BRYOPHYTA (Muscineae).
I. HEPATICAE (Liverworts). II. MUSCI (Mosses).
The Bryophyta are small plants, varying in size from 1 mm. to about
30 cm., creeping or erect, having a thalloid, or more usually a
foliose body, consisting of a cell-mass exhibiting in most cases a
distinct internal differentiation. They possess no true roots and no
true vascular tissue. The life-history of the members of the group is
characterised by a well-marked and definite alternation of generations.
The Moss or Liverwort plant is the sexual generation (_gametophyte_),
and as a result of the fertilisation of an egg-cell the asexual or
spore-bearing generation (_sporophyte_) is produced. The sporophyte
never exhibits a differentiation into stem and leaves. Asexual and
vegetative reproduction are effected by means of spores, bulbils, or
detached portions of the plant-body. Sexual reproduction is by means of
biciliate antherozoids produced in _antheridia_ and egg-cells formed
singly in _archegonia_.
In the Bryophytes the distinguishing characteristics are more
constant and well-defined than in the Thallophytes. In the former the
plant never consists of a single cell or coenocyte, but is always
multicellular, and exhibits in most cases a definite physiological
division of labour as expressed in the histological differentiation
of distinct tissue-systems. In the Thallophytes there is no true
alternation of generation in the same sense as in the Mosses and
Liverworts and in the higher plants. In the Bryophytes the sexual
reproduction has reached a higher stage of development and a much
greater constancy as regards the nature of the reproductive organs. On
the germination of the spore there is usually formed a fairly distinct
structure known as the _protonema_, from which the Moss or Liverwort
developes as a bud[449].
╭ MARCHANTIALES.
I. HEPATICAE. ┤ ANTHOCEROTALES.
╰ JUNGERMANNIALES.
The vegetative plant-body possesses a different organisation on the
ventral and dorsal side; it has the form of a thalloid creeping plant
(Thalloid Liverworts), or of a delicate stem with thin appendages or
leaves without a midrib (Foliose Liverworts). In most cases the body
of the plant is made up of parenchymatous tissue, showing but little
internal differentiation; in one or two genera a few strengthening or
mechanical fibres occur among the thinner walled ground-tissue. On the
germination of the spore, a feebly developed protonema is produced,
which gives rise to the Liverwort plant. Reproduction as in the group
Bryophyta.
[Sidenote: DETERMINATION OF LIVERWORTS.]
The Liverworts have a very wide geographical distribution, and are
specially abundant in moist shady situations; they grow on stones
or damp soil, and occur as epiphytes on other plants. _Marchantia_,
_Pellia_, and _Jungermannia_ are among the better known British
representatives of the class.
Considering the soft nature of the body of recent Liverworts, it is
not surprising that they are poorly represented in a fossil state. In
the absence of the sexual reproductive organs, or of the sporophytes,
which have scarcely ever been preserved, exact identification is almost
hopeless. The difficulties already referred to in dealing with the
algae, as regards the misleading similarity between the form of the
thallus and the bodies of other plants, have to be faced in the case of
the Liverworts. Many of the thalloid Liverworts, if preserved in the
form of a cast or impression without internal structure or reproductive
organs, could hardly be distinguished from various genera of algae
in which the thallus has the form of a forked plate-like body. Such
genera as _Pellia_, _Marchantia_, _Lunularia_, _Reboulia_, and others
bear a striking resemblance to _Fucus_, _Chondrus_ and many other algae.
Imperfect specimens of certain Lichens, not to mention some of
the Polyzoa, might easily be mistaken for Liverworts. Among the
higher plants, there are some forms of the _Podostemaceae_ which
simulate in habit both thalloid and foliose Liverworts as well as
Mosses[450]. The members of this Dicotyledonous family are described
as water-plants with a Moss- or Liverwort-like form; they occur on
rocks in quickly-flowing water in the tropics. In one instance a
recent Podostemaceous genus has been described as a member of the
Anthocerotales; the genus _Blandowia_[451], referred to by Willdenow as
a Liverwort, has since been recognised as one of the _Podostemaceae_.
