specimens of short, calcareous tubes which Munier-Chalmas has shewn
are no doubt the isolated segments of an alga practically identical
with the recent _Cymopolia_. A section[318] through one of the
fossil segments presents precisely the same features as those which
are represented in fig. 33, A. The habit of the Eocene alga and its
minute structure were apparently almost identical with those of the
recent species, _Cymopolia barbata_. The two drawings of _Cymopolia_
reproduced in fig. 33, A and B, have been copied from Munier-Chalmas’
note in the _Comptes Rendus_[319]; the corresponding figures given
by this author of the Eocene species (_Cymopolia elongata_ Deb.) are
practically identical with figs. A and B, and show no points of real
difference. The segments of the thallus of the fossil species, as
figured by Defrance[320], appear to be rather longer than those of the
recent species. The calcareous investment of the axial cell of the
thallus was traversed by regular verticils of branches or ‘leaves’; the
central branch of each whorl terminates in an oval sporangial cavity,
exactly as in fig. 33, A and B; and from the top of this branch there
is given off a ring of slender prolongations which terminate on the
surface of the calcareous tube as regularly disposed depressions, which
were no doubt originally occupied by their swollen distal ends as in
the recent species.
_Vermiporella._
This generic name was proposed by Stolley for certain branched and
curved tubes found in Silurian boulders from the North German
drift[321]. The tubes have a diameter of ·5–1 mm., and are perforated
by radial canals which probably mark the position of verticils of
branches given off at right angles to the central axis. The surface of
the tubes is divided into regular hexagonal areas.
The resemblance of these Silurian fossils to _Diplopora_ and other
genera favours their inclusion in the Verticillate Siphoneae.
_Sycidium._ Fig. 32, B.
The fossils included in this genus were first described by Sandberger
from the middle Devonian rocks of the Eifel, and referred by him to the
animal kingdom. More recently Deecke has suggested the removal of the
genus to the calcareous Siphoneae, and such a view appears perfectly
reasonable, although without more data it is not possible to speak with
absolute certainty.
_Sycidium melo._ (Sandb.) Fig. 32, _B._ The specimen represented in
fig. 32, B (i), (ii), drawn from Deecke’s figures[322], has the form of
a small oval calcareous body, 1 mm. in transverse diameter and 1–1·3
mm. in longitudinal diameter. It is pointed at one end and flattened at
the other. At the flatter end there is a circular depression, continued
into a funnel-shaped cavity, and on the walls of this cavity there are
18–20 radially disposed ribs, which extend over the surface of the
whole body. A series of transverse ribs intersects the vertical ribs at
right angles. The calcareous wall is perforated by numerous whorls of
circular pores, and the internal cavity is a simple undivided space.
Each of these oval bodies (fig. 33, B) is probably the segment of a
thallus, and the perforations in the wall may have been originally
occupied by lateral prolongations from the unseptate axial cell of the
thallus. _Sycidium_ bears a fairly close resemblance to the Tertiary
_Ovulites_.
_Diplopora._ Fig. 35, A and B.
This genus of algae is characteristic of Triassic rocks, and is
especially abundant in Muschelkalk and Lower Keuper limestones of the
Alps, Silesia, and elsewhere. The thallus, or rather the calcareous
portion of the thallus, has the form of a thick-walled tube, with a
diameter of about 4 mm., and occasionally reaching a length of 50 mm.
At one end the tube has a rounded and closed termination, and the
wall is pierced throughout its whole length by regular whorls of fine
canals. _Diplopora_ agrees with _Cymopolia_ in its main features.
[Illustration: FIG. 35. A, B, _Diplopora._ × 2. C, D, _Gyroporella_
(after Benecke. × 4). E, Calcareous segments of _Penicillus_, from
a specimen in the British Museum. × 5. F, a single segment of
_Ovulites margaritula_ Lam. × 4. G, _Confervites chantransioides_
Born. (after Bornemann. × 150).]
Fig. 35, A, affords a diagrammatic view of a _Diplopora_ tube, and
shews the arrangement of the numerous whorls of canals. In fig. 35,
B, a piece of limestone is represented containing several Diploporas
cut across transversely and more or less obliquely. In an obliquely
transverse section of a tube perforated by horizontal canals the
cavities of the canals necessarily appear as holes or discontinuous
canals in the substance of the calcareous wall. The manner of
occurrence of the specimens points to the abundance of this genus
in the Triassic seas, and suggests that the calcareous tubes of
_Diplopora_ may have been important factors in the building up of
limestone sediments[323]. In many instances no doubt the carbonate of
lime of the thallus has been dissolved and recrystallised, and the
original form completely obliterated. As in the rocks built up largely
of calcareous Florideae (p. 185) which have lost their structure, it
is a legitimate inference that some of the limestone rocks which shew
no trace of organic structure may have been in part derived from the
calcareous incrustation of various algal genera.
_Gyroporella._ Fig. 35, C and D.
In this genus from the Alpine Trias the structure of the calcareous
tube is very similar to that in _Diplopora_, but in _Gyroporella_ the
canals form less distinct whorls and are closed externally by a small
plate, as seen in figs. 35, C and D.
As Solms-Laubach has pointed out, the branch-systems of _Diplopora_,
_Gyroporella_ and other older genera are much simpler than in the
Tertiary genera _Dactylopora_ and others[324].
A species of _Gyroporella_, _G. bellerophontis_, has recently been
described by Rothpletz[325] from Permian rocks in the Southern Tyrol.
