Bryophytes: Botany Notes on Bryophytes | Plants

The below mentioned article provides a note on bryophytes.

Fossil of Hepaticopsida:

Fossils of the Hepaticopsida were previously not known before the Carboniferous while the lower vascular plants (the Pteridophytes) were known from a much earlier age. This fact, naturally, gave rise to speculations. Recently however, Hueber (1961) has discovered Hepaticites devonicus (Fig. 495A) from Devonian rocks in New York State.

This is a thallose Jungermanniale resembling Pallavicinia and definitely be­longs to the Metzgerineae.

Upper Carboniferous rocks show the presence of the genus Hepaticites side by side with the moss genus Muscites, Five species of Hepaticites (foliose and thallose Jungermanniales) are described by Walton (1925, 1928) from this age, viz., H. willsi, H. langi, H. lobatus, H. kidstoni (Fig. 495B) and H. metzgerioides.

These are known only in the vegetative stage. Of these, H. kidstoni is a foliose Jungermanniale with a central axis and four rows of leaves of which two rows are larger and two smaller. The other species resemble the anacrogynous Jungermanniale genera, Treubia, Aneura (Riccardia) and Metzgeria. Some Hepaticites are also known from the Permian.

During Mesozoic, more hepatics are known. Nine more species of Hepaticites (amaures, selenotus, laevis, rosenkrantzii, glebosus, arcuatus, haiburensis, hymenoptera and warncotti) are named by Harris from the Triassic and Jurassic of Greenland and Britain and these also seem allied to the anacrogynous Jungermanniales.

These rocks also show the first definite presence of the Marchantiales in Ricciopsis, florinii, Ricciopsis scanica and Marchantiolites porosus.

Of these, Ricciopsis resembles Riccia while Marchantiolites shows chambered thallus with air pores. The best known Bryophyte fossil, however, is Naiadetia lanceolata (Fig. 496) from the Upper Triassic (Rhaetic) of England which has been described in detail by Harris (1938). This is the only hepatic whose-reproductive parts (gemmae and archegonia) and sporophyte are known.

This seems to be allied to the Riellaceae of the Sphaerocarpales but has an upright axis with spirally arranged (3/8 phyllotaxy) leafy appendages reminding the Calobryales. The plants are 1 to 3 cm high and there are unicellular rhizoids from the base of the stem.

The stem tissue is parenchymatous and is not differentiated. The leaves are linear at the base of the stem, lanceolate in the middle and somewhat rounded on the top. They are one cell thick and without midrib. There are multicellular gemmae in terminal gemmae cups (Fig. 496B & G). The archegonia (Fig. 496D) are lateral at extra-axillary positions.

They are naked at first but then become surrounded by a perianth. The sporophyte shows a spherical capsule and a minute foot but no seta (Fig. 496E). Columella or elaters are absent. The capsule opened irregularly.

The Devonian fossil Sporogonites, sometimes supposed to be allied to the Psilophytales has been considered to be a hapatic by Andrews (1959)’.

The still younger Mesozoic rocks, i.e., the Cretaceous, shows the presence of more Marchantiales in four species of the genus Marchantites (blairmorertsis, sewardi, hallei and yukononsis) from N. and S. America. Jungermanniales are represented by Jungermannites cretaceus. Hepaticopsida sub-fossils identical with the present-day genera begin to appear in the modern Cainozoic age beginning from Miocene onwards.

Bryopsida:

Bryopsida fossils are found beginning from the Carboniferous side by side with the Hepaticopsida. The specimens are fragmentary and incomplete like most Hepaticopsida fossils. Two species of Muscites (bertrandi and polytrichaceous) occur in the Upper Carboni­ferous (France).

These rather large Specimens show stems with anatomy definitely like that of moss, simple leaves with single veins and rhizoids with oblique walls. But their affinity is uncertain and, although it is suspected, it cannot be definitely said that M. polytrichaceous is related with the modern Polytrichaceae.

It can only be said that they are types of acropcarpic mosses. A suspected moss capsule (Capsulites gondwanensis) has been reported from the Indian Permo-Triassic Gondwana by Saksena.

Neuberg (1956) has reported 14 species in 9 genera from the Lower and Upper Permian of the U.S.S.R. Three of these genera (Protosphagnum, Vorentartnularia and Jungajia) have leaves which are somewhat like Sphagnum. But, distinct veins are present so that their relationship with Sphagnum is doubtful and they should be placed in a new order Protosphagnales as suggested.

They may as well be related to the Polytrichales or the Dicranales.

In this assemblage Neuberg has also described Intia which seems near Mnium or Bryum but no sporophyte was found. The other genera named by Neuberg are Salairia, Uskatia, Polyssaiuria, Bajdaieira and Buchtia. Mesozoic moss fossils are rarer than the Hepaticopsida fossils of that time.

There is one species of Muscites (M. lesquereuxi—still of uncertain affinity) and specimens of Sphagnum sp. leaves from Greenland-—both from Cretaceous. More moss species appear during Cainozotc. Jungermannites occur during the Tertiary.

