Chapter VII: Thallophyta (3)

In an article contributed to _Science-Progress_ in 1895 I ventured to express doubts as to the correctness of the conclusions of MM. Renault and Bertrand[339]. Since then Prof. Bertrand has very kindly demonstrated to me many of his microscopic preparations of various Bogheads, and I am indebted to Prof. Bayley Balfour of Edinburgh for an opportunity of examining a series of sections of the Scotch Boghead. The examination of these specimens has convinced me of the difficulties of the problems which many investigators have tried to solve, but it has by no means led me to entirely adopt the views expressed by MM. Bertrand and Renault.

[Sidenote: BOGHEAD.]

The Boghead or Torbanite of Scotland was rendered famous by a protracted lawsuit tried in Edinburgh from July 29th to August 4th, 1853. A lease had been granted by Mr and Mrs Gillespie, of Torbanehill, in Fifeshire, to Messrs James Russell and Son, coal-masters of Falkirk, of “the whole _coal_, ironstone, iron-ore, limestone, and fire-clay (but not to comprehend copper, or any other minerals whatsoever, except those specified) with lands of Torbanehill[340].” After the Boghead had been worked for two years the Gillespies challenged the right of Messrs Russell, and argued that the valuable _mineral_ Torbanite was not included among the substances named in the agreement. The defendants maintained that it was a _coal_, known as gas-, cannel- or parrot-coal. A verdict was given for the defendants. Some of the scientific experts who gave evidence at the trial considered that the Boghead afforded indications of organic structure, while others regarded it as essentially mineral in origin.

The Torbanite or Boghead is a close-grained brown rock, of peculiar toughness and having a subconchoidal fracture. It contains about 65% carbon, with some hydrogen, oxygen, sulphur, and mineral substances. A thin section examined under the microscope presents the appearance of a dark and amorphous matrix, containing numerous oval, spherical and irregularly shaped bright orange-yellow patches. Fig. 36, 1 shows the manner of occurrence of the yellow bodies in a piece of Scotch Boghead, as seen in a slightly magnified horizontal section. Under a higher power the light patches in the figure reveal traces of a faint radial striation, which in some cases suggests the occurrence of a number of oval or polygonal cells.

The Autun Boghead possesses practically the same structure. The yellow bodies are often sufficiently abundant to impart a bright yellow colour to a thin section. If the section is vertical the coloured bodies are seen to be arranged in more or less regular layers parallel to the plane of bedding.

The Kerosene shale of New South Wales agrees closely with the Scotch and French Boghead; it is approximately of the same geological age, and is largely made up of orange or yellow bodies similar to those of the European Boghead, but much more clearly preserved.

The nature and manner of formation of the various forms of coal should be dealt with in a later chapter devoted to the subject of plants as rock-builders, but in view of the recent statements as to the algal nature of these bituminous deposits it may not be out of place to state briefly the main conclusions of the French authors.

MM. Renault and Bertrand regard each of the yellow bodies in the European and Australian Boghead as the thallus of an alga. To the form which is most abundant in the Kerosene shale they have given the generic name of _Reinschia_, while that in the Scotch and French Boghead is named _Pila_.

_Reinschia._ Fig. 36, 3.

A section of a piece of Kerosene shale at right angles to the bedding appears to be made up of fairly regular layers of flattened elliptical sacs of an orange or yellow colour. Each sac or thallus is about 300µ in length and 150µ broad (fig. 36, 3). A single row of cells constitutes the wall surrounding the central globular cavity. The cells are more or less pyriform in shape, and the cell-cavities are filled with a dark substance, described by Renault and Bertrand as protoplasm, and the cell-walls are fairly thick. In some of the larger specimens there are often found a few smaller sacs enclosed in the cavity of the partially disorganised mother-thallus. In the larger specimens the wall is usually invaginated in several places, giving the whole thallus a lobed or brain-like appearance. The supposed alga, which makes up ⁹⁄₁₀ths of the contents of a block of Kerosene shale, is named _Reinschia Australis_; it is regarded by the authors of the species as nearly related to the Hydrodictyaceae or Volvocineae.

In the Kerosene shale from certain localities in New South Wales Bertrand recognises a second form of thallus, which he refers to the genus _Pila_, characteristic of the European Bogheads.

_Pila_. Fig. 36, 2.

The “thallus” characteristic of the Scotch Boghead has been named _Pila scotica_, and that of the Autun Boghead, _Pila bibractensis_.

In the latter form, which has been studied in more detail by MM. Renault and Bertrand, the thallus consists of about 6–700 cells, and is irregularly ellipsoidal in form, from ·189–·225mm. in length, and ·136–·160mm. broad. The surface-cells are radially disposed and pyramidal in shape, the internal cells are polygonal in outline and less regularly arranged (fig. 36, 2). The Pila thalli make up ¾ths of the mass in an average sample of the Autun Boghead. The Autun Boghead often contains siliceous nodules, and sections of these occasionally include cells of a _Pila_ in which the protoplasmic contents and nuclei have been described by the French authors. The evidence for the existence of these supposed nuclei is, however, not entirely satisfactory; sections of silicified thalli which were shown to me by Prof. Bertrand did not satisfy me as to the minute histological details recognised by Bertrand and Renault.

The species of _Pila_ are compared with the recent genus _Celastrum_, and regarded as most nearly allied to the Chroococcaceae or Pleurococcaceae among recent algae. Prof. Bornet[341] has suggested _Gomphosphaeria_ as a genus which presents a resemblance to the Autun _Pila_.

In addition to the Bogheads of Autun, Torbanehill, and New South Wales, there are similar Palaeozoic deposits in Russia, America, and various other parts of the world. Full details of the structure of Boghead and the supposed algae referred to _Reinschia_, _Pila_, and other genera will be found in the writings of Bertrand and Renault[342].