The thallus is tubular in form and has a diameter of ·5–1 mm.
_Dactylopora._
The genus _Dactylopora_ was founded by Lamarck[326] on some fossil
specimens from the Calcaire Grossier and included among the Zoophytes.
D’Orbigny afterwards included it among the Foraminifera, and the
structure of the calcareous body has been described by Carpenter[327]
and other writers on the Foraminifera. In a specimen of _Dactylopora
cylindracea_ Lam. from the Paris basin, for which I am indebted to
Munier-Chalmas, the tubular thallus measures 4 mm. in diameter; at the
complete end it is closed and bluntly rounded. The wall of the tube is
perforated by numerous canals, and contains oval cavities which were no
doubt originally occupied by sporangia. The shape of the specimens is
similar to that of _Diplopora_, but the canals and cavities present a
characteristic and more complex appearance, when seen in a transverse
section of the wall, than in the older genus _Diplopora_. Gümbel has
given a detailed account of this Tertiary genus in his memoir on _Die
sogenannten Nulliporen_[328]; he distinguishes between _Dactyloporella_
and _Gyroporella_ by the existence of cavities in the calcareous wall
of the tube in the former genus, and by their absence in the latter.
The oval cavities in a _Dactyloporella_ were originally occupied by
sporangia; in _Diplopora_ and _Gyroporella_ the sporangia were probably
borne externally and on an uncalcified portion of the thallus.
• • • • •
In addition to the few examples of fossil species described above there
are numerous others of considerable interest, which illustrate the
great wealth of form among the Tertiary and other representatives of
the Verticillate Siphoneae.
Reference has already been made to _Vermiporella_ as an example
of a Silurian genus. Other genera have been described by Stolley
from Silurian boulders in the North-German drift under the names
_Palaeoporella_, _Dasyporella_ and _Rhabdoporella_[329]; the latter
genus is compared with the Triassic _Diplopora_, and the two preceding
with the recent _Bornetella_.
Schlüter has transferred a supposed Devonian Foraminiferal genus,
_Coelotrochium_[330], to the list of Palaeozoic Siphoneae.
Munier-Chalmas regards some of the fossils described by Saporta under
the name of _Goniolina_[331], and classed among the inflorescences of
pro-angiospermous plants, as examples of Jurassic Siphoneae. The shape
and surface-features of some of the examples of _Goniolina_ suggest a
comparison with Echinoid spines, but the resemblance which many of the
forms in the Sorbonne collection present to large calcareous Siphoneae
is still more striking. A comparison of Saporta’s fig. 5, Pl. xxxiii.
and fig. 4, Pl. xxxii. in volume iv. of the _Flore Jurassique_, with
the figures given by Solms-Laubach[332] and Cramer[333] of species of
_Bornetella_ brings out a close similarity between _Goniolina_ and
recent algae; the chief difference being the greater size of the fossil
forms. The possibility of confounding Echinoid spines with calcareous
Siphoneae is illustrated by Rothpletz[334], who has expressed the
opinion that Gümbel’s _Haploporella fasciculata_ is not an alga but the
spine of a sea-urchin.
Among Cretaceous forms, in addition to _Goniolina_, which passes
upwards from Jurassic rocks, _Triploporella_[335] and other genera have
been recorded.
_Uteria_[336] is an interesting type of Tertiary genera; it occurs
in the form of barrel-shaped rings, which are probably the detached
segments of a form in which the central axial cell was encrusted
with carbonate of lime, but the sporangia and the whorls of branches
differed from those of _Cymopolia_ in being without a calcareous
investment.
_b._ CONFERVOIDEAE.
Without attempting to describe at length the fossil forms referred to
this division of the Chlorophyceae, there is one fossil which deserves
a passing notice. Brongniart in 1828[337] instituted the generic term
_Confervites_ for filamentous fossils resembling recent species of
confervoid algae. Numerous fossils have been referred to this genus
by different authors, but they are for the most part valueless and
need not be further considered. In 1887 Bornemann described some new
forms which he referred to this genus from the Cambrian rocks of
Sardinia. He describes the red marble of San Pietra, near Masne,
as being in places full of the delicate remains of algae having the
form of branched filaments, and appearing in sections of the rock as
white lines on a dark crystalline matrix. In fig. 35, G, one of these
Sardinian specimens is represented. This form is named _Confervites
Chantransioides_[338]; the thallus consists of branched cell-filaments,
having a breadth of 6–7µ, and composed of ovate cells. It is possible
that this is a fragment of a Cambrian alga, but the figures and
descriptions do not afford by any means convincing evidence. From
post-Tertiary beds various genera, such as _Vaucheria_ and others, have
been recorded, but they possess but little botanical value.
C. INCERTAE SEDIS.
_Fossils in Boghead ‘Coal’ referred by some authors to
the Chlorophyceae._
During the last few years much has been written by two French authors,
Dr Renault and Prof. Bertrand, on the subject of the so-called Boghead
of France, Scotland, and other countries. They hold the view that the
formation of the extensive beds of this carbonaceous material was due
to the accumulation and preservation of enormous numbers of minute
algae which lived in Permo-Carboniferous lakes.
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.
[Illustration: FIG. 36, 1. Section of a piece of Scotch Torbanite.
Slightly enlarged. 2. _Pila bibractensis_ from the Autun Boghead, ×
282 (after Bertrand). 3. _Reinschia Australis_, from the Kerosene
shale of New South Wales, × 592 (after Bertrand).]