Some Musettes fossils and that of Palaeohypnum (possibly a pleurocarpic moss) occur during Eocene. Oligocene shows Muscites yal- lournensis from Australia—the first definite moss fossil with a capsule. During Miocene there are fossils of Muscites, Polytrichites (possibly belonging to Polytrichales), Plagiupodopsis (possibly acrocarpous) and Palaeohypnum.

In the newer Pliocene and Pleistocene rocks a large number of subjossils (i.e., not always fully fossilised) of modern moss genera are known. In India, Sphagnum peats of recent origin are known from Kashmir and the Nilgiri Hills.

The above record of the fossils throws little light on the evolution of the Bryophyta. It is only known that there is evidence of the thallose Jungermanniales (anacrogynous) in the Devonian and during the Upper Carboniferous both the Hepaticopsida (possibly the anacrogynous Jungermanniales) and the Bryopsida (acrocarpic mosses) are found.

The Marchantiales, Sphagnum and pleurocarpic mosses are known later. We are still in the dark about the Anthocerotopsida except that some Tertiary spores have been assigned to this order. In this connection, a note should be kept of the contention by Naumova (1949) that fossil spores found in the Cambrian rocks represent both the Bryophytes and the Pteridophytes.

If further verified, this would take down the origin of the Bryophytes to a much earlier age.

Origin of Bryophytes:

With little help from the fossil records, one has to rely on speculative theories based on comparative morphology, etc., on the origin of the Bryophytes.

Compara­tive study of the plant groups show that the Bryophytes have forms more complex than the Algae (or Thallophytes generally) but they are less developed than the Pteri­dophytes although there is a tendency of differentiation of the gametophytic body which is not found in the Pteridophytes.

So, if one takes for granted that the more complex forms developed from the simpler ones, one may suppose that the Bryophyta developed out of the Algae and the lower vascular plants (i.e., the Pteridophytes) developed out of the Bryophytes.

But, fossil history unveiled so far, suggests that the Pteridophytes were present during the Cambrian while there is no evidence of the presence of any Bryophyte before the Devonian.

So, there is the alternative that the Pterido­phytes developed first out of the Algae and the Bryophytes evolved later by degene­ration (Regressive evolution) of the Pteridophytes.

Most convincing, however, is the view which is being gradually explained that there was a common origin of all the land plants—the non-vascular Bryophytes and all the vascular plants beginning from the Pteridophytes. This common ancestor (or a plexus of ancestors) probably evolved out of the algae. But, there is a second problem.

Within the Bryophyta there are some simple, flat, thallose forms like Riccia and Sphaerocarpos and there are the erect, appa­rently more advanced, leafy gametophytic forms like the Calobryales among the Hepatics and the group of Musci (Bryopsida). There is also Anthoceros with a simple gametophyte but an advanced erect sporophyte. Which of these is the most ancient?

This problem has given rise to two dominant views on the evolution of the Bryophytes:

(1) The Up-grade or the Progressive Evolution Theory which says that the first Bryophytes were of the simple thallose type with simple sporophytes, the complex forms developed by progressive evolution;

(2) The Down-grade or the Regressive Evolution Theory which says that some erect, more complicated form was the first Bryophyte evolved. Other forms then developed by reduction in different lines.

Pteridophytic Origin:

Scott (1911) supposed that the Bryophytes evolved by the degeneration of Pteri­dophytes on the basis of the similarity of the stomata in the land plants, in the sporophytes of Anthoceros and on the neck or the apophysis of mosses. Kashyap (1919) also thought that the Marchantiales, Jungermanniales and Anthocerotales, arose out of the Pteridophytes (from a stock like Equisetum) in three separate lines.

But the dissi­milarities, viz., the complete absence of lignin and vascular bundles in the Bryophytes, etc. are rather too great to suggest any origin of the Bryophytes out of the Pterido­phytes.

The discovery of the simple vascular plants, Psilophytales showed that their sporophytes, specially those of Homeophyton (Homea), bear striking relations with those of Anthoceros, Sphagnum and Andreaea. The doubt whether Sporogonil.es exuberans is a Vascular Plant or a Bryophyte is discussed later. Such a simple group of vascular plants might have given rise to the Bryophytes by reduction.

Another way of explaining the same phenomenon is by supposing that both the Bryophytes and the Pteridophytes developed out of the algae and they arose out of a common ancestor (evolved from algae) which was rather a Pteridophyte than a Bryophyte.

Haskell, Takhtajan, Proskauer, Mehra etc., were or are all impressed with the points of similarity between the Anthoceros and the Psilophyton sporophytes.

Proskauer (I960, 1961) even suggested the evolution of Anthoceros-like Bryophytes out of Homeophyton pointing out to the presence of incomplete spiral thickenings (not lignified) in the cells of the outer layers of Dendroceros columella which simulates the tapetum of Horneophyton. He also suggests the origin of the Marchantials out of Pteridophytes.

Mehra (1968, 1969) strongly refutes the possibility of such origin of Bryophytes without lignin and with free archegonia (Hepaticopsida, Bryopsida) as against lignified Pteridophytes with embedded archegonia. The course of evolution was the other way.

