Chapter VII: Thallophyta (1)

The divisions of the plant kingdom dealt with in the following chapters of Volume I. are taken in their natural sequence, beginning with the lowest and passing gradually to the highest groups. The list of the classes and families included in Chapters VII.–XI. is given in the table of contents preceding Chapter I.

Thallophytes are of the simplest type, but they exhibit a very wide range as regards both the structure and differentiation of the vegetative body and the methods of reproduction. In some cases the individual consists of a minute simple cell which multiplies by cell-division; in others the body or thallus is made up of a number of similar units, while in a great number of forms there is a well-marked physiological division of labour, as expressed both in the external division of the thallus into distinct organs corresponding in function to the root, stem, and leaves of the higher plants, and further in the high degree of histological differentiation of the tissues. In other thallophytes, again, the thallus is a _coenocyte_ either unseptate or incompletely septate; that is, the individual consists of a single cell differing from a true plant-cell, in the stricter sense of the term, in possessing several nuclei, in other words, the thallus is divided up into compartments by transverse septa, but each division contains more than one nucleus. Such coenocytic plants may show well-marked external differentiation of the thallus into members or parts subserving different functions.

A similar wide range is covered by the methods of reproduction among thallophytes.

I. PERIDINIALES.

The organisms included under this head are of little importance from a palaeontological point of view, but a brief reference may be made to them as a section of the Thallophyta.

The Peridiniales include very small single-celled organisms which have often been described as occupying a position on the borderland between animals and plants, lying on the “shadowy boundary between animal and vegetable life.” The individuals are rarely naked, more frequently they are covered with a cellulose or mucilaginous investment which has frequently the form of two or more minute armour-like plates of a limiting membrane. The chromatophores are green, yellow, brown or colourless. Simple division is the usual method of reproduction, but spores have been described as occurring in some species. The motile forms are provided with cilia. The Peridiniaceae, a section of the Peridiniales, are regarded as nearly related to the Diatoms.

The Peridiniales play an important rôle in the Plankton flora of the sea and freshwater lakes, and have a world-wide distribution. In the narrative of the Challenger cruise they are described as occasionally filling the tow-nets with a yellow coloured slime[165]. Some genera, such as _Ceratium_, are found in enormous numbers off the British coast.

As an example of the occurrence of fossil representatives of the Peridiniaceae reference may be made to one of two species of _Peridinium_ described by Ehrenberg in 1836. These were found in a siliceous rock described as Cretaceous in age from Delitzsch in Saxony. A comparison of Ehrenberg’s figures of the fossil species _Peridinium pyrophorum_ Ehrenb.[166], with those of the recent species _Peridinium divergens_ Ehrenb., as given by Schütt[167] and other writers, brings out clearly the very close resemblance if not identity of the two forms. Bütschli[168] in his account of the Dinoflagellata in Bronn’s _Thier-Reich_ confirms Ehrenberg’s determination of _Peridinium pyrophorum_, and points out its striking agreement with the recent species.

II. COCCOSPHERES AND RHABDOSPHERES.

(Organisms of doubtful affinity.)

Our knowledge of these minute calcareous organisms is derived from Huxley’s description of coccoliths from the Atlantic in 1857, and from the accounts of Wallich, John Murray, and other writers. In the first volume of the narrative of the Challenger cruise[169] and in the volume on deep-sea deposits[170] these minute forms of life are figured and described. In the latter volume both genera are spoken of as extremely abundant in the surface waters of the tropical and temperate regions of the open ocean, and as forming an important constituent of the Globigerine ooze; they are said to occur entangled in the gelatinous substance of the Radiolarians, Diatoms, and Foraminifera, and are very common in the stomachs of Salps, Pteropods and other pelagic animals. Rhabdospheres are rare in regions where the temperature of the water sinks below 65° F.; the Coccospheres occur in tropical and temperate latitudes, and extend further north and south than the Rhabdospheres. As regards their botanical position, John Murray expresses the view that they are in all probability pelagic algae.

In the interesting memoir by Schütt on the _Pflanzenleben der Hochsee_[171] there occurs a short reference to the forms described in the Challenger Reports, but they were not obtained by the staff of the Hensen Plankton Expedition and Schütt’s remarks are not based therefore on personal observations. While admitting the existence of such bodies, he points out that Zoologists have referred Coccospheres and Rhabdospheres to the algae as organisms which cannot be included in any group of animals, and Schütt is unable to recognise a sufficient reason for referring them to this class of plants. It is suggested indeed that they may be purely inorganic structures.

_A_, Coccosphere × 1300. _B_, Rhabdosphere × 900. _C_, Portion of
the same × 1300. _D_, Rhabdosphere of another type, in optical
section × 1900. _E_, The same in surface view × 1900. _F_, End of
one of the trumpet-shaped appendages of _E_.]

The most recent account of these two genera is by Messrs G. Murray and Blackman a short notice in _Nature_ for April 1, 1897[172]. Numerous examples of Coccospheres and Rhabdospheres were obtained by Capt. Milner of the R.M.S. Para during a voyage to Barbados by allowing the sea water to enter the feed-pipe of the boiler through a fine muslin net. All the forms described in the Challenger Reports were met with, and an examination of the material by means of extremely high objectives has confirmed the original account of the genera, and added some points to our previous knowledge.

_Coccospheres_ (fig. 25 _A_). Spherical bodies of exceedingly small size, consisting of a central protoplasmic vesicle covered with overlapping circular calcareous scales, each of which is attached to the minute cell by a button-like projection. The scales are frequently found detached and are then spoken of as Coccoliths.

_Rhabdospheres_ (fig. 25 _B_–_F_). Spherical bodies, extremely minute, consisting of a single cell, on the surface of which are embedded numerous calcareous plates bearing long blunt spines (fig. 25, _C_) or beautiful trumpet-like appendages (fig. 25, _D_–_F_). The detached plates of Rhabdospheres are known as Rhabdoliths.

