as forming but a small sub-division of the frondose ferns. But they deserve to form a special cla.s.s, because they represent important transitional forms from the Pterideae and Lepidophytes towards higher plants, and must be regarded as among the direct progenitors of the flowering plants.

The fifth and last cla.s.s is formed by the _Scale Ferns_ (Lepidophytes, or Selagines). In the same way as the Ophioglossae arose out of the frondose forms, the scale ferns arose out of the Ophioglossae. They were more highly developed than all other ferns, and form the transition to flowering plants, which must have developed out of them. Next to the frondose ferns they took the largest part in the composition of the palaeolithic fern forests. This cla.s.s also contains, as does the cla.s.s of reed ferns, three nearly related but still very different orders, of which only one now exists, the two others having become extinct towards the end of the Carboniferous period. The scaled ferns still existing belong to the order of the club-mosses (Lycopodiaceae). They are mostly small, pretty moss-like plants, whose tender, many-branched stalk creeps in curves on the ground like a snake, and is densely encompa.s.sed and covered by small scaly leaves. The pretty creeping Lycopodium of our woods, which mountain tourists twine round their hats, is known to all, as also the still more delicate Selaginella, which under the name of creeping moss is used to adorn the soil of our hot-houses in the form of a thick carpet. The largest _club-mosses_ of the present day are found in the Sunda Islands, where their stalks rise to the height of twenty-five feet, and attain half a foot in thickness. But in the primary and secondary periods even larger trees of this kind were widely distributed, the most ancient of which probably were the progenitors of the pines (Lycopodites). The most important dimensions were, however, attained by the cla.s.s of scale trees (Lepidodendreae), and by the seal trees (Sigillarieae). These two orders, with a few species, appear in the Devonian period, but do not attain their immense and astonishing development until the Carboniferous period, and become extinct towards the end of it, or in the Permian period directly following upon it. The scale trees, or Lepidodendreae, were probably more closely related to club-mosses than to Sigillarieae. They grew into splendid, straight, unbranching trunks which divided at the top into numerous forked branches. They bore a large crown of scaly leaves, and like the trunk were marked in elegant spiral lines by the scars left at the base of the leaf stalks which had fallen off. We know of scale-marked trees from forty to sixty feet in length, and from twelve to fifteen feet in diameter at the root. Some trunks are said to be even more than a hundred feet in length. In the coal are found still larger acc.u.mulations of the no less highly developed but more slender trunks of the remarkable seal trees, Sigillarieae, which in many places form the princ.i.p.al part of coal seams. Their roots were formerly described as quite a distinct vegetable form (under the name of Stigmaria). The Sigillarieae are in many respects very like the scale-trees, but differ from them and from ferns in general in many ways. They were possibly closely related to the extinct Devonian _Lycopterideae_, combining characteristic peculiarities of the club-mosses and the frondose ferns, which Strasburger considers as the hypothetical primary form of flowering plants.

In leaving the dense forests of the primary period, which were princ.i.p.ally composed of frond ferns (Lepidodendreae and Sigillarieae), we pa.s.s onwards to the no less characteristic pine forests of the secondary period. Thus we leave the domain of the Cryptogamia, the plants forming neither flowers nor seeds, and enter the second main division of the vegetable kingdom, namely, the sub-kingdom of the _Phanerogamia_, _flowering plants_ forming seeds. This division, so rich in forms, containing the princ.i.p.al portion of the present vegetable world, and especially the majority of plants living on land, is certainly of a much more recent date than the division of Cryptogamia. For it can have developed out of the latter only in the course of the palaeolithic period. We can with full a.s.surance maintain that, during the whole archilithic period, hence during the first and longer half of the organic history of the earth, no flowering plants as yet existed, and that they first developed during the primary period out of Cryptogamia of the fern kind. The anatomical and embryological relation of Phanerogamia to the latter is so close, that from it we can with certainty infer their genealogical connection, that is, their true blood relationship. Flowering plants cannot have directly arisen out of thallus plants, nor out of mosses; but only out of ferns, or Filicines.