The resemblance between some of the foliose _Hepaticae_ and genera
of Mosses is often very close. In certain Mosses, such as _Hookeria
pennata_[452], the large two-ranked leaves suggest the branches of a
_Selaginella_.
[Illustration: FIG. 48. A. _Tristichia hypnoides_ Spreng. From a
specimen in the British Museum. B. _Podocarpus cupressina_ Br.
and Ben. (After Brown and Bennett[453].) C. _Selaginella Oregana_
Eat. From a plant in the Cambridge Botanic Garden. A, B and C very
slightly reduced.]
The plant reproduced in fig. 48 A (_Tristichia_), one of the
Podostemaceae, might easily be mistaken for a foliose Liverwort if
found as a fragmentary fossil. Such species of _Selaginella_ as _S.
Oregana_ Eat. and _S. rupestris_ Spring (fig. 48 C) have a distinctly
moss-like habit and do not present a very obvious resemblance to the
more typical and better known Selaginellas. The twig of a _Podocarpus_
(_P. cupressina_)[454] in fig. 48 B affords an instance of a conifer
which simulates to some extent certain of the larger-leaved Liverworts;
it bears a resemblance also to some fossil fragments referred to
_Selaginellites_ or _Lycopodites_. A small fossil specimen figured by
Nathorst[455] from Japan as possibly a _Lycopodites_ may be compared
with a coniferous twig, and with some of the larger Liverworts, _e.g._
species of _Plagiochila_[456]. _Podocarpus cupressina_ is, however,
chiefly instructive as an example of the striking differences which are
met with among species of the same genus; it differs considerably from
the ordinary species of _Podocarpus_, and might well be identified as a
member of some other group than that of the Coniferae.
We have no records of Palaeozoic Hepaticae. The fossils which Zeiller
has figured in his _Flore de Brive_ as _Schizopteris dichotoma_
Gümb.[457] and _S. trichomanoides_ Göpp. bear a resemblance to some
forms of hepatics, but there is no satisfactory evidence for removing
them from the position assigned to them by the French writer. In
Mesozoic rocks a few specimens are known which bear a close resemblance
as regards the form of the thalloid body to recent Liverworts, but
the identification of such fossils cannot be absolutely trusted. Two
French authors, MM. Fliche and Bleicher[458], have described a plant
from Lower Oolite rocks near Nancy as a species of _Marchantia_,
_M. oolithus_, but they point out the close agreement of such
forked laminar structures to algae and lichens. From Tertiary and
Post-Tertiary beds a certain number of fossil species have been
recorded, but they possess no special botanical interest.
ORDER MARCHANTIALES.
The plant-body is always thalloid, bearing rhizoids on the lower
surface, and having an epidermis with pores limiting the upper or
dorsal surface.
_Marchantites._
This convenient generic name was proposed by Brongniart in 1849[459];
it may be briefly defined as follows:
Vegetative body of laminar form, with apparently dichotomous
branches, and agreeing in habit with the recent thalloid Hepaticae,
as represented by such a genus as _Marchantia_.
The name _Marchantites_ is preferable to _Marchantia_, as the latter
implies identity with the recent genus, whereas the former is used in a
wide sense and refers rather to a definite form of vegetative body than
to a particular generic type.
1. _Marchantites erectus_ (Leckenby). Fig. 49. This species may be
described as follows: The thalloid body is divided into spreading
dichotomously branched segments, obtusely pointed apically. The
slightly wrinkled surface shows a distinct and comparatively broad
darker and shorter median band, with lighter coloured and thinner
margins.
[Illustration: FIG. 49. _Marchantites erectus_ (Leck.). From the
type-specimen in the Woodwardian Museum. Nat. size.]
In 1864 Leckenby described this plant from the Lower Oolite beds
of the Yorkshire coast near Scarborough, as _Fucoides erectus_,
regarding it as a fossil alga. I recently pointed out that the general
appearance and mode of occurrence of the specimens suggest a liverwort
rather than an alga, and proposed the substitution of the genus
_Marchantites_[460]. It would, however, be unwise to speak with any
great confidence as to the real affinities of the fossil.