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.
[Illustration: FIG. 37, _A._ Section of a recent _Lithothamnion_ (after
Rosanoff[350], × 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].”
[Illustration: FIG. 38. Solenopora compacta (Billings). A. Tangential
section. × 100. B. Vertical section. × 50. (After Brown.)]
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.
[Illustration: FIG. 39. _Nematophycus Logani_ (Daws.). A. Part of a
transverse section from a specimen in the British Museum. (Nat.
size.) B. Transverse section from specimens in Mr Barber’s
possession. C. Longitudinal section. (B and C × 160.) D. Transverse
section showing a radial space. E. Transverse section; a few
‘cells’ more highly magnified. D and E from a specimen in the
British Museum.]
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_.
[Illustration: FIG. 40. _Nematophycus Storriei_ Barb. Longitudinal
section, from a photograph by Mr C. A. Barber. × 45.]
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.
Before describing a few examples of fossil fungi, it is important
to consider the general question of their manner of occurrence and
determination. Considering the small size and delicate nature of most
fungi, it is not surprising that we have but few satisfactory records
of well-defined fossil forms. The large leathery sporophores of
_Polyporus_ and other genera of the Basidiomycetes, which are familiar
objects as yellow or brown brackets projecting from the trunks of
diseased forest trees, have been found in a fairly perfect condition
in the Cambridgeshire peat-beds, and examples have been described also
by continental writers[405]. As a general rule, however, we have to
depend on the chance mineralisation or petrifaction of the hyphae of a
fungus-mycelium which has invaded the living or dead tissues of some
higher plant. In the literature on fossil plants there are numerous
recorded species of fungi founded on dark coloured spots and blotches
on the impression of a leaf. Most of such records are worthless; the
external features being usually too imperfect to allow of accurate
identification. The occurrence of recent fungi as discolourations on
leaves is exceedingly common, and the characteristic _perithecia_ or
compact and more or less spherical cases enclosing a group of sporangia
in certain Ascomycetous species, might be readily preserved in a fossil
condition.
[Sidenote: ASCOMYCETES.]
Some examples of possible Ascomycetous fungi have been recently
recorded by Potonié from leaves and other portions of plants of
Permian age. There is a distinct superficial resemblance between the
specimens he figures and the fructifications of recent Ascomycetes,
but in the absence of internal structure, it would be rash to do more
than suggest the probable nature of the markings he describes. For
one of the fungus-like impressions Potonié proposes the generic name
_Rosellinites_; he compares certain irregularly shaped projections on
a piece of Permian wood with the perithecia of _Rosellinia_, a member
of the Sphaeriaceae, and describes them as _Rosellinites Beyschlagii_
Pot.[406] Various other records of similar Ascomycetes-like fossils may
be found in palaeobotanical literature[407], but it is unnecessary to
examine these in detail. Unless we are able to determine the nature of
the supposed fungus by microscopical methods our identifications cannot
in most cases be of any great value.
An example of the perithecia of a fungus (_Rosellinia congregata_
[Beck])[408] has been recorded from the Oligocene of Saxony, which
would appear to rest on a more satisfactory basis than is often the
case. In this particular instance the small projections on a piece of
fossil coniferous stem present a form which naturally suggests a fungus
perithecium. In cases where the black spots on a fossil stem or leaf
possess a definite form and structure, it is perfectly legitimate to
refer them to a group of fungi; but in very many instances the forms
referred to such genera as _Sphaerites_ and others are of little or
no value. Many forms of scale-insects and galls on leaves present an
obvious superficial resemblance to epiphyllous fungi, and might readily
be mistaken for the fructifications of certain Ascomycetous species. As
examples of scale-insects simulating fungi, reference may be made to
such genera of the Coccineae as _Aspidiotus_, _Diaspis_, _Lecanium_,
_Coccus_, and others. The female insects lying on the surface of a
leaf, if preserved as a fossil impression, might easily be mistaken for
perithecia[409].
Another pitfall in fossil mycology may be illustrated by a description
of a supposed fungus, _Sclerotites Salisburiae_[410], Mass. on a
Tertiary _Ginkgo_ leaf. The figure given by Massalongo represents a
_Ginkgo_ leaf with well marked veins, the lamina between the veins
being traversed by short discontinuous and longitudinally-running
lines; the latter are referred to as the fungus. In a recent _Ginkgo_
leaf one may easily detect with the naked eye a number of short lines
between and parallel to the veins, which if examined in section
are found to be secretory canals. There can be little doubt that
_Sclerotites Salisburiae_ owes its existence to the preservation of
these canals.
The list of fossil fungi given by Meschinelli in Saccardo’s _Sylloge
Fungorum_[411] includes certain species which are of no botanical
value, and should have no place in any list which claims to be
authentic.
Among the numerous examples of fossil ‘fungi’ which have no claim to
be classed with plants, there are some which are in all probability
the galleries of wood-eating animals. The radiating grooves frequently
found on the inner face of the bark of a pine tree made by species of
the beetle _Bostrychus_ might be mistaken for the impressions of the
firm strands of mycelial tissue of some Basidiomycetous fungus.
In some notes on fossil fungi by J. F. James[412] contributed to
the American Journal of Mycology in 1893, it is pointed out that a
supposed fungus described by Lesquereux from the Lower Coal-Measures
as _Rhizomorpha Sigillariae_[413], bears a strong likeness to some
insect-burrows, such as those of _Bostrychus_.