Algal Origin:

The predominant view, however, is that the Bryophytes arose directly out of the Algae. The first Bryophyte-like land plant is not definitely known but this hypothe­tical plant has been given different names like Prohepatics or Protobryophyta (Cavers and Smith). Church (1919) in his essay on the ‘Thalassiophyta’ explains that the first formed algae were planctonic, i.e., free-floating.

Then they came near the shallow shores when they become benthic, i.e., attached to the shallow sea bottom like the modern seaweeds. Then, some sort of land plant (Thalassiophyta) arose which had the green appearance of the Chlorophyta, the complicated tissue and organ differen­tiation of the Phaeophyta and an isomorphic alternation of generations as in Dictyota.

Proponents of the theory of the algal origin of Bryophytes are not, however, unanimous about whether this origin was monophyletic or polyphyletic. But one point seems clear: The origin of the first land plants or the Archegoniatae (Bryophyta and Pteridophyta) was probably one story.

From the algae arose some sort of primitive stock, (Archegoniate Algae of Cavers, Thalas­siophyta of Church, the Chaetophoraceoos ancestor pictured by Fritsch and Mehra or Proto-Archegoniatae of Mehra), which gave rise both to the Bryophytes and the Pterido­phytes.

Fritsch had already explained the possibility of the evolution of erect land plants out of heterotrichous chaetophoraceous algae showing isomorphic alter­ation of generations (such an ideal ancestor is suggested in Fritschiella). This ancestor became an erect land plant by elaborating the erect portion and by diminishing the prostrate past into rhizoids.

The Bryophytic life cycle was established by the elaboration of the gametophytic plant and the sporophytic plant becoming a parasite on the gametophyte. Mehra’s conception of the origin of the Bryophytes (and also of the Pteridophytes described more is shown in Fig. 497.

The origin of the Land Plants which also involve the origin of the Bryophytes is explained there. The telome theory, which is better explained after a clear conception of the Pteridophytes is obtained, is also explained there.

But, although it seems acceptable to most that this first land plant, the ancestor of Bryophytes as well as Vascular Plants, arose from the algae, possibly Chlorophyta, there is great diversity of opinion as to the appearance of this hypothetical pre-bryophytic plant because of the two contradictory views about the evolution of the Bryophytes— the Up-grade and Down-grade theories.

Up-Grade or Progressive Evolution Theory:

Most classifications of Bryophytes begin with the simpler forms and end with the complex forms although there is a large number of Bryologists now holding that evolu­tion took place the other way (regressively).

According to the Up-grade view, the first evolved Bryophyte was of the hypo­thetical Sphaero-Riccia type which combined the very simple thalloid gametophyte of Sphaerocarpos with the simple bag-like sporophyte of Riccia.

Cavers (1910-11) developed this view and the phylogenetic tree proposed by him is shown in Figure 497. Campbell 1940, etc. also supported this view but Campbell added that the primitive form could also be like Riccardia or Metzgeria of the anacrogynous Jungermanniales. Smith (1955) also favours this Up-grade view.

This primitive plant in one direction developed the Marchantiales with differentiation of the tissues of the gametophytic thallus as well as that of the sporophyte. In another direction the same primitive plant evolved into the. Junger­manniales by retention of the simplicity of the gametophytic thallus (lack of airpores, etc.) but by developing wings and then lateral leafy expansions.

The Sphaerocarpales is a side line from this branch.

Finally, in this Junger­manniales direction, when a third row of muci­lage hairs on the ventral surface became modified into leaves, the erect Calobryales were evolved.

Such a plant might have easily given rise to the Sphagnales and the other mosses. The development of three rows of leaves and the erect habit is also to be associated with the change in the apical meristematic cells from two cutting faces to three cutting faces which first occurs in the acrogynous Jungermanniales.

The Anthocerotopsida developed as another side line of the Jungermanniales by elaboration of the sporophyte and retention of the primitive simple gametophyte.

According to this view, the primitive Bryophyte developed out of the Algae had a haplobiontic life cycle, i.e., the plant was a gametophyte within which a transient sporophyte involved only meiosis directly after the sexual fusion. The sporophytic plant was gradually evolved afterwards by a lengthening of the sporophytic phase so that a regular sporophyte was intercalated between sexual fusion and meiosis.

This is strictly in accordance with the antithetic theory first proposed by Celakovsky (1874) and then developed by Bower, Cavers, Campbell and Chamberlain. The origin is to be sought in the green algae.

During the migration to land, in the first phase, the life history is pictured as in Oedogonium, a haploid plant with the diploid stage represented by a unicellular zygote which after a period of-rest develops four haploid spores by reduction division.

In the second phase of evolution the form was probably similar to Coleochaete where the zygote develops four haploid cells, which, by further division, develops 16 so 32 zoospores.

In the next phase, one may suppose that the zygote did not undergo reduction division all at once but first became multi­cellular by mitosis, thus initiating the first embryo and the first sporophyte where the ultimate cells become spore mother cells.

In the fourth or the final phase, the outer cells of this spherical sporophyte form a sterile jacket while the inner cells become sporogenous —a stage which is attained in Riccia.