The example shown in the figure is the type-specimen of Leckenby[461]:
the breadth of the branches is about 3 mm. Under a low magnifying power
the surface shows distinct and somewhat oblique wrinklings, the general
appearance being very similar to that of some recent forms of the genus
_Marchantia_.
A closely allied species has recently been described from the
Wealden beds of Ecclesbourne, near Hastings, on the Sussex coast, as
_Marchantites Zeilleri_ Sew.[462].
In a recent monograph on Jurassic plants from Poland, apparently
containing much that is of the greatest value, but which is
unfortunately written in the Polish language, Raciborski[463] describes
a new species of thalloid Liverwort under the name of _Paleohepatica
Rostafinski_. The specimens are barren plants larger than any Jurassic
species hitherto described; they agree closely in habit with Saporta’s
Tertiary species _Marchantites Sezannensis_.
2. _Marchantites Sezannensis Saporta._ Fig. 50. The body is broadly
linear and dichotomously branched, with a somewhat undulating margin.
Midrib on the dorsal surface depressed, but more prominent on the
ventral surface. The upper surface is divided into hexagonal areas, in
each of which occurs a central pore. There are two rows of scales along
the median line on the lower surface. Stalked male receptacles.
Brongniart[464] first mentioned this fossil hepatic, which was found in
the calcareous travertine of Sézanne of Oligocene age in the Province
of Marne. The specimens figured by Saporta[465] show very clearly
the characters of one of the _Marchantiaceae_, and in this case we
have the additional evidence of the characteristic male receptacles
which are given off from a point towards the apex of the lobes, and
arise from a slight median depression. In one of Saporta’s figures
(reproduced in fig. 50 _A_) there are represented some median scars
which may mark the position of cups similar to those which occur on
recent species of _Marchantia_, and in which gemmæ or bulbils are
produced.
[Illustration: FIG. 50. _Marchantites Sezannensis_ Sap. _A._ Surface
view of the thallus; _g_, ? cups with gemmæ. _B._ A male branch.
_C._ A portion of _A_ magnified to show the surface features.
(After Saporta.)]
The collection of Sézanne fossils in the Sorbonne includes some very
beautiful casts of _Marchantites_ in which the structural details
are preserved much more perfectly than in the examples described by
Saporta. In a few specimens which Prof. Munier-Chalmas recently showed
me the reproductive branches were exceedingly well shown. The fossils
occur as moulds in the travertine, and the museum specimens are in the
form of plaster-casts taken from the natural moulds.
Several species of Liverworts belonging to the Marchantiales and
Jungermanniales have been recorded from the amber of North Germany,
of Oligocene age. These appear to be represented by small fragments,
such as are figured by Göppert and Berendt[466] in their monograph
on the amber plants, published in 1845. The determinations have since
been revised by Gottsche[467], who recognises species of _Frullania_,
_Jungermannia_, and other genera.
╭ SPHAGNALES.
II. MUSCI. ┤ ANDREAEALES.
╰ BRYALES.
The plant-body (gametophyte) in the Musci consists of a stem bearing
thin leaves, usually spirally disposed, rarely in two rows. The
internal differentiation of the stem is generally well marked, and
in some cases is comparable in complexity with the structure of the
higher plants. A protonema arises from the spore, having the form of a
branched filamentous, or more rarely a thalloid structure. Reproduction
as in the group Bryophyta.
Mosses like Liverworts have an extremely wide distribution, and occur
in various habitats. In many districts vast tracts of country are
practically monopolised by peat-forming genera, such as _Sphagnum_ and
other Mosses. Some genera are found on rocks at high altitudes in dry
regions, a few grow as saprophytes, and many occur either as epiphytes
on the leaves and stems of other plants, or carpeting the ground under
the shade of forest trees.
[Sidenote: DETERMINATION OF MOSSES.]
