Fig. 98.—Longitudinal Diagram, showing the arrangement of the elongated sporangia on the sporophylls
a, Main axis, round which the sporophylls are inserted;S, sporangium;s, leaflike end of sporophyll.
The similarity between theLepidodendronand the modern Lycopod cone has been pointed out already (p. 67), and it is this which forms the principal guarantee that they belong to the same family, though the size and wood development of the palæozoic and the modern plants differ so greatly.
The large group of the Lepidodendra included some members whose fructifications had advanced so far beyond the simple sporangial cones described above as to approach very closely to seeds in their construction. This type was described onp. 75,fig. 54, in a series of female fructifications, so that its essential structure need not be recapitulated.
Fig. 99.—Transverse Section through Cone ofLepidodendron
A, Main axis with woody tissue;st, stalks of sporophylls cut in oblique longitudinal direction;s, tips of sporophylls cut across;S, sporangia with a few groups of spores. (Microphoto.)
The section shown infig. 100is that cut at right angles to that in which the sporangia are shown infig. 98, viz. tangential to the axis. A remarkable feature of the plant is that there were also round those sporangia which bore the numerous small spores (corresponding to pollen grains) enclosing integument-like flaps similar to those shown infig. 100,sp. f.
Fig. 100.—Section through one Sporangium ofLepidocarpon
sp, Sporophyll;sp.f., flaps of sporophyll protecting sporangium;S, large spore within the sporangium wallw;s, the three aborted spores of the tetrad to whichSbelongs.
This type of fructification is the nearest approach to seed and pollen grains reached by any of the Pteridophytes, and its appearance at a time when the Lycopods were one of the dominant familiesis suggestive of the effect that such a position has on the families occupying it, however lowly they may be. The simple Pteridophyte Lycopods had not only the tall trunks and solid woody structure of a modern tree, but also a semblance of its seeds. Whether this line of development ever led on to any of the higher families is still uncertain. The feeling of most specialists is that it did not; but there are not wanting men who support the view that the lycopod affinity evolved in time and entered the ranks of the higher plants, and indeed there are many points of superficial likeness between the palæozoic Lycopods and the Coniferæ. Judged from their internal structure, however, the series through the ferns and Pteridosperms leads much more convincingly to the seed plants.
In their roots, or rather in the underground structures commonly called roots, the Lepidodendrons were also remarkable. Even more symmetrically than in their above-ground branching, the base of their trunks divided; there were four main large divisions, each of which branched into two and these into two again. These structures were calledStigmaria, and were common to all species ofLepidodendronand also the group ofSigillaria(seefig. 102). On these horizontally running structures (well shown in thefrontispiece) small appendages were borne all over their surface in great profusion, which were, both in their function and microscopic structure, rootlets. They left circular scars of a characteristic appearance on the big trunks, of which they were the only appendages. These scars show clearly on the fragments along the ledge to the left of the photograph. The exact morphological nature of the big axes is not known; their anatomy is not like that of roots, but is that of a stem, yet they do not bear what practically every stem, whether underground or not, has developed, namely leaves, or scales representing reduced leaves. Their nature has been commented on previously (p. 69), and we cannot discuss the point further, but must becontent to consider them as a form of root-bearing stem, practically confined to the Lycopods and principally developed among the palæozoic fossils of that group.
Fig. 101.—Transverse Section through a Rootlet ofStigmaria
oc, Outer cortex;s, space;ic, inner cortex;w, wood of vascular strand (wood only preserved);px, protoxylem group.
In microscopic structure the rootlets are extremely well known, because in their growth they have penetrated the masses of the tissues of other plants which were being petrified and have become petrified with them. The mass of decaying vegetable tissue on which the living plants of the period flourished were everywhere pierced by these intrusive rootlets, and they are found petrified inside otherwise perfect seeds, in the hearts of woody stems, in leaves and sporangia, and sometimes even inside each other!Fig. 95shows such a rootrlying in the space left by the decay of the soft tissue of the inner cortex in an otherwise excellently preservedLepidodendronstem (see alsofig. 101). Infig. 101their simple structure is seen. They are often extremely irregular in shape, owing to the way they seem to have twisted and flattened themselves in order to fit into the tissues they were penetrating. No root hairs seem to have been developed in these rootlets, but otherwise their structure is that of a typical simple root, and very like the swamp-penetrating rootlets of the living Isoetes.