The Kerosene shale of New South Wales affords the most striking and well-preserved examples of the cellular orange and yellow bodies referred to as the globular thalli of algae. It is almost impossible to conceive a purely inorganic material assuming such forms as those which occur in the Australian Boghead. On the other hand, it is hardly less easy to understand the possibility of such explanations as have been suggested of the organic origin of these characteristic bodies.

The ground-mass or matrix of the Boghead is referred to a brown ulmic precipitate thrown down on the floor of a Permian or Carboniferous lake, probably under the action of calcareous water. In this material there accumulated countless thalli of minute gelatinous algae, which probably at certain seasons completely covered the surface of the waters, as the _fleurs d’eau_ in many of our fresh-water lakes. In addition to the thalli of _Reinschia_ and _Pila_ the Bogheads contain a few remains of various plant fragments, pollen-grains, and pieces of wood. Fish-scales and the coprolites of reptiles and fishes occur in some of the beds. On a piece of Kerosene shale in the Woodwardian Museum, Cambridge, there are two well-preserved graphitic impressions of the tongue-shaped fronds of _Glossopteris Browniana_, Brongn. There can be little doubt that the beds of Boghead were deposited under water as members of a regular sequence of sedimentary strata. The yellow bodies which form so great a part of the beds are practically all of the same type. _Reinschia_ and _Pila_ cannot always be distinguished, and it would seem that there are no adequate grounds for instituting two distinct genera and referring them to different families of recent algae.

Stated briefly, my conclusion is that the algae of the French authors may be definite organic bodies, but it is unwise to attempt to determine their affinities within such narrow limits as have been referred to in the above _résumé_. The structure of the bituminous deposits is worthy of careful study, and it is by no means impossible that further research might lead us to accept the view of the earlier investigators, that the brightly coloured organic-like bodies may be inorganic in origin.

D. RHODOPHYCEAE. (FLORIDEAE. RED ALGAE.)

The thallus of the members of this group assumes various forms, and consists of branched cell-filaments of a more or less complex structure. Cells of the thallus contain a red colouring matter in addition to the green chlorophyll. The reproduction is asexual and sexual; the formation of asexual reproductive cells (_tetraspores_) in groups of four in sporangia is a characteristic method of reproduction. Sexual reproduction is effected by means of distinct male and female cells.

With the exception of a few fresh-water genera all the red algae are marine. The Rhodophyceae, like the Cyanophyceae and Chlorophyceae, include a shell-boring form which has been found in the common razor-shell[343]. Several genera live as endophytes in the tissues of other algae. The recent species of this section of algae are characteristic of temperate and tropical seas. One subdivision of the red algae, the Corallinaceae, is extremely important from a geological point of view and must be dealt with in some detail.

CORALLINACEAE.

The thallus is usually encrusted with carbonate of lime; it is of a branched cylindrical form in the well-known _Corallina officinalis_, Linn. of the British coasts, of an encrusting and foliaceous type, in the genus _Lithophyllum_, and of a more coral-like form in the genus _Lithothamnion_. The reproductive organs occur in conceptacles, having the form of small depressed cavities in the thallus, or projecting as warty swellings above the surface of the plant. Asexual reproduction is by means of tetraspores formed in conceptacles resembling those containing the sexual cells. The Corallinaceae may be subdivided into the two families Melobesieae and Corallineae[344].

=Melobesieae.= Thallus encrusting, leaf- or coral-like;
unsegmented.
(_Melobesia_, _Lithophyllum_, _Lithothamnion_.)

=Corallineae.= Cylindrical filamentous and segmented thallus.
(_Amphiroa_ and _Corallina_.)

The genus _Corallina_ is the best known British representative of the Corallinaceae. With other members of the group it was long regarded as a coralline animal, and it is only comparatively recently that the plant-nature of these forms has been generally admitted. _Lithophyllum_, _Lithothamnion_, _Melobesia_, and other genera of the Corallinaceae and some of the Siphoneae play a very important part in the building and cementing of coral-reefs. The pink or rose-coloured calcareous thallus of some of these calcareous algae or Nullipores imparts to coral-reefs a characteristic appearance. In some cases, indeed, the coral-reefs are very largely composed of algae. Saville Kent[345] describes the Corallines or Nullipores of the Australian Barrier-reef as furnishing a considerable quota towards the composition of the coral rock. Mr Stanley Gardiner, who accompanied the coral-boring expedition to the island of Funafuti, has kindly allowed me to quote the following extract from his notes, which affords an interesting example of the importance of calcareous algae as reef-building organisms. “It is quite a misnomer to speak of the outer edge of a reef like this (Rotuma Island) as being formed of coral. It would be far better to call it a Nullipore reef, as it is completely encrusted by these algae, while outside in the perfectly clear water, 10 to 15 fathoms in depth, the bottom has a most brilliant appearance from masses of red, white and pink Nullipores, with only a stray coral here and there.”

Agassiz[346] has given an account of the occurrence of immense masses of Nullipores (_Udotea_, _Halimeda_ etc.) in the Florida reefs; his description is illustrated by good figures of these algae.

In the Mediterranean there are true Nullipore reefs, which are interesting geologically as well as botanically. Walther[347] has described one of these limestone-banks in the Gulf of Naples which occurs about 1 kilometre from the coast and 30 metres below the surface of the water. Every dredging, he says, brings up numberless masses of _Lithothamnion fasciculatum_ (Lamarck), and _L. crassum_ (Phil.). Between the branches of the algae, gasteropods and other animals become completely enclosed by the growing plants, while diatoms, foraminifera, and other forms of life are abundant. Water percolating through the mass gradually destroys the structure of the algal thalli, and in places reduces the whole bank to a compact structureless limestone.

The same author[348] has also called attention to the importance of _Lithophyllum_ as a constructive element in the coral-reefs off the Sinai peninsula.

_Lithothamnion_ a typical genus of the Corallinaceae may be briefly described.

_Lithothamnion._ Fig. 37.

