CHAPTER VIII.
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].
| I. HEPATICAE. | MARCHANTIALES. 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.
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.
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.
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.
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.
| II. MUSCI. | SPHAGNALES. 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.
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.
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.