In addition to the text-figures of Coccospheres and Rhabdospheres in the Challenger Reports, the same structures are shown in samples of globigerine ooze figured in Plate XI. of the Monograph on deep-sea deposits. In a recent number of _Nature_ Messrs Dixon and Joly[173] have announced the discovery of Coccoliths and Coccospheres in the coastal waters off South County Dublin. They estimate that in one sample of water taken about three miles from the Irish coast there were 200 Coccoliths in each cubic centimetre of sea water.

The interest of these calcareous bodies from a palaeobotanical point of view lies in the fact that similar forms have been recognized in the Chalk and the Upper Lias. Sorby, in his memorable Address delivered before the Geological Society in 1879, refers to the abundance of Coccoliths in sections of chalk which he examined[174]. Rothpletz[175] has recently recorded the occurrence of numerous Coccoliths, 5–12 µ in diameter, associated with the skeleton of a horny sponge (_Phymatoderma_) of Liassic age.

The question of the nature of Coccospheres and Rhabdospheres cannot be regarded as definitely settled. It has been shown by J. Murray, and more recently by G. Murray and V. H. Blackman, that on the solution of the calcareous material by a weak acid there remains a small gelatinous body apparently protoplasmic in nature. We may at least express the opinion that Schütt’s suggestion as to their being inorganic must be ruled out of court. It would appear that they are extremely minute unicellular organisms characterised by a delicate calcareous armour consisting of numerous plates or scales. We know nothing as to their life-history, and cannot attempt to determine their affinities with any degree of certainty until further facts are before us. It is not improbable that they are algae of an extremely minute size, and the evidence so far obtained would lead us to regard them as complete individuals rather than the reproductive cells of some larger organism. Mr George Murray is of opinion that they are certainly algae, but he considers that they cannot be included in any existing family. It is conceivable that they may be minute eggs or reproductive cells of animals or plants, but on the whole the balance of probability would seem to be in favour of regarding them as autonomous organisms.

III. SCHIZOPHYTA.

A. SCHIZOPHYCEAE (CYANOPHYCEAE).
B. SCHIZOMYCETES (Bacteria).

In this group are included small single-celled plants of an extremely low type of organisation, in which reproduction takes the form of multiplication by simple cell-division, or the formation of spores. The characteristic method of reproduction by division has given rise to the general term Fission-plants for this lowest sub-class in the vegetable kingdom. In many cases the members of this sub-class contain chlorophyll, and associated with it a blue-green colouring matter; such plants are classed together as the Blue-green algae, Cyanophyceae, or Schizophyceae. Others, again, are destitute of chlorophyll, and may be conveniently designated Schizomycetes or Fission-fungi. Seeing how close is the resemblance and relationship between the members of the sub-class, it has been the custom to include them as two parallel series under the general head, Schizophyta, rather than to incorporate them among the Algae and Fungi respectively.

A. SCHIZOPHYCEAE (CYANOPHYCEAE or Blue-green Algae).

_Chroococcaceae._ Thallus of a single cell, the cells may be either free, or more usually joined together in colonies enveloped by a common gelatinous matrix, formed by the mucilaginous degeneration of the outer portion of the cell-walls. Reproduction by means of simple division or resting cells.

_Nostocaceae._ Thallus consists of simple or branched rows of cells in which special cells known as _heterocysts_ often occur. Reproduction by means of germ-plants or _hormogonia_, or by resting cells specially modified to resist unfavourable conditions.

In both families the individuals are surrounded by a gelatinous envelope, which in some genera assumes the form of a conspicuous and comparatively resistant sheath. Marine, freshwater, and aerial forms are represented among recent genera. Several species occur as endophytes, living in the tissues or mucilage-containing spaces in the bodies of higher plants. In addition to the frequent occurrence of blue-green algae in freshwater streams and on damp surfaces, certain forms are particularly abundant in the open sea[176], and in lakes or meres[177] where they are the cause of what is known in some parts of the country as “the breaking of the meres” (“Fleurs d’eau”). From the narrative of the cruise of the Challenger, we learn that the Oscillariaceae are especially abundant in the surface waters of the ocean. The “sea sawdust” so named by Cook’s sailors[178], and the same floating scum collected by Darwin[179], affords an illustration of the abundance of some of these blue-green algae in the sea.

Another manner of occurrence of these plants has been recorded by different writers, which is of special importance from the point of view of fossil algae. On the shores of the Great Salt Lake, Utah, there are found numerous small oolitic calcareous bodies thrown up by the waves[180]. These are coated with the cells of _Glœocapsa_ and _Glœotheca_, two genera of the Chroococcaceae. Sections of the grains reveal the presence of the same forms in the interior of the calcareous matrix, and it has been concluded, on good evidence that the algae are responsible for the deposition of the carbonate of lime of the oolitic grains. By extracting the carbonic acid which they require as a source of food, from the waters of the lake, the solvent power of the water is decreased and carbonate of lime is thrown down. In similar white grains from the Red Sea[181] there is a central nucleus in the form of a grain of sand, and cells of Chroococcaceae occur in the surrounding carbonate of lime as in the Salt Lake oolite. Prof. Cohn of Breslau in 1862 demonstrated the importance of low forms of plant life in the deposition of the Carlsbad “Sprudelstein[182].” On the bottom of Lough Belvedere, near Mullingar in Ireland[183], there occur numerous spherical calcareous pebbles, of all sizes up to that of a filbert. From a pond in Michigan (U.S.A.)[184] similar bodies have been obtained varying in diameter from one to three and a half inches. In the former pebbles a species of _Schizothrix_, one of the Nostocaceae occurs in abundance, in the form of chains of small cells enclosed in the characteristic and comparatively hard tubular sheath, and associated with _Schizothrix fasciculata_ there have been found _Nostoc_ cells and the siliceous frustules of Diatoms. In the Michigan nodules the same _Schizothrix_ occurs, associated with _Stigonema_ and _Dichothrix_, other genera of the Nostocaceae. One of the Michigan pebbles is shown in section in fig. 32 _D_.