Philippi[349] was the first writer to describe this and other genera as plants. He gave the following definition of _Lithothamnion_:

“Stirps calcarea rigida, e ramis cylindricis vel compressiusculis
dichotoma ramosis constans.”

The thallus of _Lithothamnion_ grows attached to the face of a rock or other foundation, and forms a hard, stony mass, assuming various coralline shapes. The exposed face may have the form of numerous short branches or of an irregular warty surface.

In section (fig. 37, A.) the lower part of the thallus is seen to be made up of rows of cells radiating out from a central point, and the upper portion consists of vertical and horizontal rows of cells. The whole body is divided up into a large number of small cells by anticlinal and periclinal walls, and possesses an evident cellular as distinct from a tubular structure. Conceptacles containing reproductive organs are either sunk in the thallus or project above the surface. The two types of structure in a single thallus are shown in fig. 37, A, also a conceptacle containing tetraspores.

, × 200). _B._ Section of _Lithothamnion suganum_,
Roth (after Rothpletz[351], × 100). _C._ A conceptacle with
tetraspores from a Tertiary _Lithothamnion_ (after Früh[352], ×
300). _D._ _Sphaerocodium Bornemanni_ Roth, (after Rothpletz, ×
150).]

In the closely allied _Lithophyllum_ the thallus is encrusting, and in section it presents the same appearance as the lower part of a Lithothamnion thallus.

Species of _Lithothamnion_ occur in the Mediterranean Sea, and are abundant in the arctic regions[353], while on the British coasts the genus is represented by four species[354]. Some large specimens of _Lithothamnion_ and _Lithophyllum_ are exhibited in one of the show-cases in the botanical department of the British Museum. For the best figures and descriptions of recent species reference should be made to the works of Hauck, Rosanoff, Rosenvinge, Kjellman and Solms-Laubach[355].

It is to be expected that such calcareous algae as _Lithothamnion_ should be widely represented by fossil forms. In addition to the botanical importance of the data furnished by the fossil species as to the past history of the Corallinaceae, there is much of geological interest to be learnt from a study of the manner of occurrence of both the fossil and recent representatives. As agents of rock-building the coralline algae are especially important. The late Prof. Unger[356] in 1858 gave an account of the so-called Leithakalk of the Tertiary Vienna basin, and recognised the importance of fossil algae as rock-forming organisms. The Miocene Leithakalk, which is widely used in Vienna as a building stone[357], consists in part of limestone rocks consisting to a large extent of _Lithothamnion_.

Since the publication of Unger’s work several writers have described numerous fossil species of _Lithothamnion_ from various geological horizons. A few examples will suffice to illustrate the range and structure of this and other genera of the Corallinaceae. In dealing with the fossil species it is often impossible to make use of those characters which are of primary importance in the recognition of recent species. The fossil thallus is usually too intimately associated with the surrounding rock to admit of any use being made of external form as a diagnostic feature. The size and form of the cells must be taken as the chief basis on which to determine specific differences. In the absence of conceptacles or reproductive organs it is not always easy to distinguish calcareous algae from fossil Hydrozoa or Bryozoa. In many instances, however, apart from the nature and size of the elements composing the thallus, the conceptacles afford a valuable aid to identification. An example of a fossil conceptacle containing tetraspores is shown in fig. 37, C; it is from a Tertiary species of _Lithothamnion_, described by Früh from Montévraz in Switzerland.

• • • • •

1. _Lithothamnion mamillosum_ Gümb. Fig. 32, A (i) and (ii). (p. 155.) This species was first recorded by Gümbel[358] from the Upper Cretaceous (Danian) rocks of Petersbergs, near Maëstricht, on the Belgian frontier. It was originally described as a Bryozoan. The thallus has the form of an encrusting calcareous structure bearing on its upper surface thick nodular branches, as shown in fig. 32, A (ii); in section, A (i), the thallus consists of a regular series of rectangular cells.

The specific name _mamillosum_ has also been given to a recent species by Hauck[359], but probably in ignorance of the existence of Gümbel’s Cretaceous species.

• • • • •

2. _Lithothamnion suganum_ Roth. Fig. 37, B. The section of this form given in fig. 37, B shows three oval conceptacles filled with crystalline material. The two lower conceptacles originally communicated with the surface of the thallus, but as in recent species the deeper portions of the algal body became covered over by additions to the surface, forming merely dead foundations for new and overlying living tissues.

The cells of the thallus have a breadth of 7–9µ, and a length of 9–12µ.

The specimen was obtained from a Lithothamnion bank, probably of Upper Oligocene age, in Val Sugana[360], in the Austrian Tyrol.

Numerous other species of Jurassic, Cretaceous and Tertiary age might be quoted, but the above may suffice to illustrate the general characters and mode of occurrence of the genus. It is important that the student should become familiar with the _Lithothamnion_ and _Lithophyllum_ types of thallus, in view of their frequent occurrence in crystalline limestone rocks and in such comparatively recent deposits as those of upraised coral-reefs. The coral-rock of Barbados and other West-Indian islands affords a good illustration of the manner of occurrence of fossil coralline algae in association with corals and other organisms[361].

In the fossil species of _Lithothamnion_ hitherto recorded there do not appear to be any important features in which they differ from recent forms; the geological history of the genus so far as it is known, favours the view that the generic characters are of considerable antiquity.

_Solenopora._ Fig. 38.

Mr A. Brown[362], of Aberdeen, has recently brought forward good evidence for including various calcareous fossils, described by several authors under different names and referred to various genera of fossil animals, in the genus _Solenopora_, which he places among the coralline algae.

Species of this genus have been described from England, Scotland, Esthonia, Russia, and other countries. The geological range of _Solenopora_ appears to be from Ordovician to Jurassic rocks; in some cases it is an important constituent of beds of limestone.