The Stigmarian axes and their rootlets are very commonly found in the “underclays” and “gannister” beds which lie below the coal seams (seep. 25), and they may sometimes be seen attached to a bit of the trunk growing upwards through the layers. They and the aerial stems of Lepidodendron are perhaps the commonest and most widely known of fossil plants.
Before leaving the palæozoic Lycopods another genus must be mentioned, which is also a widely spread and important one, though it is less well known than its contemporary. The genusSigillariais best known by its impressions and casts of stems covered by leaf scars. The stems were sometimes deeply ribbed, and the leaf scars were arranged in rows and were more or less hexagonal in outline, as is seen infig. 102, which shows a cast and its reverse of the stem of a typicalSigillaria.
Fig. 102.—Cast and Reverse of Leaf Scars ofSigillaria. InAthe shape of the leaf bases is clearly shown, the central markings in each being the scar of the vascular bundle and parichnos
In its primary woodSigillariadiffered fromLepidodendronin being more remote from the type with a primary solid stele. Its woody structure was that of a ring, in some cases irregularly broken up into crescent-shaped bundles. The secondary wood was quite similar to that ofLepidodendron.
Stigmariaand its rootlets belong equally to the two plants, and hitherto it has been impossible to tell whetherany given specimen ofStigmariahad belonged to aLepidodendronor aSigillaria. Between the two genera there certainly existed the closest affinity and similarity in general appearance.
These two genera represent the climax of development of the Lycopod family. In the Lower Mesozoic some large forms are still found, but all through the Mesozoic periods the group dwindled, and in the Tertiary little is known of it, and it seems to have taken the retiring position it occupies to-day.
The horsetails of to-day all belong to the one genus,Equisetum, among the different species of which there is a remarkably close similarity. Most of the species love swampy land, and even grow standing up through water; but some live on the dry clay of ploughed fields. Wherever they grow they usually congregate in large numbers, and form little groves together. They are easily recognized by their delicate stems, branching in bottle-brush fashion, and the small leaves arranged round them in whorls, with their narrow teeth joined to a ring at the base. At the end of some of the branches come the cones, with compactly arranged and simple sporophylls all of one kind. In England most plants of this family are but a few inches or a foot in height, though one species sometimes reaches 6 ft., while in South America there are groves of delicate-stemmed plants 20 ft. high.
The ribbed stems and the whorls of small, finely toothed leaves are the most important external characteristicsof the plants, while in their internal anatomy the hollow stems have very little wood, which is arranged in a series of small bundles, each associated with a hollow canal in the ground tissue.
The family stands apart from all others, and even between it and the group of Lycopods there seems to be a big gap across which stretch no bonds of affinity. Has the group always been in a similar position, and stood isolated in a backwater of the stream of plant life?
Fig. 103.—Impression of Leaf Whorl ofEquisetitesfrom the Mesozoic Rocks, showing the narrow toothed form of the leaves. (Photo.)
In the late Tertiary period they seem to have held much the same position as they do now, and we learn nothing new of them from rocks of that age. When, however, we come to the Mesozoic, the members of the family are of greater size, though they appear (to judge from their external appearance) to have been practically identical with those now living in all their arrangements. In some beds their impressions are very numerous, but unfortunately most are without any indication of internal structure. Fossils from the Mesozoic are calledEquisetites, a name which indicates that they come very close to the living ones in their characters. In the Lower Mesozoic some of these stems seem to have reached the great size of a couple of feet in circumference, but to have no essential difference from the others of the group.
When, however, we come to the Palæozoic rocks we find many specimens with their structure preserved, and we are at once in a very different position as regards the family.