In the simpler Mosses, the stem consists of a parenchymatous
ground-tissue with a few outer layers of thicker-walled and smaller
cells. In others there is a distinct central cylinder which occupies
the axis of the stem, and consists of long and narrow cells; in the
more complex forms the structure of the axial tissues suggests the
central cylinder or _stele_ of higher plants. The genus _Polytrichum_,
so abundant on English moors, illustrates this higher type of stem
differentiation. In a transverse section of the stem the peripheral
tissue is seen to be composed of thick-walled cells, passing internally
into large parenchymatous tissue. The axial part is occupied by a
definite central cylinder consisting in the centre of elongated
elements with dark-coloured and thick walls having thin transverse
septa; surrounding this central tissue there are thinner walled
elements, of which some closely agree in form with the sieve-tubes of
the higher plants. The central tissue may be regarded as a rudimentary
type of xylem, and the surrounding tissue as a rudimentary phloem. Each
leaf is traversed by a median conducting strand which passes into the
stem and eventually becomes connected with the axial cylinder.
The fertilisation of the egg-cell gives rise to the development of a
long slender stalk terminating distally in a large spore-capsule. In
section the stalk or seta closely resembles the leafy axis of the moss
plant. Considering the fairly close approach of some of the mosses to
the higher plants as regards histological characters, it is conceivable
that imperfectly petrified stems of fossil mosses might be mistaken for
twigs of Vascular Cryptogams.
Like Liverworts, Mosses have left very few traces of their existence
in plant-bearing rocks. Without the aid of the characteristic
moss-‘fruit’ or sporogonium it is almost impossible to recognise
fossil moss-plant fragments. In species of the tropical genera
_Spiridens_ and _Dawsonia_, e.g. _S. longifolius_[468] Lind. or _D.
superba_[469] Grev. and _D. polytrichoides_[470] R. Br., the plant
reaches a considerable length, and resembles twigs of plants higher in
the scale than the Bryophytes. The finer branches of species of the
extinct genus _Lepidodendron_ are extremely moss-like in appearance.
Again, _Cyathophyllum bulbosum_ Muell[471], with its two kinds of
leaves arranged in rows, is not at all unlike species of _Selaginella_
or the hepatic genus _Gottschea_. It is by no means improbable that
some of the Palaeozoic specimens described as twigs of _Lycopodites_,
_Selaginites_, or _Lepidodendron_, may be portions of mosses. The
fertile branches of _Lycopodium phlegmaria_ in a fossil condition might
be easily mistaken for fragments of a moss. In some conifers with
small and crowded scale-leaves there is a certain resemblance to the
stouter forms of moss stems. Such possible sources of error should be
prominently kept in view when we are considering the value of negative
evidence as regards the geological history of the Musci.
A recent writer[472] on mosses has expressed the opinion that no doubt
the Musci played an exceedingly important rôle in past time. Although
we have no proof that this was so, yet it is far from improbable, and
the absence of fossil mosses must no doubt be attributed in part to
their failure to be preserved in a fossil state.
In the numerous samples of Coal-Measure vegetation preserved in
extraordinary perfection in the calcareous nodules of England, no
certain trace of a moss has so far been discovered. The most delicate
tissue in the larger Palaeozoic plants has often been preserved, and
in view of such possibilities of petrifaction it might appear strange
that if moss-like plants existed no fragments had been preserved.
Their absence is, however, no proof of the non-existence of Palaeozoic
mosses, but it is a fact which certainly tends towards the assumption
that mosses were probably not very abundant in the Coal Period forests.
Epiphytic mosses frequently occur on the stems and leaves of ferns
and other plants in tropical forests. Such small and comparatively
delicate plants would, however, be easily rubbed off or destroyed in
the process of fossilisation, and it is extremely rare to find among
petrified Palaeozoic plants the external features well preserved. It is
probable that the forests extended over low lying and swampy regions,
and that, in part, the trees were rooted in a submerged surface. Under
such conditions of growth there would not be the same abundance of
Bryophytes as in most of our modern forests.
To whatever cause the absence of mosses may be best attributed, it is
a fact that should not be too strongly emphasised in discussions on
plant-evolution.
_Muscites._
This comprehensive genus may be defined as follows:—
Stem filiform, simple or branched, bearing small sessile leaves, with
a delicate lamina, without veins or with a single median vein, arranged
in a spiral manner on the stem.
_Muscites_[473] is one of those convenient generic designations
which limited knowledge and incomplete data render necessary in
palaeontology. Fossil plants which in their general habit bear a
sufficiently striking resemblance to recent mosses, may be included
under this generic name.
