Passing from the direct evidence obtained from the presence of fungal hyphae in petrified tissues, we must draw attention to the indirect evidence of fungal action afforded by many fossil plants. It is important to be familiar with at least the more striking effects of fungal ravages in recent wood in order that we may escape some of the mistakes to which pathological phenomena may lead us in the case of fossils[424].
The gradual dissociation of the elements in a piece of fossil wood owing to the destruction of the middle lamellae, the occurrence of various forms of slit-like apertures in the walls of tracheids (fig. 42, E) and the production of a system of fine parallel striation on the walls of a vessel are among the results produced by parasitic and saprophytic fungi. With the help of a ferment secreted by its hyphae, a fungus is able to eat away either the thickening cell layers or the middle lamellae or both, and if, as in fig. 42, A, only the middle lamellae are left one might easily regard such tissue in a fossil condition as consisting of delicate thin-walled elements. The oblique striae on the walls of a tracheid may often be due to the action of a ferment which has dissolved the membrane in such a manner as to etch out a system of spiral lines, probably as a consequence of the original structure of the tracheids. In distinguishing between the woods of Conifers the presence of spiral thickening layers in the wood element is an important diagnostic character, and it is necessary to guard against the confusion of purely secondary structures, due to fungal action, with original features which may be of value in determining the generic affinity of a piece of fossil wood.
Under this name Renault has recently described a filamentous fungus endophytic in the cavities of the scalariform tracheids of a Lepidodendron[425]. The mycelium has the form of slender branched hyphae with transverse septa. Numerous ovoid and more or less spherical sporangia occur as terminal swellings of the mycelial threads. The long axis of the ovoid forms measures 12–15 µ, and the shorter axis 9–10 µ; the contents may be seen as a slightly contracted mass in the sporangial cavity. In some of the sporangia one sees a short apical prolongation in the form of a small elongated papilla, as shown in fig. 43, 1. Renault refers this fungus to the Chytridineae, and compares it with Cladochytrium, Woronina, Olpidium, and other recent genera.
In the immediate neighbourhood of two of the sporangia shown in the uppermost tracheid of fig. 43, 1, there are seen a few minute dark dots which are described as spores petrified in the act of escaping from a lateral pore. This interpretation strikes one as lacking in scientific caution.
The sporangia of Hyphochytrium infestans[426], as figured by Fischer in Rabenhorst’s work bear a close resemblance to those of the fossil. It would seem very probable that Renault’s species may be reasonably referred to the Chytridineae, as he proposes.
In an address to the Geologists’ Association delivered by Mr Carruthers in 1876 a brief reference, accompanied by a small-scale drawing, is made to the discovery of a fungus in the scalariform tracheids of a Lepidodendron from the English Coal-Measures[427]. In the following year Worthington Smith published a fuller account of the fungus, and proposed for it the above name[428], which he chose on the ground of a close similarity between the mycelium and reproductive organs of the fossil form and recent members of the Peronosporeae. In Smith’s description the mycelium is described as bearing spherical swellings containing zoospores. These spherical organs are fairly abundant and not infrequently met with in sections of petrified plant-tissues from the English Coal-Measures; they may be oogonia or sporangia, or in some cases mere vesicular expansions of a purely vegetative hypha. No confirmation has been given to the supposed spores referred to by Smith. Prof. Williamson and others have carefully examined the specimens, but they have failed to detect any trace of reproductive cells enclosed in the spherical sacs[429]. The mycelium does not appear to show any satisfactory evidence of its being septate as figured by Smith.
The example shown in fig. 41 E has been drawn from one of the Williamson specimens: it illustrates the form and manner of occurrence of the characteristic swellings. It is probable that some at least of the vesicles are either sporangia or oogonia, but we cannot speak with absolute confidence as to their precise nature. The general habit and structure of the fungus favour its inclusion in the class of Phycomycetes. The occurrence of several of the vesicles close together on short hyphal branches, as shown in Williamson’s figures, suggests the spherical swellings on vegetative hyphae, but it is impossible to speak with absolute confidence. There is a close resemblance between this English form and one recently described by Renault as Palaeomyces gracilis Ren.[430]; the two fossils should probably be placed in the same genus.