Most probably the scaled ferns, or Lepidophyta, and more especially amongst these the Lycopodiaceae, forms closely related to the Selaginella of the present day, have been the direct progenitors of the Phanerogamia.

On account of its anatomical structure and its embryological development, the sub-kingdom of the Phanerogamia has for a long time been divided into two large branches, into the _Gymnosperms_, or plants with naked seeds, and the _Angiosperms_, or plants with enclosed seeds.

The latter are in every respect more perfect and more highly organized than the former, and developed out of them only at a late date during the secondary period. The Gymnosperms, both anatomically and embryologically, form the transition group from Ferns to Angiosperms.

The lower, more imperfect, and the older of the two main cla.s.ses of flowering plants, that of the _Archispermeae_, or _Gymnosperms_ (with naked seeds), attained its most varied development and widest distribution during the mesolithic or secondary epoch. It was no less characteristic of this period, than was the fern group of the preceding primary, and the Angiosperms of the succeeding tertiary, epoch. Hence we might call the secondary epoch that of Gymnosperms, or after its most important representatives, the era of Pine Forests. The Gymnosperms are divided into three cla.s.ses: the Coniferae, Cycadeae, and Gnetaceae. We find fossil remains of the pines, or Conifers, and of the Cycads, even in coal, and must infer from this that the transition from scaled ferns to Gymnosperms took place during the Coal, or possibly even in the Devonian period. However, the Gymnosperms play but a very subordinate part during the whole of the primary epoch, and do not predominate over Ferns until the beginning of the secondary epoch.

Of the two cla.s.ses of Gymnosperms just mentioned, that of the _Palm Ferns_ (Zamiae, or Cycadeae) stands at the lowest stage, and is directly allied to ferns, as the name implies, so that some botanists have actually included them in the fern group. In their external form they resemble palms, as well as tree ferns (or tree-like frond ferns), and are adorned by a crown of feathery leaves, which is placed either on a thick, short trunk, or on a slender, simple trunk like a pillar. At the present day this cla.s.s, once so rich in forms, is but scantily represented by a few forms living in the torrid zones, namely, by the coniferous ferns (Zamia), the thick-trunked bread-tree (Encephalartos), and the slender-trunked Caffir bread-tree (Cycas). They may frequently be seen in hot-houses, and are generally mistaken for palms. A much greater variety of forms than occurs among the still existing palm ferns (Cycadeae) is presented by the extinct and fossil Cycads, which occurred in great numbers more towards the middle of the secondary period, during the Jura, and which at that time princ.i.p.ally determined the character of the forests.

The cla.s.s of _Pines_, or _coniferous trees_ (Coniferae), has preserved down to our day a greater variety of forms than have the palm ferns.

Even at the present time the trees belonging to it-cypresses, juniper trees, and trees of life (Thuja), the box and ginko trees (Salisburya), the araucaria and cedars, but above all the genus Pinus, which is so rich in forms, with its numerous and important species, spruces, pines, firs, larches, etc.-still play a very important part in the most different parts of the earth, and almost of themselves const.i.tute extensive forests. Yet this development of pines seems but weak in comparison with the predominance which the cla.s.s had attained over other plants during the early secondary period, that of the Trias. At that time mighty coniferous trees-with but proportionately few genera and species, but standing together in immense ma.s.ses of individuals-formed the princ.i.p.al part of the mesolithic forests. This fact justifies us in calling the secondary period the "era of the pine forests," although the remains of Cycadeae predominate over those of coniferous trees in the Jura period.[2]

From the pine forests of the mesolithic, or secondary period, we pa.s.s on into the leafy forests of the caenolithic, or tertiary period, and we arrive thus at the consideration of the sixth and last cla.s.s of the vegetable kingdom, that of the _Metaspermae_, _Angiospermae_, or _plants with enclosed seeds_. The first certain and undoubted fossils of plants with enclosed seeds are found in the strata of the chalk system, and indeed we here find, side by side, remains of the two cla.s.ses into which the main cla.s.s of Angiosperms is generally divided, namely, the _one seed-lobed plants_, or _monocotylae_, and the _two seed-lobed plants_, or _dicotylae_. However, the whole group probably originated at an earlier period during the Trias. For we know of a number of doubtful and not accurately definable fossil remains of plants from the Oolitic and Trias (sic) periods, which some botanists consider to be Monocotylae, whilst others consider them as Gymnosperms. In regard to the two cla.s.ses of plants with enclosed seeds, the Monocotylae and Dicotylae, it is exceedingly probable that the Dicotyledons developed out of the Gnetaceae, but that the Monocotyledons developed later out of a branch of the dicotyledons.