Down-Grade or Regressive Evolution Theory:

Origin of Bryophytes out of the Pteridophytes is a regressive view already dis­cussed. Cronquist, Takhtajan and Zimmermann (1966) still hold this view retaining the Bryophytes in between Pteridophytean groups, being a derivative. Proskauer (1960) has pointed out some vascular affinities between Anthocerotales and Psilophytales.

Mehra (1953) explained these by a very close and common origin of Anthocerotales and Psilophytales through Anthorhyniaceae (Fig. 498). But, whatever the origin, according to this view the most primitive Bryophyta had the most complicated ap­pearance within the group.

Wettstein (1903—1908) first suggested that the primitive Bryophyte had an erect gametophyte like that of the mosses. Later, change to dorsiventrality gave rise to the acrogynous Jungermanniales, reduction of leaf development to the anacrogynous Jungermanniales and then to the Marchantiales. Anthocerotales also arose similarly from a side line.

Church (1919) and Evans (1939) also think that the primitive Bryophyte was an erect plant with three rows of leaves and a pyramidal apical cell with three cutting faces.

The nearest approach to such a plant is to be found in the Calobryales and Naiaiatia among Hepatics and in some acrocarpic mosses. This primitive plant, later, deve­loped dorsiventrality because of general weakness of structure and gradually evolved the different types of Hepatics by regressive evolution.

In most Hepatics, although the gametophyte is generally dorsiventral, the reproductive shoots are radial and erect. This is because in the plant world the reproductive structures are the most con­servative. The reduction view found support in Goebel, Kashyap (1919), Harris (1938), Mehra (1953 onwards), Fulford (1965) and others.

Kashyap (1919), from this extensive studies on Indian Hepatics, was convinced of reduction in the evolution of the Hepatics and some of his arguments are noted in connection with the discussion of the Marchantiales that follow.

He suggested that the slaterophore of the Jungermanniales is derived by the reduction of the columella of the more elaborate Anthocerotales. This is further reduced to the elaters and then vanish.

According to the most adherents of the regressive view, the primitive ancestor arose from the algae and it had a homologous type of alternation of generations. The homologous view of alternation of generations in Bryophytes was first advocated by Pringsheim (1876), and then affirmed by Scott (1896), Church (1919), Zimmermann (1930), Bold (1938), Evans (1939), Fritsch (1945), etc.

There are many genera among the advanced algae showing two alternating isomorphic forms, one a gametophyte and the other a sporophyte. According to this view, these two generations evolved differently when plants migrated to land and one became the gametophyte plant body of the Bryophytes while the other specialised as a sporophyte.

In the Bryophytes, the latter became a parasite on the former. This view has a further confirmation in the presence of apospory and apogamy within the Bryophytes. Fritsch explained that this tendency towards developing an erect plant is already seen in the Chaetophorales (Chlorophyta) in their heterotrichous habit where some filaments are prostrate while others are erect.

Some Chaetophorales (e.g., Fritschiella and Trentepohlia) show the organisation of an apical growing point in the erect branches. If these tendencies are combined with the tissue development of the Phaeophyta, it is conceivable to have a plant, still an alga, but having the characters of Bryophytes.

According to Fritsch (1945), the Bryopsida (moss) shows the prostrate habit in the protonema and the erect habit in the game college botany tophytic plant.

Recent studies of Allsopp and Mitra (1958) show that even the moss protonema is heterotrichous, resembling the Chaetophorales so that the resemblance is more than what was supposed by Fritsch. Church’s ‘Thalassiophyta’ hypothesis also pictures a similar origin of the first Bryophytic land plant.

The sex organs of the Chlorophyta are generally unicellular. Davis (1903) explained that both the antheridia and the archegonia of the Bryophytes might have evolved from some multicellular algal gametangium by progressive sterilisation.

Zimmermann (1930—49) on the development of his Telome Theory, postulates that both the Bryophyta and the Psilophyta (the most primitive vascular plant according to him) developed out of the algae, perhaps independently.

The Psilophyta evolved first, by a change of the isomorphic life cycle into a heteromorphic one, the sporophyte forming an erect body of several telomes and mesomes. The Bryophyta arose at a later time probably directly out of the Algae. Here also the life cycle became heteromorphic, the sporophyte became parasitic on the gametophyte and was reduced to a single telome.

The latest view of Cronquist, Takhtajan and Zimmermann is that the Bryophytes, arising as reduction of some higher plant forms, should be placed within the Embryophytes as a division in between the Psilophytales and the Psilotales. But such a position is disliked by many as it suggests an origin of the Bryophytes-out of the Pteridophytes with vascular strands containing lignin.

Thus, according to the regressive view, the primitive Bryophyte had an erect gametophyte with three rows of leaves and with a sporophyte like that of Anthoceros. Reversion to prostrate form gave rise to the Hepatics, of which the Anthocerotopsida represents the most primitive type.

Elaboration of the sporophyte with development of the peristome structure gave rise to the mosses. Mehira (1968), however, suggests a different origin for the Anthocerotales (Fig. 498).

Fig. 498. Explaining Condensation Theory.