[Sidenote: OOLITIC STRUCTURE.]

The connection between the well-known oolitic structure, characteristic of rocks of various ages in all parts of the world, and the presence of algal cells is of the greatest interest from a geological point of view. In recent years considerable attention has been paid to the structure of oolitic rocks, and in many instances there have been found in the calcareous grains tubular structures suggestive of simple cylindrical plants, which have probably been concerned in the deposition of the carbonate of lime of which the granules consist. In 1880 Messrs Nicholson and Etheridge[185] recorded the occurrence of such a tubular structure in calcareous nodules obtained from a rock of Ordovician age in the Girvan district of Scotland. These Authors considered the tubes to be those of some Rhizopod, and proposed to designate the fossil _Girvanella_.

_Girvanella_ (fig. 26).

Messrs Nicholson and Etheridge defined the genus as follows:—

“Microscopic tubuli, with arenaceous or calcareous (?) walls,
flexuous or contorted, circular in section, forming loosely
compacted masses. The tubes, apparently simple cylinders, without
perforations in their sides, and destitute of internal partitions
or other structures of a similar kind.”

Since this diagnosis was published very many examples of similar tubular fossils have been described by several writers in rocks from widely separated geological horizons. The accompanying sketch (Fig. 26), drawn from a micro-photograph kindly lent to me by Mr Wethered of Cheltenham, who has made oolitic grains a special subject of careful investigation, affords a good example of the occurrence of such tubular structures in an oolitic grain of Silurian age from the Wenlock limestone of May Hill, Gloucestershire[186]. In the centre is a crystalline core or nucleus round which the tubules have grown, and presumably they had an important share in the deposition of the calcareous substance. The nature of _Girvanella_, and still more its exact position in the organic world, is quite uncertain; it is mentioned rather as _à propos_ of the association of recent Cyanophyceae with oolitic structure, than as a well-defined genus of fossil algae.

In the typo description of the calcareous nodules from Michigan, Murray speaks of the _Schizothrix_ filaments at the surface of the pebbles as fairly intact, while nearer the centre only sheaths were met with. It is conceivable that in some of the tubular structures referred to _Girvanella_ we have the mineralised sheaths of a fossil Cyanophyceous genus[187]. The organic nature of these tubules has been a matter of dispute, but we may probably assume with safety that in some at least of the fossil oolitic grains there are distinct traces of some simple organism which was in all likelihood a plant. Some authors have suggested that _Girvanella_ is a calcareous alga which should be included in the family Siphoneae[188]. As a matter of fact we must be content for the present to leave its precise nature as still _sub judice_, and while regarding it as probably an alga, we may venture to consider it more fittingly discussed under the Schizophyta than elsewhere.

Wethered[189] would go so far as to refer oolitic structure in general to an organic origin. While admitting that a Girvanella-like structure has been very frequently met with in oolitic rocks, it would be unwise to adopt so far-reaching a conclusion. It is at least premature to refer the formation of all oolitic structure to algal agency, and the evidence adduced is by no means convincing in every case. The discovery of _Girvanella_ and allied forms in rocks from the Cambrian[190], Ordovician, Silurian, Carboniferous, Jurassic and other systems is a striking fact, and lends support to the view that oolitic structure is in many cases intimately associated with the presence of a simple tubular organism. Among recent algae we find different genera, and representatives of different families, growing in such a manner and under such circumstances as are favourable to the formation of a ball-like mass of algal threads, which may or may not be encrusted with carbonate of lime. Similarly as regards oolitic grains of various sizes, and the occurrence in rocks of calcareous nodules, the tubular structure is not always of precisely the same type, and cannot always be included under the genus _Girvanella_.

Several observers have recorded the occurrence of low forms of plant-life in the waters of thermal springs. It has been already mentioned that Cohn described the occurrence of simple plants in the warm Carlsbad Springs, and fission-plants of various types have been discovered in the thermal waters of Iceland, the Azores[191], New Zealand, the Yellowstone Park, Japan, India, and numerous other places.

A few years ago Mr Weed, of the geological survey of the United States, published an interesting account of the formation of calcareous travertine and siliceous sinter in the Yellowstone Park district[192]. This author emphasizes the important rôle of certain forms of plants in the building up of the calcareous and siliceous material. Among other forms of frequent occurrence, _Calothrix gypsophila_ and a species _Leptothrix_ are mentioned, the former being a member of the Nostocaceae, allied to _Rivularia_, and the latter a genus of Schizomycetes. In many of the springs there are found masses of algal jelly like those previously described by Cohn in the Carlsbad waters. Sections of such dried jelly showed a number of interlaced filaments with glassy silica between them. Weed refers to the occurrence of small gritty particles in this mucilaginous material. These are calcareous oolitic granules which are eventually cemented together into a compact and firm mass of travertine by the continued deposition of carbonate of lime. The presence of the plant filaments is often difficult to recognise in the “leathery sheet of tough gelatinous material,” or in “the skeins of delicate white filaments” which make up the travertine deposits.

[Sidenote: BORINGS IN SHELLS.]

Under the head of _Cyanophyceae_, mention should be made of the recent genus _Hyella_[193], which occurs as a perforating or boring alga in the calcareous shells of molluscs. On dissolving the carbonate of lime of shells perforated by this alga, the latter is isolated and appears to consist of rows of small cells, with possibly some sporangia containing spores. Other boring algae have been recorded among the Chlorophyceae, and recently a member of the Rhodophyceae[194] has been found living in the substance of calcareous shells. Such examples are worthy of note in view of the not infrequent occurrence of fossil corals, shells and fish-scales, which have evidently been bored by an organism resembling in form and manner of occurrence these recent algal borers.