First in the Permian we meet with the importantgenus of plant calledCalamites, which were very abundant in the Coal Measures. Many of the Calamites were of great size, for specimens with large trunks have been found 30 ft. and more long, which when growing must certainly have been much taller than that. The number of individuals must also have been very great, for casts and impressions of the genus are among the commonest fossils. They were, in fact, one of the dominant groups of the period. Like the Lycopods, the Equisetaceæ reached their high-water mark of development in the Carboniferous period; at that time the plants were most numerous, and of the largest size and most complicated structure that they ever attained.
Fig. 104.—Small Branches attached to stouter Axis ofCalamites. Photo of Impression
As will be immediately suspected from analogy with the Lycopods, they differed from the modern members of the family in their strongly developed anatomy, and in the strength and quantity of their secondary wood.Yet in their external appearance they probably resembled the living genus in all essentials, and the groves of the larger ones of to-day growing in the marshes probably have the appearance that the palæozoic plants would have had if looked at through a reversed opera glass.
Fig. 104is a photograph of some of the small branches of a Calamite, in which the ribbed stem can be seen, and on the small side twigs the fine, pointed leaves lying in whorls.
In most of the fossil specimens, however, particularly the larger ones, the ribs are not those of the true surface, but are those marked on theinternal castof the pith.
Fig. 105.—Transverse Section ofCalamitesStem with Secondary Woodwformed in Regular Radial Rows in a Solid Ring
c, Canals associated with the primary bundles;p, cells of the pith, which is hollow with a cavityl,cor, Cortex and outer tissues well preserved. (Microphoto.)
Among tissue petrifactions there are many Calamite stems of various stages of growth. In the very young ones there are only primary bundles, and these little stems are like those of a living Equisetum in their anatomy, and have a hollow pith and small vascular bundles with canals associated. The fossil forms, however, soon began to grow secondary wood, which developed in regular radial rows from a cambium behind the primary bundles and joined to a complete ring.
A stem in this stage of development is seen infig. 105, where only the wood and internal tissues are preserved. The very characteristic canals associated with the primary bundles are clearly shown. The amount of secondarywood steadily increased as the stems grew (there appear to have been no “annual rings”) till there was a very large quantity of secondary tissue of regular texture, through which ran small medullary rays, so that the stems became increasingly like those of the higher plants as they grew older. It is the primary structure which is the important factor in considering their affinity, and that is essentially the same as in the other members of the family in which secondary thickening is not developed. As we have seen already in other groups of fossils, secondary wood appears to develop on similar lines whenever it is needed in any group, and therefore has but little value as an indication of systematic position. This important fact is one, however, which has only been realized as a result of the study of fossil plants.
Fig. 106.—Diagram of the Arrangement of the Bundles at the Node of aCalamite, showing how those of consecutive internodes alternate
n, Region of node
Fig. 107.—Leaf ofCalamitesin Cross Section
v, Vascular bundles;s, cells of sheath, filled with blackened contents;p, palisade cells;e, epidermis.
The longitudinal section of the stems, when cut tangentially, is very characteristic, as the bundles run straight down to each node and there divide, the neighbouring halves joining so that the bundles of each node alternate with those of the ones above and below it (seefig. 106).
The leaves which were attached at the nodes were naturally much larger than those of the present Equisetums, though they were similarly simple and undivided. Their anatomy is preserved in a number of cases (seefig. 107), and was simple, witha single small strand of vascular tissue lying in the centre. They had certain large cells, sometimes very black in the fossils, which may have been filled with mucilage.
Fig. 108.—Transverse Section of Young Root ofCalamites
w, Wood of axis;l, spaces in the lacunar cortex, whose radiating strandsrare somewhat crushed;ex, outermost cells of cortex with thickened wall.
Fig. 109.—Diagram of Cone ofCalamites
A, Main axis;br, sterile bracts;sp, sporophylls with four sporangiaSattached to each, of which two only are seen.
The young roots of these plants have a very characteristic cortex, which consists of cells loosely built together in a lacelike fashion, with large air spaces, so that they are much like water plants in their appearance (seefig. 108). Indeed, so unlike the old roots and the stems are they, that for long they were called by another name and supposed to be submerged stems, but their connection withCalamitesis now quite certain. As their woody axis develops, the secondary tissue increases and pushes off the lacelike cortex, and the roots become very similar in their anatomy to the stems. Both have similar zones of secondary wood, but the roots do not have those primary canals which are so characteristic of the stems, and thereby they can be readily distinguished from them.