[Illustration: FIG. 51. _Muscites polytrichaceus_ Ren. and Zeill.
(after Renault and Zeiller).]
1. _Muscites polytrichaceus_ Renault and Zeiller. In this species
the stems are about 3–4 cm. long and 1·3 m. broad, usually simple,
but sometimes giving off a few branches, and marked externally by
very delicate longitudinal grooves. The leaves are alternate, closely
arranged, lanceolate, with an acute apex, gradually narrowed towards
the base, 1–2 mm. long, traversed by a single median vein.
One of the French specimens, on which the species was founded[474],
is shown in fig. 51, and the form of the leaves is more clearly seen
in the small enlarged piece of stem. The authors of the species point
out that the tufted habit of the specimens, their small size, and the
membranous character of the leaves, all point to the Musci as the
Class to which the plant should be referred in spite of the absence of
reproductive organs.
Among recent mosses, the genus _Rhizogonium_,—one of the
_Mniaceae_,—and _Polytrichum_ are spoken of as offering a close
resemblance to the fossil form. The type-specimen was found in the
Coal-Measures of Commentry, and is now in the Museum of the École des
Mines in Paris; the figure given by MM. Renault and Zeiller faithfully
represents the appearance of the plant.
• • • • •
It has been suggested[475] that some small twigs figured by
Lesquereux[476] from the Coal-Measures of North America as _Lycopodites
Meeki_ Lesq., may possibly be mosses. The specimens do not appear
to be at all convincing, and cannot well be included as probable
representatives of Palaeozoic Musci. _Lycopodites Meeki_ Lesq. bears a
close resemblance to the recent _Selaginella Oregana_ shown in fig. 48,
C.
From Mesozoic rocks we have no absolutely trustworthy fossil mosses.
The late Prof. Heer[477] has quoted the occurrence of certain fossil
Caterpillars in Liassic beds as indicative of the existence of mosses,
but evidence of this kind cannot be accepted as scientifically sound.
In 1850 Buckman[478] described and figured a few fragments of plants
from a freshwater limestone at the base of the Lias series near
Bristol. Among others he described certain specimens as examples of a
fossil Monocotyledon, under the generic name _Najadita_. Mr Starkie
Gardner[479] subsequently examined the specimens, and suggested that
the Lias fragments referred to _Najadita_ should be compared with the
recent freshwater moss _Fontinalis_. In this opinion he was supported
by Mr Carruthers and Mr Murray of the British Museum. In a footnote
to the memoir in which this suggestion is made, Gardner refers to a
moss-capsule from the same beds, which he had received from Mr Brodie.
Through the kindness of the latter gentleman, I have had an opportunity
of examining the supposed capsule, and have no hesitation in describing
it as absolutely indeterminable. It is in the form of an irregularly
oval brown stain on the surface of the rock, with the suggestion of a
stalk at one end, but there are no grounds for describing the specimen
as a moss-capsule, or indeed anything else. The type-specimens figured
by Brodie and subsequently referred to a moss are now in the British
Museum; they are small and imperfect fragments of slender stems
bearing rather long oval leaves which might well have belonged to a
moss. The material is however too fragmentary to allow of accurate
diagnosis or determination.
2. _Muscites ferrugineus_ (Ludg.). This species possesses a slender
stem bearing crowded ovate-acuminate leaves. The capsules are
cup-shaped, borne on a short stalk, with a circular opening without
marginal teeth. This fossil was first figured and described by
Ludwig[480] from a brown ironstone of Miocene age at Dernbach in
Nassau. The author of the species placed it in the recent genus
_Gymnostomum_, and Schimper[481] afterwards changed the generic name to
_Sphagnum_, at the same time altering the specific name to _Ludwigi_.
The evidence is hardly strong enough to justify a generic designation
which implies identity with a particular recent genus, and it is a much
safer plan to adopt the non-committal term _Muscites_, at the same time
retaining Ludwig’s original specific name. Without having examined
the type-specimen it is impossible to express a definite opinion as
to the accuracy of the description given by Ludwig; if the capsule is
correctly identified it is the oldest example hitherto recorded of a
fossil moss-sporogonium.