“A new fungus from the Coal-Measures” described by Herzer in 1893[414]
may probably be referred to animal agency. In any case there is no
evidence as to the fungoid nature of the object represented in the
figure accompanying Herzer’s description.
[Sidenote: BASIDIOMYCETES.]
More trustworthy evidence of fossil fungi is afforded by the marks
of disease in petrified tissue and by the presence of true mycelia.
In examining closely the calcareous and siliceous plant-tissues from
the Coal-Measures and other geological horizons, one occasionally
sees fine thread-like hyphae ramifying through the cells or tracheal
cavities; in many cases the hyphae bear no reproductive organs and
cannot as a rule be referred to a particular type of fungus. If the
hyphal filaments are unseptate, they most likely belong to some
Phycomycetous species; or if they are obviously septate the Mesomycetes
or the Mycomycetes are the more probable groups. Occasionally there
may be found indications of the characteristic _clamp-connections_ in
the septate filaments; a small semicircular branch, which is given off
from a mycelium immediately above a transverse wall, bends round to
fuse with the filament just below the septum, thus serving as a small
loop-line connecting the cell-cavity above and below a cross wall. Such
clamp-connections are usually confined to the hyphae of Basidiomycetes
and thus serve as a useful aid in identification. A good example of a
clamp-connection in a fossil mycelium is figured by Conwentz[415] in
his monograph on the Baltic amber-trees of Oligocene age. The stout
and thick type of hypha found in some fossil woods agrees closely with
that of _Polyporus_, _Agaricus melleus_ and other well-known recent
Basidiomycetes.
In a section of a piece of lignified coniferous wood recently brought
by Col. Feilden from Kolguev island[416], the brown and stout hyphae
of a fungus are clearly seen as distinct dark lines traversing the
tracheal tissue. The occurrence of septa and the large diameter of the
mycelial branches at once suggest a comparison with such recent forms
as _Agaricus melleus_, _Polyporus_ and other Basidiomycetes. The age of
the Kolguev wood is not known with any certainty.
The vesicular swellings such as those represented in fig. 41, A, B, D
and E, may easily be misinterpreted. Such spherical expansions in a
mycelium, either terminal or intercalary, may be sporangia, oogonia or
large resting-spores, or non-fungal cell-contents, and it is usually
impossible in the absence of the contents to determine their precise
nature. Hartig[417] and others have drawn attention to the occurrence
of such bladder-like swellings in the mycelia of recent fungi, which
have nothing to do with reproductive purposes; under certain conditions
the hyphae of a fungus growing in the cavity of a cell or trachea may
form such vesicles, and these, as in fig. 42, D, _m_ may completely
fill up the cavity of a large tracheid.
Some good examples of bladder-like swellings, such as occur in the
mycelium of _Agaricus melleus_ and other recent fungi, have been
figured by Conwentz[418] in fossil wood of Tertiary age from Karlsdorf.
The swellings in this fossil fungus might easily be mistaken for
oogonia or sporangia; especially as they are few in number and
spherical in form.
A similar appearance is presented by a mass of tyloses in the cavity of
an old vessel or tracheid; and vesicular cell-contents, as in the cells
of fig. 41, A, 2–5, may closely simulate a number of thin-walled fungal
spores or sporangia.
A good example of such a vesicular tissue, in addition to that already
quoted, is afforded by a specimen of an Eocene fern, _Osmundites
Dowkeri_ Carr.[419] described by Carruthers in 1870. The ground-tissue
cells contain traces of distinct fungal hyphae (fig. 41, B), and in
many of the parenchymatous elements the cavity is completely filled
with spherical vesicles; in other cases one finds hyphae in the centre
of the cell while vesicles line the wall, as shewn in fig. 41, B.
Carruthers refers to these bladders as starch grains, and this may
be their true nature; their appearance and abundant occurrence in
the parenchyma certainly suggest vesicular cell-contents rather than
fungal cells. I could detect no proof of any connection between the
hyphae and bladders, and the absence of the latter in the cavities of
the tracheids, fig. 41, C, favoured the view of their being either
starch-grains or other vacuolated contents similar to that in the cells
of the Portland Cycad (fig. 41, A) referred to on p. 88.
[Sidenote: PATHOLOGY OF FOSSIL TISSUES.]
The vacuolated cell-contents partially filling the cells in fig. 41, D,
present a striking resemblance to the contents of the cells 2–5 in fig.
41, A. In fig. D the frothy and contracted substance might be easily
mistaken for a parasitic or saprophytic fungus, but this resemblance
is entirely misleading. It is by no means uncommon to find the cells
of recent plants occupied by such vacuolated contents, especially in
diseased tissues in which a pathological effect produces an appearance
which has more than once misled the most practised observers.
In the important work recently published by Renault on the
Permo-Carboniferous flora of Autun, there is a small spore-like body
described as a teleutospore, and classed with the Puccineae[420].
We have as yet no satisfactory evidence of the existence of this
section of Fungi in Palaeozoic times, and Renault’s description of
_Teleutospora Milloti_ from Autun might be seriously misleading if
accepted without reference to his figure. The fragment he describes
cannot be accepted as sufficient evidence for the existence of a
Palaeozoic _Puccinia_.