• • • • •

_Solenopora compacta_ (Billings). Fig. 38. This species was originally described by Billings as _Stromatopora compacta_, and afterwards defined by Nicholson and Etheridge. The thallus forms sub-spheroidal masses, from the size of a hemp-seed to that of an orange. The external surface is lobulate; the fractured surface has a porcellaneous and sometimes a fibrous appearance, and is usually white or light brown in colour. In vertical section (fig. 38, B) the cells are elongated and arranged in a radiating and parallel fashion; they often occur in concentric layers. The cells have a diameter of about ¹⁄₁₇ mm. and possess distinctly undulating walls, as seen in a tangential section (fig. 38, A). Brown describes certain larger cells in the thallus (fig. 38, A) as sporangia[363], but it is difficult to recognise any distinct sporangial cavities in the drawing. The example figured is from the Trenton limestone of Canada; a variety of the same species has been recorded from the Ordovician rocks of Girvan in Ayrshire. There appear to be good reasons for accepting Brown’s conclusion that _Solenopora_ belongs to the Corallinaceae rather than to the Hydrozoa, among which it was originally included. After comparing _Solenopora_ with recent genera of Florideae, Brown concludes that “the forms of the cells and cell-walls, the method of increase, and the arrangement of the tissue cells in the various species of _Solenopora_ bear strong evidence of relationship between that genus and the calcareous algae[364].”

The importance of the calcareous Rhodophyceae has been frequently emphasised by recent researches, and our knowledge of the rock-building forms is already fairly extensive. We possess evidence of the existence of species of different genera in Ordovician seas, as well as in those of the Silurian, Triassic, Jurassic, and more recent periods. It is reasonable to prophesy that further researches into the structure of ancient limestones will considerably extend our knowledge of the geological and botanical history of the Corallinaceae.

Numerous fossils have been described as examples of other genera[365] of Rhodophyceae than those included in the Corallinaceae, but these possess little or no scientific value and need not be considered.

E. PHAEOPHYCEAE (BROWN ALGAE).

Olive-brown algae, thallus often leathery in texture, composed of cell-filaments or parenchymatous tissue, in some cases exhibiting a considerable degree of internal differentiation. The sexual reproductive organs may be either in the form of passive egg-cells and motile antherozoids or of motile cells showing no external sexual difference.

With one or two exceptions all the genera are marine. They have a wide distribution at the present day, and are especially characteristic of far northern and extreme southern latitudes. The gigantic forms _Lessonia_, _Macrocystis_ and others already alluded to, belong to this group; also the genus _Sargassum_, of which the numberless floating plants constitute the characteristic vegetation of the Sargasso Sea.

Palaeobotanical literature is full of descriptions of supposed fossil representatives of the brown algae, but only a few of the recorded species possess more than a very doubtful value; most of them are worthless as trustworthy botanical records. Many of the numerous impressions referred to as species of _Fucoides_ and other genera present a superficial resemblance to the thallus of the common Bladder-wrack and other brown seaweeds. Such similarity of form, however, in the case of flat and branched algal-like fossils is of no scientific value. In many instances the impressions are probably those of an alga, but they are of no botanical interest. The flat and forked type of thallus of _Fucus_, _Chondrus crispus_ (L.) and other members of the Phaeophyceae is met with also among the red and green algae, to say nothing of its occurrence in the group of thalloid Liverworts, or of the almost identical form of various members of the animal kingdom. The variety of form of the thallus in one species is well illustrated by the common _Chondrus crispus_ (L.). This alga was described by Turner[366] in his classic work on the _Fuci_ under the name of _Fucus crispus_ as “a marine Proteus.” It affords an interesting example of the different appearance presented by the same species under different conditions, and at the same time it furnishes another proof of the futility of relying on imperfectly preserved external features as taxonomic characters of primary importance.

An example of a supposed Jurassic _Fucus_ is shown in fig. 49, and briefly described in the Chapter dealing with fossil Bryophytes.

Several species of Flysch Algae have recently been referred by Rothpletz[367] to the Phaeophyceae under the provisional generic name _Phycopsis_, but they are of no special botanical interest.

The extremely interesting genus _Nematophycus_ has lately been assigned by a Canadian author[368] to a position in the Phaeophyceae. Although the particular points on which he chiefly relies are not perhaps thoroughly established, there are certain considerations which lead us to include _Nematophycus_ as a doubtful member of the present group of algae.

_Nematophycus._

The stem attains a diameter of between 2 and 3 feet in the largest specimens; it is made up either of comparatively wide and loosely arranged tubes pursuing a slightly irregular vertical course accompanied by a plexus of much narrower tubes, or of tubes varying in diameter but not divisible into two distinct types. Rings of growth occur in some forms but not in others. Radially elongated or isodiametric spaces occur in the stem tissues in which the tubes are less abundant.

Reproductive organs unknown, with the possible exception of some very doubtful bodies described as spores.

In 1856 Sir William Dawson proposed the generic name _Prototaxites_ for some large silicified trunks discovered in the Lower and Middle Devonian rocks of Canada. A few years later the same writer[369] published a detailed account of the new fossils and arrived at the conclusion that the Devonian stem showed definite points of affinity with the recent genus _Taxus_, and the generic name suggests that he regarded it as the type of Coniferous trees belonging to the sub-family Taxineae. The reasons for this determination were afterwards shown by Carruthers to be erroneous. Dawson thought he recognised pits and spiral thickenings in the walls of the tubular elements, as well as pointed ends in some of the latter. The spiral markings were in reality small hyphal tubes passing obliquely across the face of the wider tubes, and the apparent ends of the supposed tracheids were deceptive appearances due to the fact that the tubes had in some cases been cut through in an oblique direction. In 1870 Carruthers[370] expressed the opinion that Dawson’s _Prototaxites_ was a “colossal fossil seaweed” and not a coniferous plant. The same author[371] in 1872 published a full and able account of the genus, and conclusively proved that _Prototaxites_ could not be accepted as a Phanerogam; he brought forward almost convincing evidence in favour of including the genus among the algae. The name _Prototaxites_ was now changed for that of _Nematophycus_. Carruthers compares the rings of growth in the fossil stems with those in the large Antarctic _Lessonia_ stems, but he regards the histological characters as pointing to the Siphoneae as the most likely group of recent algae in which to include the Palaeozoic genus.