The examples referred to below and originally recorded by Cash and Hick no doubt belong to the same type as Smith’s Peronosporites.
The sketches reproduced in fig. 44 have been drawn from specimens originally described by Cash and Hick in 1878[431]. The sections were cut from a calcareous nodule from the Halifax Coal-Measures containing fragments of various plants and among others a piece of cortical tissue, probably of a Lepidodendron or Stigmaria. In a transverse section of this tissue one sees under a moderately high power that the cells have become partially separated from one another by the destruction of the middle lamellae (fig. 44 A). The cell-cavities and the spaces between the isolated cells contain numerous fine fungal hyphae, which here and there terminate in spherical swellings. One such swelling is shown under a low power in fig. 44 A, in the middle uppermost cell, and more highly magnified in fig. 44 B. In fig. C there are two such swellings (the larger one having a diameter of ·003 mm.) in contact, but the connection does not appear to be organic. The cell-walls of the infected tissue present a ragged and untidy appearance, and in places (e.g. fig. 44 D) the membrane has been pierced by some of the mycelial branches.
This fungus bears a close resemblance to Peronosporites antiquarius, but it is impossible to determine its precise botanical position without further data. In Cash and Hick’s paper in which the above fungus is briefly dealt with, some small spore-like bodies are figured which the authors speak of as possibly a Myxomycetous fungus[432]. There is however no sound reason for such a supposition.
As examples of Ascomycetous fungi found in silicified wood of Tertiary age, two species may be quoted from Felix.
The mycelium and conidia of this form were discovered in some Eocene silicified wood from Perekeschkul near Baku, on the shores of the Caspian. The conidia are elliptical or pyriform in shape and divided by a transverse septum into two cells. No traces were found of any special conidiophores. The mycelium consists of septate branched hyphae, rendered conspicuous by a brown colouration. Felix compares the fossil with the recent genera Cephalothecium and Cladosporium.
The conidia of this form were found to be fairly abundant in the silicified tissue investigated by Felix; they occur usually in chains of 2 to 6 conidia having an ovoid or flask-shaped form, with a thick membrane (fig. 43, 4). The mycelium consists of branched hyphae divided into long cylindrical cells by transverse septa; occasional instances were found of an H-shaped fusion between lateral branches of parallel hyphae.
Felix compares this species with examples of the genera Haptographium and Dematium of the family Sphaeriaceae; it was found in the woody tissue of a dicotyledonous stem from Perekeschkul.
The object represented in fig. 41 F consists of a stalked spherical sac bearing a number of radiating arms which are divided distally into delicate terminations. We find similar bodies figured by Williamson[435] in his IXth and Xth Memoirs on the Coal-Measure plants; he includes some of them under the generic term Zygosporites, and compares them with the zygospores of the freshwater algae Desmideae. Hitherto these spore-like fossils have only been recorded as isolated spheres, but in the example shown in fig. 41 F there is a distinct tubular and thin-walled stalk attached to the Zygosporites. The specimen was found in the partially disorganised cortical tissue of a Lyginodendron stem from the English Coal-Measures. It is difficult to decide as to the precise nature of the fossil, but the presence of the hyphal stalk points to a fungus rather than an alga as the most probable type of plant with which to connect it. It may possibly be a sporangium of a fungus comparable with the common mould Mucor, or it may be a zygospore formed by the conjugation of two hyphae of which only one has been preserved.
For an example of a fossil representative of the Basidiomycetes we may turn to the excellent monograph by Conwentz on the Baltic amber trees, and quote one of the forms which he has described.
In several preparations of the wood preserved by petrifaction in amber Conwentz found distinct indications of the ravages of a fungus, which suggested the presence of the recent species Polyporus vaporarius Fr. With the help of the indirect evidence afforded by the pathological effects as seen in the tissues of the host-plant, and the direct evidence of the fungal mycelium Conwentz was led to this identification.
The mycelium is brown in colour, in part thick-walled, and in part with thin walls, transversely septate and not much branched. In the portion of one of Conwentz’ figures reproduced in fig. 43, 2, the rents and holes in the tracheid walls are clearly shown; they afford the indirect evidence of fungal attacks, and are of the same nature as those shown in fig. 42, B, C and E.