The fructifications of the Calamites were not unlike those of the living types of the family, though in somerespects slightly more complex. Round each cone axis developed rings of sporophylls which alternated with sterile sheathing bracts. Each sporophyll was shaped like a small umbrella with four spokes, and stood at right angles to the axis, bearing a sporangium at each of the spokes. A diagram of this arrangement is seen infig. 109.
Fig. 110.—Longitudinal Section of Part ofCalamitesCone
br, Sterile bracts attached to axis;sp, attachment of sporophylls;S, sporangia. AtXa group of four sporangia is seen round the sporophyll, which is seen ata. (Microphoto.)
A photograph of an actual section of such a cone, cut slightly obliquely through the length of the axis, is seen infig. 110, where the upper groups of sporangia are cut tangentially, and show their grouping round the sporophyll to which they are attached.
A few single tetrads of spores are enlarged infig. 111, where it will be seen that the large spores are of a similar size, but that the small ones of the tetradsare very irregular. They are aborting members of the tetrad, and appear to have been used as food by the other spores. In each sporangium large numbers of these tetrads develop and all the ripe spores seem to have been of one size.
In a species ofCalamites(C. casheana), otherwise very similar to the common one we have been considering, there is a distinct difference in the sizes of the spores from different sporangia. The small ones, however, were only about one-third of the diameter of the large ones, so that the difference was very much less marked than it was between the small and large spores of the Lycopods.
Among the palæozoic members of the group are other genera closely allied to, but differing fromCalamitesin some particulars. One of these isArchæocalamites, which has a cone almost identical with that of the living Equisetums, as it has no sterile bracts mingled with the umbrella-like sporophylls. Other genera are more complex than those described forCalamites, and even in the simple conedArchæocalamitesitself the leaves are finely branched and divided instead of being simple scales.
But no genus is so completely known as isCalamites, which will itself suffice as an illustration of the palæozoic Equisetaceæ. Though the genus, as was pointed out above, shows several important characters differing from those of Equisetum, and parallel to some extent to those of the palæozoic Lycopods, yet these features are more of a physiological nature than a systematic one, and they throw no light on the origin of the family or on its connection with the other Pteridophytes. It is in the extinct family dealt with in the next chapter that we find what some consider as a clue to the solution of these problems.
Fig. 111.—Tetrads of Spores ofCalamites
S, Normal-sized spores;a,b, &c., aborting spores.
The group to whichSphenophyllumbelongs is of considerable interest and importance, and is, further, one of those extinct families whose very existence would never have been suspected had it not been discovered by fossil botanists. Not only is the family as a whole extinct, it also shows features in its anatomy which are not to be paralleled among living stems.Sphenophyllumbecame extinct in the Palæozoic period, but its interest is very real and living to-day, and in the peculiar features of its structure we see the first clue that suggests a common ancestor for the still living groups of Lycopods and Equisetaceæ, which now stand so isolated and far apart.
Before, however, we can consider the affinities of the group, we must describe the structure of a typical plant belonging to it. The genusSphenophyllumincludes several species (for which there are no common English names, as they are only known to science) whose differences are of less importance than their points of similarity, so that one species only,S. plurifoliatum, will be described.
We have a general knowledge of the external appearance ofSphenophyllumfrom the numerous impressions of leaves attached to twigs which are found in the rocks of the Carboniferous period. These impressions present a good deal of variety, but all have rather delicate stems with whorls of leaves attached at regular intervals. The specimens are generally easy to recognize from the shape of the leaves, which are like broad wedges attached at the point (seefig. 112). In some cases the leaves are more finely divided and less fanlike, and it may even happen that on the same branchsome may be wedge-shaped like those infig. 112, and others almost hairlike. This naturally suggests comparison with water plants, which have finely divided submerged leaves and expanded aerial ones. In the case ofSphenophyllum, however, the divided leaves sometimes come at the upper ends of the stems, quite near the cones, and so can hardly have been those of a submerged part. The very delicate stems and some points in their internal anatomy suggest that the plant was a trailing creeper which supported itself on the stouter stems of other plants.