The occurrence of small ramifying tubes in recent and fossil corals, fish-scales, and bones was long ago pointed out by Quekett[195], Kölliker[196], Rose[197] and other writers[198]. These narrow tubular cavities have generally been attributed to the boring action of some parasitic organism, either a fungus or an alga. In 1876 Duncan published two important papers[199] dealing with the occurrence of such tubes in recent corals, as well as in the calcareous skeleton of _Calceolina_, _Goniophyllum_ and other Palaeozoic, Mesozoic and Tertiary species of corals. This writer attributed the formation of the cavities in the case of the fossil species to the action of a fungus which he named _Palaeachlya perforans_, and considered as very nearly related to _Achlya penetrans_ found in the “dense sclerenchyma” of recent corals. In fig. 27 A. is reproduced one of the drawings given by Rose[200] in his paper published in 1855; it shows a section of a fish-scale from the Kimeridge clay which has been attacked by a boring organism. Rose attributes the dichotomously branched canals to some “infusorial parasite.”

In the important paper by MM. Bornet and Flahault on perforating algae a full description is given of various boring forms belonging to the Chlorophyceae and the Cyanophyceae[201]. The canals which these algae produce in calcareous shells and other hard substances are of the same type as those previously described in fossil corals, fish-scales and bones. In dealing with living perforating Thallophytes the colour and other cell-contents often enable us to distinguish between algae and fungi, but in fossil specimens such tests cannot be applied. The fossil tubular borings may or may not show traces of the transverse septa and reproductive cells; it is often the case that no trace of the organism has been left, but only the canals by which it penetrated the calcareous or bony skeleton. In some of the examples of _Palaeachlya_ figured by Duncan there appear to be numerous spores in some of the sections, but it is generally a very difficult and often an impossible task to discriminate between the borings of fungi and algae in fossil material.

Fig. 27 B, which is copied from one of Bornet and Flahault’s drawings, represents a piece of Solen shell riddled with small canals made by the organism which has been named by the French authors _Ostracoblabe implexa_, and regarded by them as a fungus. Fig. 27 C represents a small piece of the vegetative body of _Ostracoblabe_ obtained from a decalcified shell. In endeavouring to determine the organism which has produced borings in fossil corals or shells, it must be borne in mind that some forms of canals or passages may have been the work of perforating sponges, but these are larger in diameter than those made by algae or fungi. By some writers[202] the tubular cavities in shells have been referred to true algae, but others consider them to be of fungal origin.

As an example of a fossil alga referred to the Cyanophyceae, the genus _Zonatrichites_[203] may be quoted. Bornemann, who first described the specimens, points out the close resemblance in habit to some members of the recent Rivulariaceae.

_Zonatrichites._

The author of the genus defines it as follows:—

“A calcareous alga, with radially arranged filaments, forming
hemispherical or kidney-shaped layers, growing on or enclosing
other bodies. Parallel or concentric zones are seen in
cross-section, formed by the periodic growth of the alga, the
older and dead layers serving as a foundation on which the young
filaments grow in radially arranged groups.”

The nodules which are apparently formed by species of this genus occur in various sizes and shapes; Bornemann describes one hemispherical mass 8 cm. broad and 4 cm. thick. In some cases the organism has given rise to oolitic spherules, which in radial section exhibit the branched tubular cells spreading in fan-shaped groups from the centre of the oolitic grain. The section parallel to the surface of a nodule presents the appearance of a number of circular or elliptical tubes cut across transversely or more or less obliquely. The resemblance between the fossil and a specimen of the recent species _Zonatrichia calcivora_ Braun, is certainly very close, but it is very difficult, in the absence of material exhibiting more detailed structure than is shown in the specimens described by Bornemann, to decide with any certainty the true position of the fossil. The figures do not enable us to recognise any trace of cells in the radiating tubes. It is possible that we have in _Zonatrichites_ an example of a Cyanophyceous genus in which only the sheaths of the filaments have been preserved. In any case it is probable that this Mesozoic species affords another instance of a fossil alga which has been responsible for certain oolitic or other structures in limestone rocks.

The species described by Bornemann was obtained from a Breccia near Lissau in Silesia, of Keuper age.

M. Renault has recently described certain minute structures in a Palaeozoic coprolite to which he gives the name _Gloioconis Borneti_[204], and which he regards as a Permian gelatinous alga similar to the well-known recent genus _Glœocapsa_. The appearances revealed in a section of the coprolite are interpreted by this author as a collection of small colonies of a unicellular gelatinous alga in various stages of development. Renault’s figure shows a spherical group of faintly outlined and cloudy bodies, most of which include one or two small dark spots. The latter are regarded as the cells of the alga, and the surrounding cloudy substance is described as the gelatinous sheath. The absence of a nucleus in these extremely minute fossil cells (8–10 µ in diameter) is referred to as an argument in favour of referring the organism to the Cyanophyceae rather than to the Chlorophyceae. It is possible that the ill-defined structure described by Renault may be a petrified alga, but there is not sufficient evidence to warrant a decided opinion; the absence of nuclei can hardly be taken seriously in such a case as this as an argument in favour of the Cyanophyceae.

[Sidenote: CYANOPHYCEAE.]

Although our exact knowledge of fossil Cyanophyceae is extremely small, it is probable that such simple forms of plants existed in abundance during the past ages in the earth’s history. Several writers have expressed the opinion that the blue-green algae may be taken as the modern representatives of those earliest plants which first existed on an archaean land-surface. The living species possess the power of resisting unfavourable conditions in a marked degree, and are able to adapt themselves to very different surroundings. Their occurrence in hot springs proves them capable of living under conditions which are fatal to most plants, and suggests the possibility of their occurrence in the heated waters which probably constituted the medium in which vegetable life began. An interesting example of the growth of blue-green algae under unfavourable conditions was recorded in 1886 by Dr Treub[205] of the Buitenzorg Gardens, Java. In 1883 a considerable part of the island Krakatoa, situated in the Straits of Sunda, between Sumatra and Java, was entirely destroyed by a terrific volcanic explosion. What remained had been reduced to a lifeless mass of hot volcanic ashes. Three years later, Treub visited the island, and found that several plants had already established themselves on the volcanic rocks. Various ferns and flowering plants were recorded in Treub’s description of this newly established flora. It seemed that the barren rocky surface had been prepared for the more highly organised plants by the action of certain forms of Cyanophyceae, which were able to live under conditions which would be fatal to more complex types.