Enough has been said to call attention to the paucity of exact data on which to generalise as to the geological history of fungi. The types selected for description or passing allusion have not been chosen in each case because of their special intrinsic value, but rather as convenient examples by which to illustrate authentic records or to serve as warnings against possible sources of error.
It would seem that we have fairly good and conclusive evidence of the existence in Permo-Carboniferous times of Phycomycetous fungi, but it is not until we pass to post-Palaeozoic or even Tertiary plants that we discover satisfactory representatives of the higher fungi or Mycomycetes. If special attention were paid to the investigation of fossil fungi, it is quite possible that our knowledge of the past history of the group might be considerably extended. It is essential that the greatest caution should be exercised in the identification of forms and in their reference to definite families; otherwise our lists of fossil species will serve to mislead, and to emphasize the untrustworthy character of palaeobotanical data. Unless we feel satisfied as to the position of a fossil fungus it is unwise to use a generic term suggestive of a definite family or recent genus. Such a name as Renault has used in one instance, Palaeomyces, might be employed as a useful and comprehensive designation.
VII. CHAROPHYTA.
It has been the general custom to include the Characeæ or Stoneworts among the Chlorophyceae (green algae), of which they form a distinctly isolated family. On the whole, it would seem better to follow the course lately adopted by Migula[437] and allow the Characeæ to rank as a family of a distinct group, Charophyta. While agreeing in many respects with plants higher in the scale than Thallophytes, the Stoneworts do not sufficiently resemble the Bryophyta to be included in that group.
The Charophyta are plants containing chlorophyll, living in fresh and brackish water; the stem is jointed, and bears at the nodes whorls of leaves, on which are borne the reproductive organs. The antheridia are spherical in shape and of complex structure, containing numerous biciliate antherozoids. The oogonia are oval in form and contain a single large egg-cell. The Chara-plant is developed from a protonema formed from the germinating oospore. Vegetative reproduction is effected by means of bulbils, accessory shoots, etc.
The Nitelleæ have not been recognised in a fossil condition. The absence or feeble development of a calcareous incrustation renders the genera of this family less likely to be preserved than such a genus as Chara.
Chareae.
Leaves and stems with or without a cortical investment. Fruit with a five-celled corona. The envelope of the ‘fruit’ and other parts of the plant are frequently encrusted with carbonate of lime.
In the genus Chara, the best known member of the family, the plant as a whole resembles in its general habit and external differentiation of parts the higher plants. The stem consists of long internodes separated by short nodes bearing whorls of leaves. Each internode consists of a long cylindrical cell, which becomes enclosed by a cortical sheath composed of rows of cells which have grown upwards and downwards from the peripheral nodal cells. The cortical cells are usually spirally twisted and impart to the stem a characteristic appearance; they are divided by transverse walls into numerous cells some of which occasionally grow out into short processes (fig. 45 c). The leaves repeat on a smaller scale the structural features of the stem, but possess a limited growth, whereas the stem has an unlimited power of growth by means of a large hemispherical apical cell. Branches arise in the axils of the leaves. The plants are either monoecious or dioecious. The oogonium is elliptical in shape, and is borne on a short stalk-cell, it contains a single oosphere. The wall of the oogonium is formed of five spirally twisted cells which have grown over it from the five peripheral cells of a leaf-node. The tips of the investing cells project at the apex in the form of a terminal crown or corona (fig. 45, E, c). The antheridia have a complex structure, and produce a very large number of motile antherozoids.
After fertilisation, the egg-cell becomes surrounded by a membrane, at first colourless, but afterwards yellow or brown. The inner cell-walls of the cells surrounding the oospore become thicker and darker in colour; the outer walls remain thin and eventually fall away. The lateral walls may or may not become thickened. In most of the Chareae a calcareous deposit is formed between the hard shell and the outer walls of the cells enveloping the oospore. This calcareous shell is developed subsequently to the thickening and hardening of the inner walls of the fruit-case. The cells of the corona and stalk do not become calcareous. In the fossil Charas, it is this calcareous shell that is preserved. In the members of the Chareae the stems are usually encrusted with carbonate of lime, and thus have a much better chance of preservation than the slightly calcareous Nitelleæ.
Chara.