Evolution and Affinities of Bryophytes:

To properly understand the interrelationships of the different bryophytic taxa, it is. necessary to have a clear comparative idea of both the gametophytic and the sporophytic forms. In the gametophytes, increase of complexity is apparent in the series Anthocerotopsida, Hepaticopsida [Riccia-Marchantia-Jungermanniales (Ana- crogynous-Acrogynous)-Calobryales and Takakiales], Bryopsida.

When sporophytes are considered, Bower has pointed out the progressive sterilisation in the series. Riccia-Sphaerocarpos-Marchantia-Jungermanmniales (Anacrogynous-Acrogynous) — Anthocerotales and Bryopsida. In Riccia the sporophyte is a simple bag of ‘spores— almost wholly fertile. In Sphaerocarpos there is a short sterile stalk. In Marchantia there is differentiation into capsule, seta and foot.

Moreover, some sterile elaters develop inside the capsule. In the Jungermanniales the sterile wall increases in thickness. Maximum differentiation is in Anthoceros and the mosses where most of the spprophyte tissue the columella and the thick, photosynthetic wall of the capsule, even a meristematic region in Anthoceros, the peristome in mosses) is sterile.

It has been stated above that according to the Regressive Theory the Hepaticopsida evolved out of the more complex Anthocerotopsida or some erect complicated primitive Bryophyta while, according to the Progressive Theory, the simplest of the Hepaticopsida was the first Bryophyte evolved.

Sphaerocarpales and the Marchantiales:

The Sphaerocarpales and the Marchantiales show the simplest plant bodies among the Bryophytes. The gametophyte of Sphaerocarpos is of the simplest type. Sphaero­carpos, Riccia and Marchantia grow by a transverse row of apical cell each with two cutting faces. The sporophytes of the Sphaerocarpales and the Marchantiales are also extremely simple, their capsule walls being only one cell in thickness.

The bag-like sporophyte of Riccia is no doubt the simplest so that Lotsy pictured a hypothetical Sphaero-Riccia embodying the gametophyte of Sphaerocarpos with the sporophyte of Riccia as the ancestral Bryophyte. The spermatozoids of Sphaerocarpales, again, closely resemble those of Marchantiales as against the Jungermanniales, bringing them closer to the former.

Considering all points, it may be said that the Sphaerocarpales and the Marchantiales represent the simplest of all the Bryophytes. Whether they are the most primitive (Progressive Theory) or the most reduced (Regressive Theory) is a matter of dispute.

Within the Marchantiales, again, the Ricciaceae is certainly the simplest family while the Marchantiaceae is the most complex considering both the gametophyte and the sporophyte.

The air pores and assimilatory filaments are well-developed in Marchan­tiaceae, die sex organs are borne on special antheridiophores and archegoniophores, the sporophytes are more elaborate in having foot, seta, elaters and have special protec­tive structures surrounding the immature sporophytes.

All these structures are much simpler in the Ricciaceae. While the proponents of the Regressive Theory like Kashyap hold that this happened by reduction, adherents of the Progressive Theory, like Bower and Smith, hold that the Marchantiales developed these characters as advance in course of evolution.

Bower held that the most primitive Bryophyte had a sporophyte like that of Riccia (Ricciaceae) where the sporophyte is wholly formed of sporogenous cells.

Evolution then took the line of progressive sterilisation of potential sporogenous tissues. The next stage is shown by the family Corsiniaceae where the sporophyte resembles that of Sphaerocarpos, sterile foot and seta are differentiated and some of the sporogenous cells remain sterile though their walls do not become thickened.

Next, in the Targioniaceae, the sterile sporogenous cells form elaters. The final stage is represented by the Marchantiaceae where the foot, seta and the elaters are very conspicuous although the wall of the capsule is still only one layer thick. As against the views of Bower, Campbell, etc., Kashyap (1919), after a close study of the Indian Hepatics, became convinced of the Regressive Theory.

He concluded that the more elaborate Marchantiales like Marchantia and Preissia are more primitive and were gra­dually reduced to the simple form of Riccia by progressive reduction as becomes ap­parent on the comparative study of the gametophytes and sporophytes of different genera of Marchantiales, viz.,

(a) Loss of assimilatory filaments in the air chamber:

The air chambers (several tiers in Rebouliaceae but one tier in Marchantiaceae) are full of assimilatory filaments which are very well developed in Reboulia, Preissia and Marchantia but are pro­gressively more reduced in hydrophilous Conocephalum and Wiesnerella. It ultimately disappears in aquatic Dumortiera.

(b) Simplification of pores:

The pores are complex barrel-shaped in Marchantia and Preissia. In Conocephalum and Reboulia, they are barrel-shaped in the discs (archegoniphore, and the antheridiophare) but simple not he thallus. In Exormotheca and Stephensoniella, they are simple both on the thallus and the discs. In Riccia, the pore status is lost, they are simply the branches of the pits.

(c) The gradual shifting of the stalkes of the antheridiophore and the archegoniophore from the terminal to a dorsal position and then to the complete elimination of the stalk. The archegonia and the young sporophytes also gradually lose their protective coverings.