In the petrified tissues of fossil plants there are occasionally found small spherical vesicles, with delicate limiting membranes, in the cavities of parenchymatous cells or in the elements of vascular tissue. Some of these spherical inclusions have been described as possibly simple forms of endophytic algae[206], such as we are now familiar with in species of the Cyanophyceae and other algae. So far, however, no recorded instance of such fossil endophytic algae is entirely satisfactory. Some of the cells figured by Williamson as possibly algae, endophytic in the tissues of Coal-Measure plants, are no doubt thin-walled vesicles which formed part of a highly vacuolated cell-contents. Examples of such vesicles in living and fossil cells are shown in fig. 42. The fact that the contents of living plant tissues have been erroneously described as endophytic organisms, should serve as a warning against describing fossil endophytes without the test of good evidence to support them.

The description of a fossil _Nostoc_ by the late Prof. Heer[207] from the Tertiary rocks of Switzerland cannot be accepted as a trustworthy example of a fossil plant, much less of a genus of recent algae. The application of recent generic names to fossils which are possibly not even organic must do more harm than good.

B. SCHIZOMYCETES (Bacteria).

It is impossible to draw a sharp line between the two subdivisions of the Schizophyta. The so-called Fission-Fungi or Bacteria differ from the Schizophyceae or Fission-Algae in the cell-contents being either colourless, blood-red or green, but never blue-green. We may regard the Bacteria, generally, as the lowest forms of plants; they are extremely simple organisms which have been derived from some primitive types which possessed the power of independent existence and contained chlorophyll—that important substance which enables a plant to obtain its carbon first-hand from the carbon dioxide of the atmosphere.

Bacteria may be briefly described as single-celled plants, and as de Bary suggested comparable in shape to a billiard ball, a lead pencil or a corkscrew[208]. A single spherical or cylindrical cell measures about 1 µ in diameter[209]. They occur either singly or in filaments, or as masses of various shapes consisting of numberless bacterial cells. The nature and manner of life of Bacteria, and their extraordinary power of successfully resisting the most unfavourable conditions, render it probable that they constitute an extremely ancient group of organisms.

The wonderful perfection of preservation of many fossil plants enables us to investigate the contents of petrified cells and to examine in minutest detail the histology of extinct plants. To those who are familiar with the possibilities of microscopical research as applied to silicified and calcified fossil tissues, it is by no means incredible that evidence has been detected of the existence of Bacteria as far back in the history of the earth as the Carboniferous and Devonian periods.

Were there no trustworthy records of the occurrence of Bacteria in Palaeozoic times, it would still be a natural supposition that these ubiquitous organisms must have been abundantly represented. It has been suggested as a probable conclusion that some forms of Bacteria, which produced chemical changes in the soil necessary for the nutrition of plants, must have existed contemporaneously with the oldest vegetation[210].

The paper-coal of Toula, which in some places reaches a thickness of 20 cm., is a plant-bed of exceptional interest. It differs from ordinary coal in being made up of numberless thin brown-papery sheets associated with a darker coloured substance largely composed of ulmic acid. Prof. Zeiller[211], in an interesting account of the papery layers, has shown that they consist of the cuticles of a Lepidodendroid plant, _Bothrodendron_. An examination of a piece of one of the sheets at once reveals the existence of a regular network of which the walls of the meshes are the outlines of the epidermal cells, the meshes being bridged across by a thin light brown membrane which represents the layer of cuticularised cell-wall of each epidermal cell. At regular intervals and disposed in a spiral arrangement, we find small gaps in the papery cuticle which mark the position of the _Bothrodendron_ leaves. These Palaeozoic cuticles are not petrified; they are only slightly altered, and have retained the power of swelling in water, being able to take up stains like recent tissues. It may reasonably be assumed that the persistent cuticles owe their preservation to a greater power of resistance to destructive agents than was possessed by the other tissues of the plant. It is by no means unlikely, as Renault[212] has recently suggested, that as the _Bothrodendron_ stem-fragments lay in the swamps or marshes the tissues were gradually eaten away by Bacteria, but the cuticles successfully resisted the attacks of the bacterial saprophytes. The same observer has described what he regards as the actual organism which effected this wholesale destruction, under the name _Micrococcus Zeilleri_. He finds, after treating the cuticles with ammonia to remove the ulmic acid, that there occur numerous minute spherical bodies, each surrounded by a thin envelope, either singly or in groups on the surface of the cuticular membrane. These vary in size from ·5µ to 1µ in diameter. I have not been able to detect any satisfactory proof of such _Micrococci_ in specimens of the paper-coal which were treated according to Renault’s method, but it is extremely probable that this unusual method of preservation of stem-cuticles is the result of selective bacterial action.

Renault believes that some of the minute spherulitic structures which are seen in sections of decayed tissues of Palaeozoic plants owe their origin, in part, to the ravages of bacteria. The disorganisation of parenchymatous cells gives rise to a gelatinous substance in which needle-like crystals of silica may be deposited, from a siliceous solution, in a matrix which has resulted from bacterial activity. In some of the sections of tissues figured by Renault[213] the outlines of a few cells are still indicated by fragments of the partially decayed wall, while in other cells the walls have been completely destroyed by Bacteria of which some are preserved in the centre of the cell-area, forming a kind of nucleus to the siliceous spherulites.

[Sidenote: BACILLI.]

In addition to the _Micrococcus_ described by Renault from the Toula paper-coal, there are a host of other forms which have been minutely diagnosed and figured by Profs. Renault and Bertrand[214]. These authors have discovered what they believe to be well-defined species of _Micrococcus_ and _Bacillus_ ranging in age from Devonian to Jurassic. The material which has afforded the somewhat startling results of their investigations consists partly of the coprolites of reptiles and fishes, and of silicified and calcified plant tissues.