The generic characters have already been described in the brief account of the family Chareae.
The generic name was proposed by Vaillant in 1719[438], and adopted by Linnaeus, who classed the Stoneworts with aquatic phanerogams. As long ago as 1623[439] a figure of Chara was published by Caspar Bauhin as a form of Equisetum. The generic name Chara has usually been applied to recent and fossil species alike. The existing species have a wide distribution; Chara foetida, A. Br., a common British form, occurs in practically all parts of the world. Stems and calcareous ‘fruit-cases’ occur fairly commonly in a fossil state, and differ but little from recent species, at least as regards essential features.
It is difficult to say at what geological horizon the Stoneworts are first represented. The first certain traces of Chara occur in Jurassic rocks, but certain spirally marked subspherical bodies have been recorded from Devonian and Carboniferous strata, which closely resemble Chara oogonia, and may be Palaeozoic representatives of the genus.
In 1889 Mr Knowlton[440] of the American Geological Survey described some ‘problematic organisms’ found in Devonian rocks at the falls of the Ohio. Examples of these fossils are shown in fig. 46 b and c; the spirally grooved body measures from 1·50 to 1·80 mm. in diameter, and about 1·70 mm. in length. The Chara-like character of the fossils had been previously suggested by Meek[441] in 1873. Without going into the arguments for or against placing these fossils in the Chareae, they may at least be mentioned as possible but not certain Palaeozoic forms of Chara or an allied genus.
In this form the ‘fruits’ are minute and subspherical, ·39-·44 mm. long, and ·35-·40 mm. broad, showing in side view 5–6 slightly oblique spiral bands. Each spiral band bears a row of slightly projecting tubercles.
This species was first described by Saporta[442] from the Oxfordian (Jurassic) rocks of the Department of Lot in France; it is compared by the author of the species with Chara Jaccardi Heer, described by Heer from the Upper Jurassic rocks of Switzerland.
The Oogonia are broadly oval, about ·5mm. in length, and at the broadest part of about the same breadth. The surface is marked by eleven or twelve bands in the form of a flattened spiral. The stems possess investing cortical cells.
This species was founded on specimens from the Wealden beds of Sussex[443], but numerous examples of Chara ‘fruits’ and stems have long been known from the uppermost Jurassic rocks of the Dorset coast and the Isle of Wight, which may probably be included in this species. These fossil Charas are abundant[444] in the Chert beds of Purbeck age seen in the cliffs near Swanage. Pieces of corticated stems from this locality are represented in fig. 45 A and B.
The cortical cells surrounding a large internodal cell are very clearly seen in the section shown in fig. 45 B, and in the longitudinal view in fig. 45 A. The resemblance of these specimens to the stems of recent Stoneworts is very striking.
The single oogonium of fig. 47 was found in the Wealden beds near Hastings.
This species is characterised by globular or somewhat elliptical oogonia, with six or seven spiral bands.
It is very abundant in the Lower Headon beds of Hordwell Cliffs on the Hampshire coast[445]. Various species of Chara are commonly met with in the Oligocene beds of the Isle of Wight and Hampshire, as well as in the Paris basin beds, and elsewhere. Well preserved ‘fruits’ and stem fragments are met with in a siliceous rock of Upper Oligocene age imported from Montmorency in the Paris basin, and used as a stone for grinding phosphates at some chemical works near Upware, a few miles from Cambridge.
Many other species of fossil Charas are known from various horizons and localities, but the above examples suffice as illustrative types. In Post-Tertiary deposits masses of Chara and plant fragments occasionally occur forming blocks of Travertine. Examples of such Chara beds have been recorded by Sharpe from Northampton[446], by Lyell[447] from Forfarshire, and by other writers from several other districts. Beds of calcareous marl are occasionally seen as whitish streaks in the peat of the Fenland[448]; these often consist in great part of Charas. A season’s growth of Chara in a shallow lake or mere in the Fens may appear as a white line in a section of peaty and other material which has been formed on the site of old pools or lakes.
The recognition of specific characters in the isolated Chara ‘fruits’ usually met with in a fossil state is exceedingly unsatisfactory; the features usually relied on in the living species are not preserved, and great care should be taken in the separation of the various forms.