Mehra (1957 to 1969), a pupil of Kashyap, supports the above hypothesis and supposes that the Marchantiales were derived from the Jungermanniales by reduction. He is of the view that although the Marchantiales have evolved by reduction of the jungermanniales stock, this does not mean that the order Marchantiales represents a single line of regressive evolution from Marchantiaceae to Ricciaceae.

He thinks (Fig. 498) that after the regressive derivation of the Marchantiales, there was a line of progressive evolution ending in the Marchantiaceae proper (i.e., Compositae). Sauteriaceae, Rebouliaceae and Exormothecaceae are the earlier branches of this line.

The exormothecaceae (Exormotheca and Stephensoniella taken out of the Gompositae, of. 2nd page of Marchantiaceae—classification) line developed the families Corsiniaceae and Targioniaceae by reduction. Ricciaceae being developed as an extreme reduction of this line. Monocleaceae was developed by reduction directly out of the Marchantiaceae stock.

Thus, according to Mehra, both Marchantia and Riccia are the latest forms in evolution in two different lines. The reduction of foliose forms to thallose forms (i.e., from Junger-manniales to Marchantiales) is expoained by Mehra (1957) by the Condensation Theory originally suggested by Kashyap. The leaf expansions of foliose forms fuse to form thallose expansions.

Within the Metzgerineae foliose Fossombronia may be experimentally induced to take up the shape of thallose Petalophyllum by the fusion of succnbuus lower edges of the foliose expan­sions into the thallus so that the upper foliose expansions look like the vertical lamellae of Petalophyllum.

Mehra suggests that from this structure, by farther condensa­tion (Fig. 498C), completely flat Stephensoniella —(Marchantiaceae—Fig. 498D) and Asterella (Fimbriaria—Rebouliaceae Fig. 498G)—hyper thalli develop which first gives off the Sphaerocarpales line and then branches into the Sauteriaceae, Marchan­tiaceae, Rebouliaceae and the Exormothecaceae (which farther develops Targioniceae, Corsiniaceae, Ricciaceae etc.) lines.

Within the Sphaerocarpales Geothallus is somewhat leafy while Sphaerocarpos is completely thallose.

Harris (1938, 1939) while discussing the fossil (Naiadatia suggested that the an­cestral Hepatics had an erect leafy shoot with spiral leaves as in mosses. Burgeff (1943) after twenty years study of mutations of Marchantia concluded that the mutants clearly show a phyletic reduction series which proves that Marchantia gave tise to the other genera by reduction.

Cavers,(Fig. 497), considered the Marchantiales as a blind line of evolution from the hypothetical Sphaero-Riccia.

Jungermanniales:

The Jungermanniales have the plant body more complex than that of the Mar­chantiales. The sporophytes have multi-layered capsule walls and a general structure which is’ also more elaborate than that of the Marchantiales.

Here again, the Metzgerineae (anacrogynous Jungermanniales) are comparatively simpler with thallose or simple foliose, prostrate gametophytes and apical growth by a cell with two cutting faces while the archegonia develop on the dorsal surface of the thallus.

The Jungermannineae (acrogynous Jungermanniales) are more complicated in having foliose gametophytes growing by apical cells having three cutting faces and with archegonia and sporophyte growing on tips of special foliose shoots which have erect tendencies.

As in other cases, the Regressive Theory holds that the simpler Metzgerineae (an­acrogynous) came out of the more complicated Jungermannineae (acrogynous) by gradual reduction of the leafy appendages and the sexual shoots. According to this view the most primitive Hepatic was an erect plant with radial symmetry like the Calobryales or like Naiadatia.

This then obtained dorsiventral symmetry by reduction of the ven­tral row of leaves which became the amphigastria in the acrogynous Jungermanniales (Lejeunea and Lepidozia), then became further reduced into the slime papillae (Plagiochila and Cephalozia) and got completely lost in Radula. Simultaneously with this change the axis became flattened and then the Metzgerineae (anacrogynous forms) was evolved.

The reproductive shoots, being more conservative in all plants, tried to retain the radial habit till the last.

The Marchantiales, the Sphaerocarpales and the Anthocerotopsidsa gametophyte developed from the Metzgerineae by reduction.

It should be remembered in this connection that thallose Jungermanniales are known from the Devonian while the foliose forms are known from the Carboniferous—earlier than the Marchantiales. Mehra (1957) has explained the origin of thallose forms from foliose forms in Metz­gerineae by compaction (as in Metzgeria by flattening of the midrib and loss of leaves) and condensation (as in Petalophyllum).

The Progressive Theory argues just the other way. Bower’s view of sterilisation of the sporophyte is further extended giving rise to the elaterophores of Pellia and Riccardia. The sporophyte also becomes more elaborate with a multi-layered capsule wall and a longer seta. There is also tendency of independence shown by the presence of chloroplastids and stomata on the sporophyte. Cavers (Fig. 497) held a similar view.

Some of the Metzgerineae like Metzgeria and Aneura (Riccardia) have so simple gametophytes that they have been associated with the most primitive Bryophyte (like Sphaero-Riccia) by Campbell.