The same author refers another Palaeozoic fungus to the Mucorineae
under the name of _Mucor Combrensis_[421]; this identification is based
on a mycelium having a resemblance to the branched thallus of _Mucor_,
but in the absence of reproductive organs such resemblance is hardly
adequate as a means of recognition.
The occurrence of hyphal cells in calcareous shells and corals has
already been alluded to.[422] In addition to the examples referred
to above, there is one which has been described by Etheridge[423]
from a Permo-Carboniferous coral. This observer records the occurrence
of tubular cavities in the calices of _Stenopora crinita_ Lonsd.,
and attributes their origin to a fungus which he names _Palaeoperone
endophytica_; he mentions one case in which a tube contains fine
spherical spore-like bodies which he compares with the spores of a
_Saprolegnia_. As pointed out above (p. 128), it is almost impossible
to decide how far these tubes in shells and corals should be attributed
to fungi, and how far to algae.
[Illustration: FIG. 41. A. Cells of _Cycadeoidea gigantea_ Sew. × 355.
B and C. Parenchymatous cells and scalariform tracheids of
_Osmundites Dowkeri_ Carr. × 230. D. Epidermal cells of _Memecylon_
(_Melastomaceae_) with vacuolated contents. E. _Peronosporites
antiquarius_ Smith, (No. 1923 in the Williamson collection). × 230.
F. _Zygosporites_. × 230. (A, B, C and E drawn from specimens in
the British Museum; D from a drawing by Prof. Marshall Ward; F from
a specimen in the Botanical Laboratory Collection, Cambridge.)]
[Illustration: FIG. 42. A, B, C. Tracheids of coniferous wood attacked
by _Trametes radiciperda_ Hart. (_Polyporus annosus_ Fr.) D and E.
Tracheids attacked by _Agaricus melleus_ Vahl. A, × 650, B–E, ×
360. (After Hartig.)]
Passing from the direct evidence obtained from the presence of fungal
hyphae in petrified tissues, we must draw attention to the indirect
evidence of fungal action afforded by many fossil plants. It is
important to be familiar with at least the more striking effects of
fungal ravages in recent wood in order that we may escape some of the
mistakes to which pathological phenomena may lead us in the case of
fossils[424].
The gradual dissociation of the elements in a piece of fossil wood
owing to the destruction of the middle lamellae, the occurrence of
various forms of slit-like apertures in the walls of tracheids (fig.
42, E) and the production of a system of fine parallel striation on
the walls of a vessel are among the results produced by parasitic and
saprophytic fungi. With the help of a ferment secreted by its hyphae,
a fungus is able to eat away either the thickening cell layers or the
middle lamellae or both, and if, as in fig. 42, A, only the middle
lamellae are left one might easily regard such tissue in a fossil
condition as consisting of delicate thin-walled elements. The oblique
striae on the walls of a tracheid may often be due to the action of a
ferment which has dissolved the membrane in such a manner as to etch
out a system of spiral lines, probably as a consequence of the original
structure of the tracheids. In distinguishing between the woods of
Conifers the presence of spiral thickening layers in the wood element
is an important diagnostic character, and it is necessary to guard
against the confusion of purely secondary structures, due to fungal
action, with original features which may be of value in determining the
generic affinity of a piece of fossil wood.
_Oochytrium Lepidodendri_, Ren. Fig. 43, 1.
Under this name Renault has recently described a filamentous fungus
endophytic in the cavities of the scalariform tracheids of a
_Lepidodendron_[425]. The mycelium has the form of slender branched
hyphae with transverse septa. Numerous ovoid and more or less spherical
sporangia occur as terminal swellings of the mycelial threads. The
long axis of the ovoid forms measures 12–15 µ, and the shorter axis
9–10 µ; the contents may be seen as a slightly contracted mass in the
sporangial cavity. In some of the sporangia one sees a short apical
prolongation in the form of a small elongated papilla, as shown in fig.
43, 1. Renault refers this fungus to the Chytridineae, and compares it
with _Cladochytrium_, _Woronina_, _Olpidium_, and other recent genera.
In the immediate neighbourhood of two of the sporangia shown in the
uppermost tracheid of fig. 43, 1, there are seen a few minute dark dots
which are described as spores petrified in the act of escaping from a
lateral pore. This interpretation strikes one as lacking in scientific
caution.
The sporangia of _Hyphochytrium infestans_[426], as figured by Fischer
in Rabenhorst’s work bear a close resemblance to those of the fossil.
It would seem very probable that Renault’s species may be reasonably
referred to the Chytridineae, as he proposes.
[Illustration: FIG. 43. 1. _Oochytrium Lepidodendri_, Ren. (After
Renault.) 2. _Polyporus vaporarius_ Fr. var. _succinea_. (After
Conwentz.) 3. _Cladosporites bipartitus_ Fel. (After Felix.) 4.
_Haplographites cateniger_ Fel. (After Felix.)]
_Peronosporites antiquarius_ W. Smith. Fig. 41, E.
In an address to the Geologists’ Association delivered by Mr Carruthers
in 1876 a brief reference, accompanied by a small-scale drawing, is
made to the discovery of a fungus in the scalariform tracheids of a
_Lepidodendron_ from the English Coal-Measures[427]. In the following
year Worthington Smith published a fuller account of the fungus, and
proposed for it the above name[428], which he chose on the ground of
a close similarity between the mycelium and reproductive organs of
the fossil form and recent members of the Peronosporeae. In Smith’s
description the mycelium is described as bearing spherical swellings
containing zoospores. These spherical organs are fairly abundant and
not infrequently met with in sections of petrified plant-tissues from
the English Coal-Measures; they may be oogonia or sporangia, or in
some cases mere vesicular expansions of a purely vegetative hypha.