The cla.s.s of _one seed-lobed plants_ (Monocotylae, or Monocotyledons, also called Endogenae) comprises those flowering plants whose seeds possess but one germ leaf or seed lobe (cotyledon). Each whorl of its flower contains in most cases _three_ leaves, and it is very probable that the mother plants of all Monocotyledons possessed a regular triple blossom. The leaves are mostly simple, and traversed by simple, straight bunches of vessels or "nerves." To this cla.s.s belong the extensive families of the rushes, gra.s.ses, lilies, irids, and orchids, further a number of indigenous aquatic plants, the water-onions, sea gra.s.ses, etc., and finally the splendid and highly developed families of the Aroideae and Pandaneae, the bananas and palms. On the whole, the cla.s.s of Monocotyledons-in spite of the great variety of forms which it developed, both in the tertiary and the present period-is much more simply organized than the cla.s.s of the Dicotyledons, and its history of development also offers much less of interest. As their fossil remains are for the most part difficult to recognize, it still remains at present an open question in which of the three great secondary periods-the Trias, Jura, or chalk period-the Monocotyledons originated.

At all events they existed in the chalk period as surely as did the Dicotyledons.

[Ill.u.s.tration:

_Haeckel History of Creation._

_PL. V._

Relative lengths of the 5 Epochs in percentages.

Quarternary Epoch 0.5 Tertiary Epoch 2.3 Secondary Epoch 11.5 Primary Epoch 32.1 Primordial Epoch 53.6 ----- _Total_ 100.0

Single-stemmed or MONOPHYLETIC PEDIGREE of the VEGETABLE KINGDOM based on Palaeontology.]

The second cla.s.s of plants with enclosed seeds, the _two seed-lobed_ (Dicotylae, or Dicotyledons, also called Exogenae) presents much greater historical and anatomical interest in the development of its subordinate groups. The flowering plants of this cla.s.s generally possess, as their name indicates, two seed lobes or germ leaves (cotyledons). The number of leaves composing its blossom is generally not three, as in most Monocotyledons, but four, five, or a multiple of those numbers. Their leaves, moreover, are generally more highly differentiated and more composite than those of the Monocotyledons; they are traversed by crooked, branching bunches of vessels or "veins." To this cla.s.s belong most of the leafed trees, and as they predominate in the tertiary period as well as, at present, over the Gymnosperms and Ferns, we may call the caenolithic period that of leafed forests.

Although the majority of Dicotyledons belong to the most highly developed and most perfect plants, still the lowest division of them is directly allied to the Gymnosperms, and particularly to the Gnetaceae. In the lower Dicotyledons, as in the case of the Monocotyledons, calyx and corolla are as yet not differentiated. Hence they are called _Apetalous_ (Monochlamydeae, or Apetalae). This sub-cla.s.s must therefore doubtless be looked upon as the original group of the Angiosperms, and existed probably even during the Trias and Jura periods. Among them are most of the leafed trees bearing catkins-birches and alders, willows and poplars, beeches and oaks; further, the plants of the nettle kind-nettles, hemp, and hops, figs, mulberries, and elms; finally, plants like the spurges, laurels, and amaranth.

It was not until the chalk period that the second and more perfect cla.s.s of the Dicotyledons appeared, namely, the _group with corollas_ (Dichlamydeae, or Corolliflorae). These arose out of the Apetalae from the simple cover of the blossoms of the latter becoming differentiated into calyx and corolla. The sub-cla.s.s of the Corolliflorae is again divided into two large main divisions or legions, each of which contains a large number of different orders, families, genera, and species. The first legion bears the name of star-flowers, or Diapetalae, the second that of the bell-flowers, or Gamopetalae.