We may pass over various notes and additional contributions by Dawson, who did not admit the corrections to his original descriptions which Carruthers’ work supplied. In 1889 an important memoir appeared by Penhallow[372] of Montreal in which he confirmed Carruthers’ decision as to the algal nature of _Prototaxites_; he contributed some new facts to the previous account by Carruthers, and expressed himself in favour of regarding the fossil plant as a near ally of the recent Laminariae. The next addition to our botanical knowledge of this genus was made by Barber[373] who described a new specific type of _Nematophycus_—_N. Storriei_—found by Storrie in beds of Wenlock limestone age near Cardiff. Solms-Laubach[374], in a recent memoir on Devonian plants, recorded the occurrence of another species of this genus in Middle Devonian rocks near Gräfrath on the Lower Rhine. Lastly Penhallow[375], in describing a new species, lays stress on the resemblance of some of the tubular elements in the stem to the sieve-hyphae of the recent seaweeds _Macrocystis_ and _Laminaria_. He concludes that the new facts he records make it clear that _Nematophycus_ “is an alga, and of an alliance with the Laminarias.” The recent evidence brought forward by Penhallow is not entirely satisfactory; the drawings and descriptions of the supposed trumpet-shaped sieve-hyphae are not conclusive. On the whole it is probably the better course to speak of _Nematophycus_ as a possible ally of the brown algae rather than as an extinct type of the Siphoneae, but until our knowledge is more complete it is practically impossible to decide the exact position of this Siluro-Devonian genus.

Solms-Laubach[376] has suggested that the generic name _Nematophyton_, used by Penhallow in preference to Carruthers’ term _Nematophycus_, is the more suitable as being a neutral designation and not one which assumes a definite botanical position. In view of the nature of the evidence in favour of the algal affinities of the fossil, the reasons for discarding Carruthers’ original name are hardly sufficient.

Before discussing more fully the distribution and botanical position of _Nematophycus_ we may describe at length one of the best known species, and give a short account of some other forms.

• • • • •

1. _Nematophycus Logani_ (Daws.). Fig. 39, A–E. The stem possesses well marked concentric rings of growth due to a periodic difference in size of the large tubular elements. The tissues consist of two distinct kinds of tubular elements, the larger tubes loosely arranged and pursuing a fairly regular longitudinal course, and having a diameter of 13–35µ; the smaller tubes, with a diameter of 5–6µ, ramify in different directions and form a loose plexus among the larger and more regularly disposed elements. Branching occurs in both kinds of tubes; septa have been recognised only in the smaller tubes. Irregular and discontinuous radial spaces traverse the stem tissues, having a superficial resemblance in their manner of occurrence to the medullary rays of the higher plants.

The best specimens of this species were obtained by Sir William Dawson from the Devonian Sandstones of Gaspé in New Brunswick. The largest stems had a diameter of 3 feet and reached a length of several feet[377]; in some examples Dawson found lateral appendages attached to the stem which he described as “spreading roots.” Externally the specimens were occasionally covered with a layer of friable coal, and internally the tissues were found to be more or less perfectly preserved by the infiltration of a siliceous solution. Most of the examples of _Nematophycus_ from Britain and Germany are much smaller and less perfectly preserved than those from Canada. The Peter Redpath Museum, Montreal, contains several very large blocks of _Nematophycus_, in many of which one sees the concentric rings of growth clearly etched out by weathering agents in a cross section of a large stem.

In fig. 39, A, a sketch is given of a thin transverse section of a stem, drawn natural size. The lines of growth are clearly seen, and as in coniferous stems the breadth of the concentric zones varies considerably. The short lines traversing the tissues in a radial direction represent the medullary-ray-like spaces referred to in the specific diagnosis. A transverse section examined under a low-power objective presents the appearance of a number of thick-walled and comparatively wide tubes loosely arranged; they may be in contact or separated from one another. If the microscope be carefully focussed through the thickness of the section the transversely-cut tubes appear to move laterally, producing a curiously dazzling effect if the objective is raised or lowered rapidly. This lateral movement is due to the undulating vertical course of the tubes. Under a higher power the lighter-coloured matrix in which the tubes are embedded shows a number of very much smaller and thinner-walled hyphal elements; some of these are cut across transversely, others more or less obliquely and others again longitudinally. These smaller tubes constitute an irregular plexus surrounding and ramifying between the larger elements. The diameter of the larger tubes decreases for a certain distance in a radial direction as seen in a transverse section, and this change in size gives rise to the appearance of concentric lines indicating periodic changes in growth.

The radial spaces are characterised by the partial absence of the larger tubes, and as seen in longitudinal sections these spaces constitute regions in which the smaller tubes branch very freely. Fig. 39, B, represents a small piece of a transverse section seen under a fairly high power. In fig. 39, C, the tubes are seen in longitudinal section. The larger elements are unseptate and not very regular in their vertical course through the stem; the smaller elements are seen as fine tubes lying between and across the larger tubes. In the sections I have examined no undoubted transverse septa were detected in any of the tubular elements.

The question as to the possible connection between the larger and smaller elements is one which is not as yet satisfactorily disposed of. Penhallow[378] regards the finer hyphal elements as branches of the larger tubes, but Barber[379], who has carefully examined good material of _Nematophycus Logani_, was unable to detect any organic connection between the two. My own observations are in accord with those of Barber. Further details and numerous figures of this species of _Nematophycus_ will be found in the memoirs of Carruthers, Penhallow and Barber.