No confirmation has been given to the supposed spores referred to
by Smith. Prof. Williamson and others have carefully examined the
specimens, but they have failed to detect any trace of reproductive
cells enclosed in the spherical sacs[429]. The mycelium does not appear
to show any satisfactory evidence of its being septate as figured by
Smith.
The example shown in fig. 41 E has been drawn from one of the
Williamson specimens: it illustrates the form and manner of occurrence
of the characteristic swellings. It is probable that some at least
of the vesicles are either sporangia or oogonia, but we cannot speak
with absolute confidence as to their precise nature. The general
habit and structure of the fungus favour its inclusion in the class
of _Phycomycetes_. The occurrence of several of the vesicles close
together on short hyphal branches, as shown in Williamson’s figures,
suggests the spherical swellings on vegetative hyphae, but it is
impossible to speak with absolute confidence. There is a close
resemblance between this English form and one recently described by
Renault as _Palaeomyces gracilis_ Ren.[430]; the two fossils should
probably be placed in the same genus.
The examples referred to below and originally recorded by Cash and Hick
no doubt belong to the same type as Smith’s _Peronosporites_.
The sketches reproduced in fig. 44 have been drawn from specimens
originally described by Cash and Hick in 1878[431]. The sections were
cut from a calcareous nodule from the Halifax Coal-Measures containing
fragments of various plants and among others a piece of cortical
tissue, probably of a _Lepidodendron_ or _Stigmaria_. In a transverse
section of this tissue one sees under a moderately high power that
the cells have become partially separated from one another by the
destruction of the middle lamellae (fig. 44 A). The cell-cavities and
the spaces between the isolated cells contain numerous fine fungal
hyphae, which here and there terminate in spherical swellings. One
such swelling is shown under a low power in fig. 44 A, in the middle
uppermost cell, and more highly magnified in fig. 44 B. In fig. C
there are two such swellings (the larger one having a diameter of ·003
mm.) in contact, but the connection does not appear to be organic.
The cell-walls of the infected tissue present a ragged and untidy
appearance, and in places (_e.g._ fig. 44 D) the membrane has been
pierced by some of the mycelial branches.
[Illustration: FIG. 44. Cells with fungal hyphae. A. A piece of
disorganised tissue, showing the separation of the cells. B. Part
of A more highly magnified. C. A single cell containing two swollen
hyphae. D. Partially destroyed cell-membranes pierced by fungal
hyphae. (Drawn from sections in the Edinburgh Botanical Museum,
originally described by Cash and Hick.)]
This fungus bears a close resemblance to _Peronosporites antiquarius_,
but it is impossible to determine its precise botanical position
without further data. In Cash and Hick’s paper in which the above
fungus is briefly dealt with, some small spore-like bodies are figured
which the authors speak of as possibly a Myxomycetous fungus[432].
There is however no sound reason for such a supposition.
As examples of Ascomycetous fungi found in silicified wood of Tertiary
age, two species may be quoted from Felix.
_Cladosporites bipartitus_ Felix[433], fig. 43, 3.
The mycelium and conidia of this form were discovered in some Eocene
silicified wood from Perekeschkul near Baku, on the shores of the
Caspian. The conidia are elliptical or pyriform in shape and divided by
a transverse septum into two cells. No traces were found of any special
conidiophores. The mycelium consists of septate branched hyphae,
rendered conspicuous by a brown colouration. Felix compares the fossil
with the recent genera _Cephalothecium_ and _Cladosporium_.
_Haplographites cateniger_ Felix[434], fig. 43, 4.
The conidia of this form were found to be fairly abundant in the
silicified tissue investigated by Felix; they occur usually in chains
of 2 to 6 conidia having an ovoid or flask-shaped form, with a thick
membrane (fig. 43, 4). The mycelium consists of branched hyphae divided
into long cylindrical cells by transverse septa; occasional instances
were found of an H-shaped fusion between lateral branches of parallel
hyphae.
Felix compares this species with examples of the genera _Haptographium_
and _Dematium_ of the family Sphaeriaceae; it was found in the woody
tissue of a dicotyledonous stem from Perekeschkul.
_Zygosporites_ sp.
The object represented in fig. 41 F consists of a stalked spherical
sac bearing a number of radiating arms which are divided distally
into delicate terminations. We find similar bodies figured by
Williamson[435] in his IXth and Xth Memoirs on the Coal-Measure plants;
he includes some of them under the generic term _Zygosporites_, and
compares them with the zygospores of the freshwater algae Desmideae.
Hitherto these spore-like fossils have only been recorded as isolated
spheres, but in the example shown in fig. 41 F there is a distinct
tubular and thin-walled stalk attached to the _Zygosporites_. The
specimen was found in the partially disorganised cortical tissue of a
_Lyginodendron_ stem from the English Coal-Measures. It is difficult
to decide as to the precise nature of the fossil, but the presence of
the hyphal stalk points to a fungus rather than an alga as the most
probable type of plant with which to connect it. It may possibly be a
sporangium of a fungus comparable with the common mould _Mucor_, or it
may be a zygospore formed by the conjugation of two hyphae of which
only one has been preserved.
[Sidenote: POLYPORUS.]