_Bacillus Permicus._ Ren. and Bert.[215] (Fig. 28 B.)

This _Bacillus_, which was discovered in sections of a Permian coprolite from Central France, has the form of cylindrical rods 12–14µ in length, and 1·3–1·5µ broad, rounded at each end. The rods occur either singly or occasionally, two or three individuals are joined end to end. Fig. 28 B represents a piece of one of Renault and Bertrand’s sections; the small rods are clearly seen lying in various directions in the homogeneous matrix of the coprolite. Each individual is said to be surrounded by an extremely minute empty space ·4µ in width, originally occupied by the Bacillus membrane, the central rod representing the mineralised cell-contents. In this example the petrifying substance was probably derived from the phosphate of calcium of bones which were attacked by Bacteria. I am indebted to Prof. Renault for an opportunity of examining specimens of this and other fossil Bacteria, and in this particular case there is undoubtedly strong evidence in favour of the author’s determination.

_Bacillus Tieghemi_ Ren.[216] and _Micrococcus Guignardi_ Ren.[217]
(Fig. 28 A.)

Renault has given the name _Bacillus Tieghemi_ to certain minute rods 6–10µ, in length, and 2·2–3·8µ broad, often containing a dark coloured spherical spore-like body 2µ in diameter, which have been found in the tissues of a Coal-Measure plant.

The name _Micrococcus Guignardi_ has been applied to more or less spherical bodies 2·2µ in diameter, also met with in silicified plants.

A portion of one of Renault’s figures is reproduced in Fig. 28 A. The faint and broken lines mark the position of the middle lamellae of parenchymatous cells from the pith of a Calamite. The tissue has been almost completely destroyed, but the more resistant middle lamellae have been partially preserved. The short and broad rods represent what Renault terms _Bacillus Tieghemi_; the small circle in the middle of some of these being referred to as a spore, and in one specimen shown in the figure, the second rod at right angles to the first is described as a small daughter-Bacillus formed by the germination of the central spore.

The isolated circles in the figure are referred to _Micrococcus_.

[Sidenote: FOSSIL BACTERIA.]

It is unnecessary to give an account of the numerous examples of _Micrococci_ and _Bacilli_ described by Renault from Devonian, Carboniferous, Permian and Jurassic rocks. We may, however, in a few words consider the general question of the existence and possible determination of fossil Bacteria.

In 1877 Prof. Van Tieghem[218] of Paris drew attention to the method of operation and plan of attack of _Bacillus amylobacter_ as a destructive agent in the decay of plant débris in water. He was able to follow the gradual disorganisation of the tissues and the various steps in the ‘butyric fermentation’ effected by this _Bacterium_. Similarly the same author[219] was able to detect the action of an allied organism in some silicified tissues from the Carboniferous nodules of Grand-Croix, a well-known locality for petrified plants near Saint-Étienne. He recognised also the traces of the _Bacillus_ itself in the partially destroyed plant tissues. The Palaeozoic Bacteria made use of some cellulose-dissolving ferment of which the action is clearly demonstrated in sections of silicified tissues. Many of the phenomena described by Renault and Bertrand as due to similar Bacterial action, afford additional evidence that the gradual disorganisation of vegetable tissues was effected in precisely the same manner as at the present day.

In some cases we have I believe trustworthy examples of the Bacteria themselves, both in coprolites and plant-tissues, but it is more than probable that some of the recorded examples are not of any scientific value. The examination of petrified tissues under the higher powers of a microscope often reveals the existence of numerous spherical particles and rod-like bodies which agree in shape with _Micrococci_ or _Bacilli_. Minute crystals of mineral substances may occur in the siliceous or calcareous matrix of a petrified plant which simulate minute organic forms. Vogelsang[220] in his important work _die Krystalliten_ has thrown considerable light on the ontogeny of crystals, and the minute globulites and other forms of incipient crystallisation might well be mistaken for Bacterial cells. Granting, however, that we have satisfactory evidence, both direct and indirect, that some forms of Bacteria lived in the decaying tissues of Palaeozoic plants, and in the intestines of reptiles and other animals, we cannot safely proceed to specific diagnoses and determinations[221].

Renault has pointed out that fossil Bacteria may often be more readily detected than living forms owing to the presence of a brown ulmic substance which results from the carbonisation of the protoplasm. He is forced to admit, however, that such diagnostic characters as are obtained by Bacteriologists by means of cultures cannot be utilised when we are dealing with fossil examples! We are told that “Partout où nous avons cherché des Bacteriaceés, nous en avons rencontré.”[222] This indeed is the danger; an extended examination of fossil sections under an immersion-lens must almost inevitably lead to the discovery of minute bodies of a more or less spherical form which _might_ be _Micrococci_. To measure, and name such bodies as definite species of _Micrococci_ is, I believe, but wasted energy and an attempt to compass the impossible.

Specialists tell us that the accurate determination of species of recent Bacteria is practically hopeless: may we not reasonably conclude that the attempt to specifically diagnose fossil forms is absolutely hopeless? “The imagination of man is naturally sublime, delighted with whatever is remote and extraordinary—”, but it is to be deplored if the fascination of fossil bacteriology is allowed to warp sound scientific sense.

IV. ALGAE.

A. DIATOMACEAE. (Diatoms.)
B. CHLOROPHYCEAE. (Green algae.)
C. RHODOPHYCEAE. (Red algae.)
D. PHAEOPHYCEAE. (Brown algae.)

The presence of chlorophyll is one common characteristic of the numerous plants included in the Algae. The generally adopted classification rests in part on an artificial distinction, namely the prevailing colour of the plant.

It must be definitely admitted, at the outset, that palaeobotany has so far afforded extremely little trustworthy information as to the past history of algae. Were we to measure the importance of the geological history of these plants by the number of recorded fossil species, we should arrive at a totally wrong and misleading estimate. By far the greater number of the supposed fossil algae have no claim to be regarded as authentic records of this class of Thallophytes. It has been justly said that palaeontologists have been in the habit of referring to algae such impressions or markings on rocks as cannot well be included in any other group. “A fossil alga,” has often been the _dernier ressort_ of the doubtful student.