• • • • •

Some specimens of silicified _Nematophycus_ stems afford particularly instructive examples of the state of preservation or method of mineralisation as a source of error in histological work. The sketches reproduced in fig. 39, D and E, were made from a section of a large specimen of _Nematophycus_ in the British Museum. In fig. D we have one of the radial spaces containing some indistinct small elements, the tissue surrounding the space appears to consist of polygonal cells suggesting ordinary parenchymatous tissue. In fig. E a few of these ‘cells’ are seen more clearly, they have black and ragged walls, and often contain very small and faint circles of which the precise nature is uncertain. The true interpretation of this form of structure was first supplied by Penhallow[380]. The black network simulating parenchymatous tissue consists of the substance of _Nematophycus_ tubes which has been completely redistributed during fossilisation and collected along fairly regular lines, as seen in figs. D and E. The original structure has been almost completely destroyed, and the material composing the walls of the large tubes has finally been rearranged as a network, interrupted here and there by the characteristic radial spaces which remain as evidence of the original _Nematophycus_ characters. It is possible in some cases to trace every gradation from sections exhibiting the normal structure through those having the appearance shown in figs. D and E to others in which the structure is completely lost. Penhallow describes this method of fossilisation in _N. crassus_ (Daws.); an examination of several specimens in the National Collection leads me to entirely confirm his general conclusions, and also to the opinion that _N. Logani_ shows exactly the same manner of mineralisation as _N. crassus_. The chief point of interest as regards this method of preservation lies in the fact that a fossil described by Dawson[381] as _Celluloxylon primaevum_, and referred to as a probable conifer, is undoubtedly a badly preserved _Nematophycus_. Penhallow examined Dawson’s specimens and obtained convincing evidence of their identity with certain forms of highly altered _Nematophycus_ stems.

• • • • •

2. _Nematophycus Storriei_ Barber. Fig. 40. The specimens on which Barber[382] founded this species were obtained by Mr Storrie from the Tymawr quarry near Cardiff, in beds of Wenlock age. The fragmentary nature of the material is largely compensated for by the excellence of the preservation. We may briefly define the species as follows:

The stem consists of separate interlacing undivided and usually unbranched tubes of varying diameter. Spaces more or less isodiametric in dimensions are scattered through the tissue. The spaces constitute regions in which the tubular elements branch freely.

The main distinguishing features of this British species are (i) the absence of two distinct and well-defined forms of tubular elements. The main part of the stem consists of thick walled tubes similar to those of _N. Logani_, but the spaces between them are occupied by thinner-walled and smaller tubes varying considerably in diameter; (ii) the form of the spaces which are not radially elongated as in _N. Logani_.

Fig. 40 shows the undulating course of the tubes as seen in a longitudinal section; the black colour of some of the elements is due to the fact that the surface of the wall is seen, while in the lighter-coloured portions of the tubes the wall has been cut through. The lighter patch about the middle of the figure shows the form of one of the spaces in which the tubes are freely branched.

In addition to the two species already described six others have been recorded, but with these we need not concern ourselves in detail. One of these species, _N. Hicksi_, was found by Dr Hicks[383] in the Denbighshire grits quarry of Pen-y-Glog near Corwen in North Wales. The position of these beds has recently been determined by Mr Lake[384] as corresponding to that of the Wenlock limestone. This species and _N. Storriei_ are both Silurian examples of the genus. It is possible, as Barber has suggested, that the specimens described under these two names should be referred to one species. The specimens found by Hicks were small and imperfectly preserved fragments; Etheridge has given a full description of their structure, and Barber has subsequently examined the material. The preservation is not such as will admit of any very precise specific diagnosis; the fragments are correctly referred to _Nematophycus_, but their specific characters cannot be clearly determined.

Solms-Laubach[385] has described some fragments of another species of _Nematophycus_ from the Devonian rocks of the Lower Rhine. His specimens are chiefly interesting as extending the geographical range of the genus, and as affording examples of a curious method of preservation. The specimens obtained were small fragments, flattened and very dark brown in colour. The tubular elements consisted of an external membrane of black coal, enclosing a central core of dark red iron-oxide. On burning the fragment on a piece of platinum foil the coal composing the wall of the tubes was removed and the deep-red casts of the tube-cavities remained[386]. The investigation of the structural characters of this imperfect material was conducted by reflected light. Under certain conditions, when it is impossible to obtain thin sections for examination by transmitted light, it is possible to accomplish much, as shown by Solms-Laubach’s work, by means of observation with direct light.

The last species to be noticed is _Nematophycus Ortoni_ recently described by Penhallow. There are no concentric rings of growth, no radial spaces and no smaller hyphae in the tissues of this type of stem. In longitudinal section, the tubes show occasional local expansions of the lumen which Penhallow compares with the ‘trumpet-hyphae’ of some recent brown algae. No actual sieve-plates or transverse walls have been detected, but the general appearance of the tubes is considered to afford distinct evidence of the original existence of such walls. The figures accompanying the description do not carry conviction as to the correctness of the reference of the tubes to imperfectly preserved sieve-hyphae.

The following list, taken, with a few alterations, from Penhallow’s memoir[387], shows the geographical and geological range of the species of _Nematophycus_ hitherto recorded.

╭ Lower Devonian of Gaspé.
_Nematophycus Logani_ (Daws.) ┤ Silurian [Wenlock] of England.
╰ Silurian of New Brunswick.

_N. Hicksi_ (Eth.) Silurian. (Wenlock) of N. Wales.

_N. crassus_ (Daws.)[388] Middle Devonian of Gaspé and New
York.

_N. laxus_ (Daws.) Lower Devonian of Gaspé.

_N. tenuis_ (Daws.) Lower Devonian of Gaspé.

_N. Storriei_ (Barb.) Silurian (Wenlock) of Wales
(Cardiff).

_N. dechenianus_ (Pied.) Upper Devonian of Germany (Gräfrath).

_N. Ortoni_ (Pen.) Upper Erian of Ohio.