For an example of a fossil representative of the Basidiomycetes we may
turn to the excellent monograph by Conwentz on the Baltic amber trees,
and quote one of the forms which he has described.
_Polyporus vaporarius_ Fr. _f. succinea_[436], fig. 43, 2.
In several preparations of the wood preserved by petrifaction in amber
Conwentz found distinct indications of the ravages of a fungus, which
suggested the presence of the recent species _Polyporus vaporarius_ Fr.
With the help of the indirect evidence afforded by the pathological
effects as seen in the tissues of the host-plant, and the direct
evidence of the fungal mycelium Conwentz was led to this identification.
The mycelium is brown in colour, in part thick-walled, and in part with
thin walls, transversely septate and not much branched. In the portion
of one of Conwentz’ figures reproduced in fig. 43, 2, the rents and
holes in the tracheid walls are clearly shown; they afford the indirect
evidence of fungal attacks, and are of the same nature as those shown
in fig. 42, B, C and E.
• • • • •
Enough has been said to call attention to the paucity of exact data on
which to generalise as to the geological history of fungi. The types
selected for description or passing allusion have not been chosen in
each case because of their special intrinsic value, but rather as
convenient examples by which to illustrate authentic records or to
serve as warnings against possible sources of error.
It would seem that we have fairly good and conclusive evidence of the
existence in Permo-Carboniferous times of Phycomycetous fungi, but
it is not until we pass to post-Palaeozoic or even Tertiary plants
that we discover satisfactory representatives of the higher fungi or
Mycomycetes. If special attention were paid to the investigation of
fossil fungi, it is quite possible that our knowledge of the past
history of the group might be considerably extended. It is essential
that the greatest caution should be exercised in the identification
of forms and in their reference to definite families; otherwise our
lists of fossil species will serve to mislead, and to emphasize the
untrustworthy character of palaeobotanical data. Unless we feel
satisfied as to the position of a fossil fungus it is unwise to use
a generic term suggestive of a definite family or recent genus. Such
a name as Renault has used in one instance, _Palaeomyces_, might be
employed as a useful and comprehensive designation.
VII. CHAROPHYTA.
CHARACEÆ. NITELLEÆ.
It has been the general custom to include the Characeæ or Stoneworts
among the Chlorophyceae (green algae), of which they form a distinctly
isolated family. On the whole, it would seem better to follow the
course lately adopted by Migula[437] and allow the Characeæ to rank
as a family of a distinct group, Charophyta. While agreeing in many
respects with plants higher in the scale than Thallophytes, the
Stoneworts do not sufficiently resemble the Bryophyta to be included in
that group.
The Charophyta are plants containing chlorophyll, living in fresh and
brackish water; the stem is jointed, and bears at the nodes whorls of
leaves, on which are borne the reproductive organs. The antheridia
are spherical in shape and of complex structure, containing numerous
biciliate antherozoids. The oogonia are oval in form and contain a
single large egg-cell. The Chara-plant is developed from a _protonema_
formed from the germinating oospore. Vegetative reproduction is
effected by means of bulbils, accessory shoots, etc.
The Nitelleæ have not been recognised in a fossil condition. The
absence or feeble development of a calcareous incrustation renders the
genera of this family less likely to be preserved than such a genus as
_Chara_.
Chareae.
Leaves and stems with or without a cortical investment. Fruit with a
five-celled _corona_. The envelope of the ‘fruit’ and other parts of
the plant are frequently encrusted with carbonate of lime.
In the genus _Chara_, the best known member of the family, the plant as
a whole resembles in its general habit and external differentiation of
parts the higher plants. The stem consists of long internodes separated
by short nodes bearing whorls of leaves. Each internode consists of
a long cylindrical cell, which becomes enclosed by a cortical sheath
composed of rows of cells which have grown upwards and downwards from
the peripheral nodal cells. The cortical cells are usually spirally
twisted and impart to the stem a characteristic appearance; they
are divided by transverse walls into numerous cells some of which
occasionally grow out into short processes (fig. 45 _c_). The leaves
repeat on a smaller scale the structural features of the stem, but
possess a limited growth, whereas the stem has an unlimited power
of growth by means of a large hemispherical apical cell. Branches
arise in the axils of the leaves. The plants are either monoecious or
dioecious. The oogonium is elliptical in shape, and is borne on a short
stalk-cell, it contains a single oosphere. The wall of the oogonium is
formed of five spirally twisted cells which have grown over it from the
five peripheral cells of a leaf-node. The tips of the investing cells
project at the apex in the form of a terminal crown or _corona_ (fig.
45, _E_, _c_). The antheridia have a complex structure, and produce a
very large number of motile antherozoids.
[Illustration: FIG. 45. _A_ and _B_. _Chara Knowltoni_ Sew. From a
section in the British Museum. _C._ Stem of _Chara foetida_ A.
Br. in transverse section (after Migula. × 18). _D._ Interior of
oogonium of _C. foetida_. _E._ Oogonium of _C. foetida_ (_D_ and
_E_ after Migula. × 50).]
After fertilisation, the egg-cell becomes surrounded by a membrane, at
first colourless, but afterwards yellow or brown. The inner cell-walls
of the cells surrounding the oospore become thicker and darker in
colour; the outer walls remain thin and eventually fall away. The
lateral walls may or may not become thickened. In most of the Chareae a
calcareous deposit is formed between the hard shell and the outer walls
of the cells enveloping the oospore. This calcareous shell is developed
subsequently to the thickening and hardening of the inner walls of the
fruit-case. The cells of the corona and stalk do not become calcareous.