Fig. 112.—Impression ofSphenophyllumLeaves attached to the Stem, showing the wedge-shaped leaflets arranged in whorls
The stems were ribbed, but unlike those of the Calamites the ribs ran straight down the stem through the nodes, and did not alternate there, so that the bundles at the node did not branch and fuse as they did inCalamites.
The external appearance of the long slender cones was not unlike that of the Calamite cones, though their internal details showed important distinctions.
In one noticeable external feature the plants differed from those of the last two groups considered, and that was in theirsize. Palæozoic Lycopods and Equisetaceæ reached the dimensions of great trees, but hitherto notreelike form ofSphenophyllumhas been discovered, and in the structure-petrifactions the largest stems we know were less than an inch in diameter.
In the internal anatomy of these stems lies one of the chief interests and peculiarities of the plants. In the very young stage there was a sharply pointed solid triangle of wood in the centre (fig. 113), at each of the corners of which was a group of small cells, the protoxylems. The structure of such a stem is like that of a root, in which the primary wood all grows inwards from the protoxylems towards the centre, and had we had nothing but these isolated young stems it would have been impossible to recognize their true nature.
Fig. 113.—Sphenophyllum, Transverse Section of Young Stem
c, Cortex, the soft tissue within which has decayed and left a space, in which lies the solid triangle of wood, with the small protoxylem groupspxat each corner. (Microphoto.)
Such very young stems are rare, for the development of secondary wood began early, and it soon greatly exceeded the primary wood in amount.Fig. 114shows a photograph of a stem in which the secondary wood is well developed. The primary triangle of wood is still to be seen in the centre, and corresponds to that infig. 113, while closely fitting to it are the bays of the first-formed secondary wood, which makes the wood mass roughly circular. Outside this the secondary wood forms a regular cylinder round the axis, which shows no sign of annual rings. The cells of the wood are large and approximately square in shape, while at the angles formed at the junction of every four cells is a group of small, thin-walled parenchyma, seefig. 115. There are no medullary rays goingout radially through the wood, such as are found in all other zones of secondary wood, and in this arrangement of soft tissue the plants are unique.
Fig. 114.—Sphenophyllum, Transverse Section with Secondary WoodW. Atcthe cork formation is to be seen. (Microphoto.)
Beyond the wood was a zone of soft tissue and phloem, which is not often preserved, while outside that was the cork, which added to the cortical tissues as the stem grew (seefig. 114,c).
Fig. 115.—Group of Wood Cellsw, showing their shape and the small soft-walled cells at the angles between themp
Petrified material of leaves and roots is rare, and both are chiefly known through the work of the French palæobotanist Renault. The leaves are chiefly remarkable for the bands of sclerized strengthening tissue, and generally had the structure of aerial, not submerged leaves. The roots were simple in structure,and, as inCalamites, had secondary tissue like that in the stems.
In the case of the fructifications it is the English material which has yielded the most illuminating specimens. The cones were long and slender, externally covered by the closely packed tips of the scales, which overlapped deeply. Between the whorls of scales lay the sporangia, attached to their upper sides by slender stalks. A diagram will best explain how they were arranged (seefig. 116). Two sporangia were attached to each bract, but their stalks were of different lengths, so that one sporangium lay near the axis and one lay outside it toward the tip of the bract.
Fig. 116.—Diagram of Arrangement of Scales and Sporangia in Cones ofSphenophyllum
A, Axis;br, bract;S, sporangium, with stalkst.
In its anatomy the stalk of the cone has certain features similar to those in the stem proper, which were among the first indications that led to the discovery that the cone belonged toSphenophyllum. There were numerous spores in each of the sporangia, which had coats ornamented with little spines when they were ripe (fig. 117, if examined with a magnifying glass, will show this). Hitherto the only spores known are of uniform size, and there is no evidence that there was any differentiation into small (male) and large (female) spores such as were found in some of the Lepidodendrons. In this respectSphenophyllumwas less specialized than eitherLepidodendronorCalamites.