In summing up our information as to the structure of _Nematophycus_ we find there are certain points not definitely settled, and which are of considerable importance. The few recorded instances of spore-like bodies by Penhallow and Barber are not satisfactory; we are still ignorant of the nature of the reproductive organs. Such instances of lateral appendages as have been referred to do not throw much light on the habit of the plant. So far as we know at present the stem of _Nematophycus_ was not differentiated internally into a cortical and central region. It may be that the specimens have been only partially preserved, and the coaly layer which occasionally surrounds a stem may represent a carbonised cortex which has never been petrified. The large and loosely arranged tubes constitute the chief characteristic feature of the genus; in some cases (_N. Logani_) there is an accompanying plexus of smaller hyphae, in others (_N. Storriei_) there is no definite division of the tissue into two sets of tubes of uniform size, and in _N. Ortoni_ the tubular elements are all of the large type.

Penhallow has recognised the branching of large tubes in _N. Logani_ and _N. crassus_ giving rise to the small hyphal elements. In most specimens, however, no such mode of origin of the smaller tubes can be detected. The spaces which interrupt the homogeneity of the tissues in some forms have been described as branching depots, on account of the frequent occurrence in these areas of much branched hyphae. The function of these spaces (fig. 39, D, and fig. 40) may be connected with aeration of the stem-tissues.

As Carruthers first pointed out the unseptate nature of the elements and the occurrence of large and small tubes forming a comparatively lax tissue suggested affinities with such recent genera as _Penicillus_, _Halimeda_, _Udotea_ and other members of the Siphoneae. In those fossil stems which possess tubes of two distinct sizes, we cannot as a rule trace any organic connection between the two sets of tubular elements. Transverse septa have been detected in the tubes of some specimens of _N. Logani_. These considerations and the large size and habit of growth of the stem leave one sceptical as to the wisdom of assigning the fossil genus to the Siphoneae. On the other hand, apart from the doubtful sieve-hyphae of Penhallow, the manner of growth of the plant, the concentric rings, marked by a decrease in the diameter of the tubes, the lax arrangement and irregular course of the elements, afford points of agreement with some recent Phaeophyceae. The stem of a _Laminaria_ (fig. 29) or of a _Lessonia_ are the most obvious structures with which to compare _Nematophycus_. The medullary region of a _Laminaria_ or _Fucus_ and of other genera presents a certain resemblance to the tissues of the fossil stems. On the whole we may be content to leave _Nematophycus_ for the present as probably an extinct type of alga, more closely allied to the large members of the Phaeophyceae than to any other recent seaweeds.

_Pachytheca._
(A fossil of uncertain affinity.)

There is another fossil occasionally associated with _Nematophycus_ and referred by many writers to the Algae, which calls for a brief notice. _Pachytheca_ is too doubtful a genus to justify a detailed treatment in the present work. Although, as I have elsewhere suggested[389], we are hardly in a position to speak with any degree of certainty as to its affinity, it is not improbable that it may eventually be shown to be an alga.

Without attempting a full diagnosis of the genus, we may briefly refer to its most striking characters.

_Pachytheca_ usually occurs in the form of small spherical bodies, about ·5 cm. in diameter, in Old Red Sandstone or Silurian rocks. In section a single sphere is found to consist of two well marked regions; in the centre, of a number of ramifying and irregularly placed narrow tubes, and in the peripheral or cortical region, of numerous regular and radially disposed simple or forked septate tubes. The tubular elements of the two regions are in organic connection.

The name was proposed by Sir Joseph Hooker for some specimens found by Dr Strickland[390] in the Ludlow bone-bed (Silurian) of Woolhope and May-Hill. Examples were subsequently recorded from the Wenlock limestone of Malvern and from Silurian and Old Red Sandstone rocks of other districts. Hicks[391] found _Pachytheca_ in the Pen-y-Glog grits of Corwen in association with _Nematophycus_, and the two fossils have been found together elsewhere. This association led to the suggestion that _Pachytheca_ might be the sporangium of _Nematophycus_, and Dawson[392], in conformity with his belief in the coniferous character of the latter plant, referred to _Pachytheca_ as a true seed.

The best sections of this fossil have been prepared with remarkable skill by Mr Storrie of Cardiff; they were carefully examined and described by Barber in two memoirs[393] published in the _Annals of Botany_, the account being illustrated by several well executed drawings and microphotographs.

Among other difficulties to contend against in the interpretation of _Pachytheca_ there is that of mineralisation. The preservation is such as to render the discrimination of original structure as distinct from structural features of secondary origin, consequent on a particular manner of crystallisation of the siliceous material, a matter of considerable difficulty.

Suggestions as to the nature of _Pachytheca_ have been particularly numerous; it has been referred to most classes of plants and relegated by some writers to the animal kingdom. The most recent addition to our knowledge of this problematic fossil was the discovery of a specimen by Mr Storrie in which the _Pachytheca_ sphere rested in a small cup, like an acorn fruit in its cupule. This specimen was figured and described by Mr George Murray[394] in 1895; he expresses the opinion that the discovery makes the taxonomic position of the genus still more obscure. Solms-Laubach briefly refers to _Pachytheca_ in connection with _Nematophycus_, and regards its precise nature almost as much an unsolved riddle now as it was when first discovered. For a fuller account of this fossil reference must be made to the contributions of Hooker[395], Barber[396] and others. The literature is quoted by Barber and more recently by Solms-Laubach[397]. There are several specimens and microscopic sections of _Pachytheca_ in the geological and botanical departments of the British Museum. The genus has been recorded from Shropshire, North Wales, Malvern, Herefordshire, Perthshire and other British localities, as well as from Canada; it occurs in both Silurian and Old Red Sandstone rocks.

_Algites._

A generic name for those fossils which in all probability belong to the class Algae, but which, by reason of the absence of reproductive organs, internal structure, or characters of a trustworthy nature in the determination of affinity, cannot be referred with any degree of certainty to a particular recent genus or family.

This term was suggested in 1894[398] as a provisional and comprehensive designation under which might be included such impressions or casts as might reasonably be referred to Algae. The practice of applying to alga-like fossils names suggestive of a definite alliance with recent genera is as a rule unsound. It would simplify nomenclature, and avoid the multiplication of generic names, if the term _Algites_ were applied to such algal fossils from rocks of various ages as afford no trustworthy data by which their family or generic affinity can be established.