Calobryales:

The Calobryales is a peculiar group of Hepatics with erect plants showing almost radial symmetry. They strongly resemble Naiadatia and simple acrocarpous mosses so that they have been held to be very near the most primitive Bryophyte by the pro­ponents of the Regressive Theory who hold that this was a plant-with radial symmetry.

The sporophyte, again, is simpler than that of the Jungermanniales and is similar to those of the Sphaerocarpales and the Marchantiales. The order is partly acrogynous and partly anacrogynous so that some Bryologists place it between the acrogynous and anacrogynous Jungermanniales. But, because of its special features, it is considered better to remove it out of the Jungermanniales as a separate order.

The growth of the antheridia and archegonia in Calobryales and Mosses by 2-sided or 3-sided apical cells and the peculiar way of archegonial development of Anthoceros makes Mehra (1957, 1958) believe that these sex organs represent reduced branches.

The rhizomatous base with mucilage hairs and without rhizoids is similar to that of the Takakiales so that Pyoskauer (1962) and others think that the latter actually should be within the Calobryales.

The proponents of the Progressive Theory, Smith for instance, hold that the Calobryales developed in the same line as the Jungermanniales, but they separated earlier so that they have retained the simplest type of sex organs among the Hepatics and a simple sporophyte but, have developed the most elaborate type of gametophyte.

Anthocerotopsida:

The Anthocerotopsida is a group of plants with such distinctive features that it has been thought necessary to take it out of the general Hepatics and to place it in a sepa­rate class parallel to the Hepaticopsida and the Bryopsida. Four genera within this class are universally recognised Anthoceros, Megaceros, Dendroceros and Notothylas.

Later systematists have proposed to break down Anthoceros into three genera: Aspiromitus, Phaeoceros and Anthoceros. Notothylas is now placed in a separate family Notothylaceae.

The Anthocerotopsida has been called a synthetic group as, apparently, it shows characters linking it to widely differing plant groups, viz., the Algae, the Hepaticopsida, the Bryopsida and the lower vascular plants (Pteridophytes).

The most striking similarity with the algae is the chloroplast. A single alga-like chloroplast is the rule within the group. The chloroplast has a tendency to increase in number in deeper layers and in Megaceros there are multiple chloroplasts so that this genus is considered nearer to the higher embryophytes. The simplicity of the gameto­phytic structure brings it at par with some of the green thalloid algae.

The general appearance of the gametophyte and those of the mature sex organs resemble the Hepaticopsida and that is why they were included within the Hepaticae. The sporophyte may be derived by extending Bower’s view of greater sterilisation and more chlorophyllose tissue tending to independence.

This is a stage just in advance of the Hepaticopsida. But, this sporophyte differs in certain points. While the primary sporogenous tissue of the Hepaticopsida is derived from the endothecium, that of the Antho­cerotopsida is derived from the amphithecium.

In this respect, Notothylas shows the nearest approach to the Hepaticopsida —specially to the simpler members like Sphaerocarpos and Cyathodium. It has a very reduced sporophyte and in some Notothylas species the endothecium also is sporogenous, there being no sterile columella.

In still other species of Notothylas the amphithecium forms only the jacket while the endothecium is entirely sporogenous —just as in the simpler Hepaticopsida. It should be noted that Cyathodium also approaches the Anthocerotopsida in having fewer chloroplasts in the cells, approach­ing Megaceros.

It is possible that the Anthocerotopsida developed out of the Hepaticopsida by way of plants like Notothylas. But, it has also been argued that Notothylas was derived from Anthoceros by reduction and further reduction might have given rise to the Hepa­ticopsida.

Cavers (Fig. 497) derived the Anthocerotopsida as a side line from the Junger­manniales branch of the Hepaticopsida just as is the case of the Sphaerocarpales but with more evolution of the sporophyte.

Anthocerotopsida is linked with the Bryopsida by the structure of the sporophyte. The amphithecial development of the sporogenous tissue is common with the Sphagnideae, and the embryos are also similar in other details.

The sporophyte of many mosses have a long continued growth, an elaborate photosynthetic tissue with regular stomata and a regular sterile columella—all in common with the Anthocerotopsida. Cavers derived the Bryopsida through the Anthocerotopsida (Fig. 497).

The green, cylindrical, upright sporophyte of unlimited growth as found in Antho­ceros and its tendency towards an independent life is a near approach to the lower vascular plants (Pteridophytes).

The establishment of a central sterile columella (a forerunner of vascular cylinder), the elaborate photosynthetic tissue with stomata and an intercalary meristematic tissue is an advance in the direction of the independent, green, vascular sporophytes of unlimited growth as found in the most primitive land plants.

The sporophyte of Anthoceros greatly resembles that of the Devonian Rhyniaceae and is short of it only by the absence of a vascular system, of completely independent life and of the branching habit. Haskell (1949) gave emphasis on the similarity bet­ween Pteridophytic plant structure as shown by Homeophyton and the Anthoceros sporo­phyte.

Proskauer (1960) noted spiral cellulose thickening lining the archesporium both outside and inside in Dendroceros. According to him this indicates the tapetal layers in Rhynia and Homeophyton.