In the actual sections ofSphenophyllumcones the numerous sporangia seem massed together in confusion, but usually some are cut so as to show the attachmentof the stalk, as infig. 117,st. As the stalk was long and slender, but a short length of it is usually cut through in any one section, and to realize their mode of attachment to the axis (as shown infig. 116) it is necessary to study a series of sections.
Fig. 117.—Part of Cone ofSphenophyllum, showing sporangiasp, some of which are cut so as to show a part of their stalksst.B, Bract. (Microphoto.)
Of the other plants belonging to the group,Bowmanites Römeriis specially interesting. Its sporangia were borne on stalks similar to those ofSphenophyllum, but each stalk had two sporangia attached to it. Two sporangia are also borne on each stalk inS. fertile. These plants help in elucidating the nature of the stalked sporangia ofSphenophyllum, for they seem to indicate a direct comparison between them and the sporophylls of the Equisetales.
There is, further, another plant, of which we only know the cone, of still greater importance. This cone(Cheirostrobus) is, however, so complex that it would take far too much space to describe it in detail. Even a diagram of its arrangements is extraordinarily elaborate. To the specialist the cone is peculiarly fascinating, for its very complexity gives him great scope for weaving theories about it; but for our purposes most of these are too abstruse.
Fig. 118.—A, Diagram of Three-lobed Bract from Cone ofCheirostrobus.a, Axis;br, the three sterile lower lobes of the bract;sp, the three upper sporophyll-like lobes, to each of which were attached four sporangiaS.B, Part of the above seen in section longitudinal to the axis. (Modified from Scott.)
Its most important features are the following. Round the axis were series of scales, twelve in each whorl, and each scale was divided into an upper and a lower portion, each of which again divided into three lobes. The lower three of each of these scale groups were sterile and bractlike, comparable, perhaps, with the bracts infig. 116; while the upper three divisions were stalks round each of which were four sporangia. Each sporophyll segment thus resembled the sporophyll ofCalamites, while the long sausage-shaped sporangia themselveswere more like those ofLepidodendron. Infig. 118is a diagram of a trilobed bract with its three attached sporophylls. Round the axis were very numerous whorls of such bracts, and as the cone was large there were enormous numbers of spore sacs.
A point of interest is the character of the wood of the main axis, which is similar to that of Lepidodendron in many respects, being a ring of centripetally developed wood with twelve projecting external points of protoxylem.
This cone[13]is the most complex fructification of any of the known Pteridophytes, whether living or fossil, which alone ensures it a special importance, though for our purpose the mixed affinities it shows are of greater interest.
To mention some of its characters:—The individual segments of the sporophylls, each bearing four sporangia, are comparable with those ofCalamites, while the individual sporangia and the length of the sporophyll stalk are similar in appearance to those ofLepidodendron. The wood of the main axis also resembles that of a typicalLepidodendron. The way the vascular bundles of the bract pass out from the axis, and the way the stalks bearing the sporangia are attached to the sterile part of the bracts, are like the corresponding features inSphenophyllum, and still more likeBowmanites.
Many other points of comparison are to be found in these plants, but without going into further detail enough has been indicated to support the conclusion thatCheirostrobusis a very important clue to the affinities of the Sphenophyllales and early Pteridophytes. It is indeed considered to have belonged to an ancient stock of plants, from which the Equisetaceæ, andSphenophylla, and possibly also the Lycopods all sprang.
Sphenophyllum,Bowmanites, andCheirostrobus, a series of forms that became extinct in the Palæozoic, remote in their structure from any living types, whoseexistence would have been entirely unsuspected but for the work of fossil botany, are yet the clues which have led to a partial solution of the mysteries surrounding the present-day Lycopods and Equisetums, and which help to bridge the chasm between these remote and degenerate families.
In the plant world of to-day there are many families including immense numbers of species whose organization is simpler than that of the groups hitherto considered. Taken all together they form, in fact, a very large proportion of the total number of living species, though the bulk of them are of small size, and many are microscopic.