In the fossil Charas, it is this calcareous shell that is preserved.
In the members of the Chareae the stems are usually encrusted with
carbonate of lime, and thus have a much better chance of preservation
than the slightly calcareous Nitelleæ.
_Chara._
The generic characters have already been described in the brief account
of the family Chareae.
The generic name was proposed by Vaillant in 1719[438], and adopted by
Linnaeus, who classed the Stoneworts with aquatic phanerogams. As long
ago as 1623[439] a figure of _Chara_ was published by Caspar Bauhin
as a form of _Equisetum_. The generic name _Chara_ has usually been
applied to recent and fossil species alike. The existing species have
a wide distribution; _Chara foetida_, A. Br., a common British form,
occurs in practically all parts of the world. Stems and calcareous
‘fruit-cases’ occur fairly commonly in a fossil state, and differ but
little from recent species, at least as regards essential features.
It is difficult to say at what geological horizon the Stoneworts
are first represented. The first certain traces of _Chara_ occur in
Jurassic rocks, but certain spirally marked subspherical bodies have
been recorded from Devonian and Carboniferous strata, which closely
resemble Chara oogonia, and may be Palaeozoic representatives of the
genus.
In 1889 Mr Knowlton[440] of the American Geological Survey described
some ‘problematic organisms’ found in Devonian rocks at the falls of
the Ohio. Examples of these fossils are shown in fig. 46 _b_ and _c_;
the spirally grooved body measures from 1·50 to 1·80 mm. in diameter,
and about 1·70 mm. in length. The Chara-like character of the fossils
had been previously suggested by Meek[441] in 1873. Without going into
the arguments for or against placing these fossils in the Chareae,
they may at least be mentioned as possible but not certain Palaeozoic
forms of _Chara_ or an allied genus.
[Illustration: FIG. 46. _a_. _Chara Bleicheri_ Sap. × 30. _b_ and _c_.
Devonian _Chara_? sp. _circa_ × 12. _d_ and _e_. _Chara Wrighti_
Forbes. _circa_ × 12.]
1. _Chara Bleicheri_, Saporta. Fig. 46, _a_.
In this form the ‘fruits’ are minute and subspherical, ·39-·44 mm.
long, and ·35-·40 mm. broad, showing in side view 5–6 slightly oblique
spiral bands. Each spiral band bears a row of slightly projecting
tubercles.
This species was first described by Saporta[442] from the Oxfordian
(Jurassic) rocks of the Department of Lot in France; it is compared by
the author of the species with _Chara Jaccardi_ Heer, described by Heer
from the Upper Jurassic rocks of Switzerland.
2. _C. Knowltoni_, Seward. Fig. 45, _a_ and _b_, and Fig. 47.
The Oogonia are broadly oval, about ·5mm. in length, and at the
broadest part of about the same breadth. The surface is marked by
eleven or twelve bands in the form of a flattened spiral. The stems
possess investing cortical cells.
This species was founded on specimens from the Wealden beds of
Sussex[443], but numerous examples of Chara ‘fruits’ and stems have
long been known from the uppermost Jurassic rocks of the Dorset coast
and the Isle of Wight, which may probably be included in this species.
These fossil Charas are abundant[444] in the Chert beds of Purbeck age
seen in the cliffs near Swanage. Pieces of corticated stems from this
locality are represented in fig. 45 _A_ and _B_.
The cortical cells surrounding a large internodal cell are very clearly
seen in the section shown in fig. 45 _B_, and in the longitudinal view
in fig. 45 _A_. The resemblance of these specimens to the stems of
recent Stoneworts is very striking.
[Illustration: FIG. 47. _Chara Knowltoni_, Sew. × 30.]
The single oogonium of fig. 47 was found in the Wealden beds near
Hastings.
3. _Chara Wrighti_, Forbes. Fig. 46, _d_ and _e_.
This species is characterised by globular or somewhat elliptical
oogonia, with six or seven spiral bands.
It is very abundant in the Lower Headon beds of Hordwell Cliffs on the
Hampshire coast[445]. Various species of _Chara_ are commonly met with
in the Oligocene beds of the Isle of Wight and Hampshire, as well as
in the Paris basin beds, and elsewhere. Well preserved ‘fruits’ and
stem fragments are met with in a siliceous rock of Upper Oligocene age
imported from Montmorency in the Paris basin, and used as a stone for
grinding phosphates at some chemical works near Upware, a few miles
from Cambridge.
Many other species of fossil Charas are known from various horizons
and localities, but the above examples suffice as illustrative types.
In Post-Tertiary deposits masses of _Chara_ and plant fragments
occasionally occur forming blocks of Travertine. Examples of such Chara
beds have been recorded by Sharpe from Northampton[446], by Lyell[447]
from Forfarshire, and by other writers from several other districts.
Beds of calcareous marl are occasionally seen as whitish streaks in the
peat of the Fenland[448]; these often consist in great part of Charas.
A season’s growth of _Chara_ in a shallow lake or mere in the Fens may
appear as a white line in a section of peaty and other material which
has been formed on the site of old pools or lakes.
The recognition of specific characters in the isolated Chara ‘fruits’
usually met with in a fossil state is exceedingly unsatisfactory; the
features usually relied on in the living species are not preserved, and
great care should be taken in the separation of the various forms.