[Sidenote: LARGE SEAWEEDS.]

Before discussing our knowledge, or rather lack of knowledge, of fossil algae at greater length, it will be well to briefly consider the manner of occurrence and botanical nature of existing forms. In the sea and in fresh water, as well as in damp places and even in situations subject to periods of drought, algae occur in abundance in all parts of the world. We find them attaining full development and reproducing themselves at a temperature of −1° C. in the Arctic Seas, and again living in enormous numbers in the waters of thermal springs. Around the coast-line of land areas, and on the floor of shallow seas algae exhibit a remarkable wealth of form and luxuriance of growth. As regards habit and structure, there is every gradation from algae in which the whole individual consists of a thin-walled unseptate vesicle, to those in which the thallus attains a length unsurpassed by any other plant, and of which the anatomical features clearly express a well-marked physiological division of labour such as occurs in the highest plants.

The large and leathery seaweeds which flourish in the extreme northern and southern seas are plants which it is reasonable to suppose might well have left traces of their existence in ancient sediments. Sir Joseph Hooker, in his account of the Antarctic flora[223], investigated during Sir James Ross’s voyage in H.M. ships Erebus and Terror, has given an exceedingly interesting description of the gigantic brown seaweeds of southern latitudes. The trunks are described as usually 5–10 feet long, and as thick as a human thigh, dividing towards the summit into numerous pendulous branches which are again broken up into sprays with linear ‘leaves.’ Hooker records how a captain of a brig employed his crew for two bitterly cold days in collecting _Lessonia_ stems which had been washed up on the beach, thinking they were trunks of trees fit for burning. On our own coasts we are familiar with the common _Laminaria_, the large brown seaweed with long and strap-shaped or digitate fronds which grows on the rocks below low-tide level. The frond passes downwards into a thick and tough stipe firmly attached to the ground by special holdfasts. A transverse section of the stalk of a fairly old plant presents an appearance not unlike that of a section of a woody plant. In the centre there is a well-defined axial region or pith consisting of thick walled, long and narrow tubes pursuing a generally vertical though irregular course, and embedded in a matrix of gelatinous substance derived from the mucilaginous degeneration of the outer portions of the cell-walls. The greater part of such a section consists, however, of regularly disposed rows of cells which have obviously been formed by the activity of a zone of dividing or meristematic elements. The occurrence of distinct concentric rings in this secondary tissue clearly points to some periodicity of growth which is expressed by the alternation of narrow and broader cells. In the Antarctic genus _Lessonia_, the stem reaches a girth equal to that of a man’s thigh, and in structure it agrees closely with the smaller stem of _Laminaria_. In these large algal stems, the cells are not lignified as in woody plants, and in longitudinal section they have for the most part the form of somewhat elongated parenchyma, differing widely in appearance from the tracheids or vessels of woody plants. At the periphery of the _Laminaria_ stem, represented in fig. 29, there occur numerous and comparatively large mucilage ducts.

In certain algae of different families the thallus is encrusted with carbonate of lime, and is thus rendered much more resistant. The Diatoms, on the other hand, possess still more durable siliceous tests which are particularly well adapted to resist the solvent action of water and other agents of destruction. It is these calcareous and siliceous forms which supply the greater part of the trustworthy data furnished by fossil algae.

[Sidenote: SCARCITY OF FOSSIL ALGAE.]

It remains to consider some of the causes to which we may attribute the scarcity of fossil algae, and the possible sources of error which beset any attempt to describe or assign names to impressions and casts simulating algal forms.

In the first place, the delicate nature of algal cells is a serious obstacle to fossilisation. Even in plants in which the woody stems have been preserved by a siliceous or calcareous solution, we frequently find the more delicate cells represented by a mass of crystalline matter without any trace of the cell-walls being preserved. In such plants as algae, where the cell-walls are not lignified, but consist of cellulose or some special form of cellulose, which readily breaks down into a mucilaginous product, the tissues have but a small chance of withstanding the wear and tear of fossilisation.

The danger of relying on external form as a means of recognition is especially patent in the case of those numerous markings or impressions frequently met with on rocks, and which resemble in outline the thallus of recent algae. Among animals, such as certain Polyzoa, the flat branching body of various algae is closely simulated, and in other plants, such as the frondose liverworts, the same thalloid and branched form of body is again met with. Some of the much dissected _Aphlebia_ leaves of ferns (e.g. _Rhacophyllum_ species) bear a striking resemblance to fossil algae; and numerous other examples might be quoted. In palaeobotanical literature we find a host of names, such as _Chondrites_, _Fucoides_[224], _Caulerpites_ and others applied to indefinite and indistinct surface markings which happen to resemble in shape certain of the better known genera of recent seaweeds.

The close parallelism in outward form displayed by different genera and families of algae is in itself sufficient argument against the use of recent generic names for fossils of which the algal nature is often more than doubtful. Were external form to be accepted as a trustworthy guide, in the absence of internal structure and reproductive organs, such a genus as _Caulerpa_[225] would afford material for numerous generic designations. A comparison of the different species of this Siphoneous green alga brings out very clearly the exceedingly protean nature of this interesting genus, and serves as one instance among many of the small taxonomic value which can be attached to external configuration. _Caulerpa pusilla_ Mart. and Her., _C. taxifolia_ (Vahl.), _C. plumaris_ Forsk., _C. abies-marina_ J. Ag., _C. ericifolia_ (Turn.), _C. hypnoides_ (R. Br.), _C. cactoides_ (Turn.), _C. scalpelli formis_ (R. Br.), and others clearly illustrate the almost endless variety of form exhibited by the species of a single genus of algae. We constantly find in the several classes of plants a repetition of the same form either in the whole or in the separate members of the vegetative body, and but a slight acquaintance with plant types should lead us to use the test of external resemblance with the greatest possible caution. To emphasize this danger may seem merely the needless reiteration of a self-evident fact, but there is, perhaps, no source of error which has been more responsible for the creation of numerous worthless species among fossil plants.