V. MYXOMYCETES (MYCETOZOA).

This class of organisms affords an interesting example of the impossibility of maintaining a hard and fast line between the animal and plant kingdom. Zoologists and Botanists usually include the Myxomycetes[399] in the text-books of their respective subjects, and the name Animal-fungi which has been applied to these organisms expresses their dual relationship. They constitute one of three groups which we may include in that intermediate zone or ‘buffer-state’ between the two kingdoms. From a palaeobotanical point of view the Myxomycetes are of little interest, but a very brief reference may be made to them rather for the sake of avoiding unnecessary incompleteness in our classification than from their importance as possible fossils.

They are organisms without chlorophyll, consisting of a naked mass of protoplasm, known as the _plasmodium_, which may attain a size of several inches. Such plasmodia creep over the surface of decaying organic substrata, and in forming their asexual reproductive cells they are converted into somewhat complex fruits containing spores. The spores produce motile swarm-cells, which eventually coalesce together to form a new plasmodium.

A few examples of fossil Myxomycetes have been recorded from the Palaeozoic and more recent formations, but none of them are entirely beyond suspicion. We may mention three examples of fossils referred to this group, but only one of these is entitled to serious consideration.

_Myxomycetes Mangini_ Ren.[400] It is not uncommon to find distinct traces of original or secondary cell-contents in well preserved petrified plant-tissues. There is often a difficulty, however, in distinguishing between the true cell-contents and the cells of some parasitic or saprophytic intruder. In some petrified corky tissue in a silicified nodule from the Permo-Carboniferous beds of Autun, Renault has recently discovered what he believes to be traces of a Myxomycetous plasmodium. The cork-cells would be without protoplasmic contents of their own, and their cavities contain a number of fine strands stretching from the cell-walls in different directions and uniting in places as irregular or more or less spherical masses. The drawings given by Renault of these irregular reticulated structures with scattered patches of what may possibly be petrified plasmodial protoplasm bear a striking resemblance to the plasmodium of a Myxomycete. A figure of the capillitium of a species of _Leocarpus_ figured by Schröter[401] in his account of the Myxomycetes in Engler and Prantl’s work is very similar to that of Renault’s ‘plasmodium.’

It is by no means inconceivable that the _Myxomycetes Mangini_ may be correctly referred to this group, but the wisdom of assigning a name to such structures may well be questioned.

• • • • •

The other two examples call for little notice. Messrs Cash and Hick[402] in a paper on fossil fungi from the Coal-Measures refer to some small spherical bodies as possibly the spores of a Myxomycete. They might be referred equally well to numerous other organisms.

Göppert and Menge[403] in their monograph on plants in the Baltic Tertiary Amber, express the opinion that an ill-defined tangle of threads which they figure may be a Myxomycete.

It would serve no useful purpose to quote other instances of possible representatives of fossil Mycetozoa; but the consideration of the above examples may serve to emphasize the desirability of refraining from converting a possibility into an apparently recognised fact by the application of definite generic and specific names.

VI. FUNGI.

The most striking difference between the fungi and algae is the absence of chlorophyll in the former, and the consequent inability of fungi to manufacture their organic compounds from inorganic material. Fungi live therefore either as parasites or saprophytes, and as the same species may pass part of its life in a living host to occur at another stage of its development as a saprophyte, it is impossible to distinguish definitely between parasitic and saprophytic forms. The vegetative body of a fungus, that is the portion which is concerned with providing nourishment and preparing the plastic food-substance for the reproductive organs, is known as the _mycelium_. It consists either of a single and branched tubular cell known as a _hypha_, or of several hyphae or thread-like elements (filamentous fungi). The hyphal filaments may be closely packed together and form a felted mass of compact tissue, which in cross section closely simulates the parenchyma of the higher plants. This pseudoparenchymatous form of thallus is particularly well illustrated by the so-called _sclerotia_; these are sharply defined and often tuberous masses of hyphal tissue covered by a firm rind and containing supplies of food in the inner hyphae. They are able to remain in a quiescent state for some time, and to resist unfavourable conditions until germination and the formation of a new individual take place. The reproductive structures assume various forms; in some of the simpler fungi (Phycomycetes) sexual organs occur, as in the parallel group of Siphoneae among the algae, but in the higher fungi the reproduction is usually entirely asexual. An interesting case has recently been recorded among the more highly differentiated fungi in which distinct sexuality has been established[404]. In addition to the reproductive organs, such as oogonia and antheridia, the asexual cells or spores are borne either in special sporangia, or they occur as exposed _conidia_ on supporting hyphae or _conidiophores_. Thick-walled and resistant resting-spores of various forms are also met with.

Without going into further details we may very briefly refer to the larger subdivisions of this group of Thallophytes.

PHYCOMYCETES. Mycelium usually consisting of a single cell.
ZYGOMYCETES, Reproduction by means of conidia, and in many
OOMYCETES, cases also by the conjugation of two similar
including hyphae or by the fertilisation of an egg-cell
=Chytridiaceae=, contained in an oogonium.
&c.

MESOMYCETES,
including the Intermediate between the Phycomycetes and
Sub-classes the higher fungi. Multicellular hyphae. No
HEMIASCI and sexual organs.
HEMIBASIDII.

MYCOMYCETES. Septate vegetative mycelium. No sexual
including the reproduction—as a general rule. Asexual conidia
Sub-classes and other forms of spores. In the Ascomycetes
ASCOMYCETES and the spores are found in characteristic
BASIDIOMYCETES. club-shaped cases or asci; in the Basidiomycetes
the spores are borne on special branches from
swollen cells known as _basidia_. The sporophore
or spore-bearing body in this group may attain a
considerable size (e.g. _Agaricus_, _Polyporus_,
&c.) and exhibit a distinct internal
differentiation.