These “lower plants” include all the mosses, and the flat green liverworts, the lichens, the toadstools, and all the innumerable moulds and parasites causing plant diseases, the green weeds growing in water, and all the seaweeds, large and small, in the sea, the minute green cells growing in crevices of the bark of trees, and all the similar ones living by millions in water. Truly a host of forms with an endless variety of structures.
Yet when we turn to the fossil representatives of this formidable multitude, we find but few. Indeed, of the fossil members of all these groups taken together we know less that is of importance and real interest than we do of any single family of those hitherto considered. The reasons for this dearth of fossils of the lower types are not quite apparent, but one which may have some bearing on it is the difficulty of mineralization. It is self-evident that the more delicate and soft-walled any structure isthe less chance has it of being preserved without decay long enough to be fossilized. As will have been understood from Chapter II, even when the process of fossilization took place, geologically speaking, rapidly, it can never have been actually accomplished quickly as compared with the counter processes of decay. Hence all the lower plants, with their soft tissue and lack of wood and strengthening cells, seem on the face of it to stand but little chance of petrifaction.
There is much in this argument, but it is not a sufficient explanation of the rarity of lower plant fossils. All through the preceding chapters mention has been made of very delicate cells, such as pith, spores, and even germinating spores (seefig. 47,p. 68), with their most delicate outgrowing cells. If then such small and delicate elements from the higher plants are preserved, why should not many of the lower plants (some of which are large and sturdy) be found in the rocks?
As regards the first group, the mosses, it is probable that they did not exist in the Palæozoic period, whence our most delicately preserved fossils are derived. There seems much to support the view that they have evolved comparatively recently although they are less highly organized than the ferns. Quite recently experiments have been made with their near allies the liverworts, and those which were placed for one year under conditions similar to those under which plant petrifaction took place, were found to be perfectly preserved at the end of the period; though they would naturally decay rapidly under usual conditions. This shows that Bryophyte cells are not peculiarly incapable of preservation as fossils, and adds weight to the negative evidence of the rocks, strengthening the presumption of their late origin.
That some of the lower plants, among the very lowest and simplest, can be well preserved is shown in the case of the fossil fungi which often occur in microscopic sections of palæozoic leaves, where they infest the higher plants as similar parasitic species do to-day.
We must now bring forward the more important of the facts known about the fossils of the various groups of lower plants.
Bryophytes.—Mosses.Of this family there are no specimens of any age which are so preserved as to show their microscopical structure. Of impressions there are a few from various beds which show, with more or less uncertainty in most cases, stems and leaves of what appear to be mosses similar to those now extant, but they nearly all lack the fructifications which would determine them with certainty. These impressions go by the name ofMuscites, which is a dignified cloak for ignorance in most cases. The few which are quite satisfactory as impressions belong to comparatively recent rocks.
Liverwortsare similarly scanty, and there is nothing among them which could throw any light on the living forms or their evolution. The more common are of the same types as the recent ones, and are calledMarchantites, specimens of which have been found in beds of various ages, chiefly, however, in the more recent periods of the earth’s history.
It is of interest to note that among all the delicate tissue which is so well preserved in the “coal balls” and other palæozoic petrifactions, there are no specimens which give evidence of the existence of mosses at that time. It is not unlikely that they may have evolved more recently than the other groups of the “lower” plants.
Characeæ.—Members of this somewhat isolated family (Stoneworts) are better known, as they frequently occur as fossil casts. This is probably due to their character, for even while alive they tend to cover their delicate stems and leaves, and even fruits, with a limy incrustation. This assists fossilization to some degree, and fossil Charas are not uncommon. Usually they are from the recently deposited rocks, and theearliest true Charas date only to the middle of the Mesozoic.
An interesting occurrence is the petrifaction of masses of these plants together, which indicate the existence of an ancient pool in which they must have grown in abundance at one time. A case has been described where masses ofCharaare petrified where they seem to have been growing, and in their accumulations had gradually filled up the pond till they had accumulated to a height of 8 feet.
The plants, however, have little importance from our present point of view.
Fungi.—Of the higher fungi, namely, “toadstools”, we have no true fossils. Some indications of them have been found in amber, but such specimens are so unsatisfactory that they can hardly afford much interest.