There is, however, another category of impressions and casts of common occurrence in sedimentary rocks which requires a brief notice. Very many of the fossil algae described in text-books and palaeobotanical memoirs have been shown to be of animal origin, and to be merely the casts of tracks and burrows. A few examples will best serve to illustrate the identity of many of the fossils referred to algae with animal trails and with impressions produced by inorganic agency.

Dr Nathorst of Stockholm has done more than any other worker to demonstrate the true nature of many of the species of _Chondrites_, _Cruziana_, _Spirophyton_, _Eophyton_, and numerous other genera. In 1867 there were discovered in certain Cambrian beds of Vestrogothia, long convex and furrowed structures in sandstone rocks which were described as the remains of some comparatively highly organised plant, and described under the generic name _Eophyton_[226]. By many authors these fossils have been referred to algae, but Nathorst has shown that the frond of an alga trailed along the surface of soft plaster of Paris produces a finely furrowed groove (fig. 30, 2) which would afford a cast similar to that of _Eophyton_. The same author has also adduced good reasons for believing that the Eophytons of Cambrian rocks may represent the trails made by the tentacles of a _Medusa_ having a habit similar to that of _Polydonia frondosa_ Ag. Impressions of _Medusae_ have been described by Nathorst from the beds in which _Eophyton_ occurs; and the specimens in the Stockholm Museum afford a remarkable instance of the rare preservation of a soft-bodied organism[227]. By allowing various animals to crawl over a soft-prepared surface it is possible to obtain moulds and casts which suggest in a striking manner the branched thallus of an alga. The tracks of the Polychaet, _Goniada maculata_ Örstd.[228], one of the Glyceridae, are always branched and very algal-like in form (fig. 30, 3). Many of the so-called fossil algae are undoubtedly mere tracks or trails of this type. In the fossil-plant gallery of the British Museum there are several specimens of small branched casts, clearly marked as whitish fossils on a dark grey rock of Lower Eocene age from Bognor; these were described by Mantell and Brongniart[229] as an alga, but there is little doubt of their being of the same category as the track shown in fig. 30, 3.

[Sidenote: FOSSILS SIMULATING ALGAE.]

The well-known half-relief casts met with in Cambrian, Silurian and Carboniferous rocks, and known as _Cruziana_ or Bilobites, are probably casts of the tracks of Crustaceans. The impression left by a King-Crab (_Limulus_) as it walks over a soft surface affords an example of this form of cast. It has been suggested that some of the Bilobites may be the casts of an organism like _Balanoglossus_[230], a worm-like animal supposed by some to have vertebrate affinities. The resemblance between some of the lower Palaeozoic Bilobites and the external features of a _Balanoglossus_ is very striking, and such a comparison is worth considering in view of the fact that soft-bodied animals have occasionally left distinct impressions on ancient sediments.

The literature on the subject of fossil algae _versus_ inorganic and animal markings is too extensive and too wearisome to consider in a short summary; the student will find a sufficient amount of such controversial writing—with references to more—in the works quoted below[231].

In the Stockholm Museum of Palaeobotany there is an exceedingly interesting collection of plaster casts obtained by Dr Nathorst in his experiments on the manufacture of fossil ‘algae,’ which afford convincing proof of the value and correctness of his general conclusions.

The pressure of the hand on a soft moist surface produces a raised pattern like a branched and delicate thallus. The well-known _Oldhamia antiqua_ Forbes and _Oldhamia radiata_ Forbes[232], from the Cambrian rocks of Ireland may, in part at least, owe their origin to mechanical causes, and we have no sufficient evidence for including them among the select class of true fossil algae. Sollas[233] has shown that the structure known as _Oldhamia radiata_ is not merely superficial but that it extends across the cleavage-planes. _Oldhamia_ is recorded from Lower Palaeozoic rocks in the Pyrenees[234] by Barrois, who agrees with Salter, Göppert and others in classing the fossil among the algae. The photograph accompanying Barrois’ description does not, however, add further evidence in favour of accepting _Oldhamia_ as a genus of fossil algae.

The burrows made by _Gryllotalpa vulgaris_ Latr., the Mole-cricket, have been shown by Zeiller to bear a close resemblance to a branch of a conifer in half-relief (fig. 30, 4), or to such a supposed algal genus as _Phymatoderma_[235].

In fig. 30, 1, we have what might well be described as a fossil alga. This is merely a cast of a miniature river-system such as one frequently sees cut out by the small rills of water flowing over a gently-sloping sandy beach. A cast figured and described by Newberry as an alga, _Dendrophycus triassicus_[236], from the Trias of the Connecticut Valley, is practically identical with the rill-marks shown in fig. 30, 1. The cracks produced in drying and contracting sediment may form moulds in which casts are subsequently produced by the deposition of an overlying layer of sand, and such casts have been erroneously referred to algal impressions[237]. Dawson[238] has figured two good examples of Carboniferous rill-marks from Nova Scotia in his paper on Palaeozoic burrows and tracks of invertebrate animals.

[Sidenote: RECOGNITION OF FOSSIL ALGAE.]

The specimen represented in fig. 31 affords an example of a fairly well-known fossil from the Wenlock limestone, originally described by Salter as _Chondrites verisimilis_ Salt, from Dudley[239]. He regarded it as an alga, and the graphitic impression agrees closely in form with the thallus of some small seaweeds. A closer examination of the fossil reveals a curious and characteristic irregular wrinkling on the graphite surface, which suggests an organism of more chitinous and firmer material than that of an alga.

A similar and probably an identical fossil is described and figured by Lapworth[240] in an appendix to a paper by Walter Keeping on the geology of Central Wales, under the name of _Odontocaulis Keepingi_ Lap. and regarded as a dendroid graptolite. In any case we have no satisfactory grounds for including these fossils in the plant-kingdom.