Even within Anthoceros, in certain species (e.g., A. Jusiformis) it has been noted that the sporophyte may live independently for some time after the gametophyte, on which it had developed, decays. The bivalved dehis­cence of the capsule of Anthoceros may be the beginning of dichotomous branching. Naturally, there are theories on the evolution of the Pteridophytes out of Anthoceros.

Mehra (1968, Fig. 487) is of opinion that the Anthocerotopsida did not originate from the same stock out of which the Hepaticopsida and the Bryopsida arose but out of a separate stock called Anthorhyniaceae. This stock gave rise to the Antherocero- topsida on one hand and the Rhyniaceous stock (which gave rise to the Psilophytales and the Pteridophytes other than Lycopsida) in the other hand.

The Anthocerotop­sida is, thus, closely related to the Psilophytopsida.

In conclusion, it may be said that the Anthocerotopsida resembles the type of plants which existed before the actual evolution of the vascular plants. If not the actual ancestor of the modern land plants, they seem to be related to this ancestor. The Hepaticopsida and the Bryopsida must also be related to this ancestor. But, the primitive Anthocerotopsida was closer to the Hepaticopsida than to the other groups.

In assigning any primitiveness to this class, it should be remembered that it is unknown as fossil and the earliest indirect evidence of its presence is in some tertiary spores.

Bryopsida:

The Bryopsida represents a very specialised group of Bryophyta in the life history of which the protonema has an important place in the gametophytic phase. The fila­mentous protonema resembles algae giving rise to speculations (e.g., that of Goebel) that the mosses might have arisen directly out of the algae.

But, these filaments are associated with highly developed sex organs and the gametophyte as well as the sporo­phyte of the Bryopsida are much more elaborate than those of the Hepaticopsida and of the Anthocerotopsida.

Moreover, the protonemata of the simpler Bryopsida, those of the Sphagnidae and the Andreaeidae, are thallose. So, there seems to be little evidence that the Bryopsoda arose directly out of the algae.

Campbell supposes that the thalloid protonema of the primitive mosses developed out of the thalloid Hepatics and the thallose form gave rise to the filamentous protonema as it was more convenient for giving rise to a large number of gametophytic plants.

Fossil evidence does not take us anywhere. It has already been seen that although the first fossil Hepaticopsida is known from the Devonian, Bryophytes appear abun­dantly only in the Upper Carboniferous and then, simultaneously, both the Hepati­copsida and the Bryopsida are equally developed.

Of the Bryopsida, the acrocarpic ones with erect, radial gametophytes are found first and the pleurocarpic ones with pros­trate, branching gametophytes followed them.

The ancestor of the Bryopsida is likely to be an erect, radial plant as found in the Calobryales of Hepaticopsida. But, the game­tophytes of all mosses are much more complicated structures than that found in any known Hepaticopsida or Anthocerotopsida.

The gametophyte of mosses is adapted well to its amphibious condition of life. The erect sporophyte of mosses has the nearest approach in the Anihocerotopsida. But, here again, the sporophyte of the mosses with its intricate peristome is such a complicated structure that it needs a very specialised origin.

Among mosses, cleistocarpic forms with no apparent peristome, seem nearer to the Hepaticopsida than to the stegocarpic forms with elaborate peristome structures. But, it is not the general opinion of the Bryologists that the different cleistocarpic genera developed separately and independently by the reduction of different stego­carpic genera.

However, it may be that some of these cleistocarpic forms, specially Archidium, are more closely related to the ancestral moss. Among stegocarpic mosses, Tetraphis has a capsule similar to that cf Andreaea with apparent valvular dehiscence. The Cavers concept of the course of evolutionjof the Bryopsida is seen in Figure 497.

Considering the manner of water conduction, mosses are divided into endohydric and ectohydric. The great majority of mosses do not conduct water through the central strand, water being absorbed by all’ the gametophytic surface. These are called ecto­hydric. In many of these no central strand is differentiated. This group is much nearer to the Hepaticopsida and the Anthocerotopsida in this respect.

The endohydric mosses (e.g., Polytrichum loc. cit., Bryum), on the other hand, have a better Organised central strand through which there is some possibility of water being actually conducted. But, as yet, it is not known how far this distinction is due to heredity and evolution and how far merely due to ecological response.

Sphagnidae and Andreaeidae:

Of the Bryopsida, the Sphagnidae, with its simpler gametophyte, and a simple sporo­phyte in which the sporogenous tissue originates entirely from the amphithecium, is the nearest approach to the Anthocerotopsida. Even its protonema is thallose like the Hepatics gametophyte plant body.

Next to the Sphagnidae is the Andreaeidae which may be considered intermediate between the Sphagnidae and the Bryidae.

The sporo­phyte and the protonema of the Andreaeidae resembles the Sphagnidae, the sporogenous tissues arches over the columella as in the Sphagnidae and the Anthocerotopsida but it is not developed out of the amphithecium as in those cases; it develops out of the endothe­cium as in the Bryidae.

The pseudopodium is similar to the Sphagnidae but the capsule does not show any peristome or operculum and is valvular in dehiscence as in the Jungermanniales.

Bryidae:

Within the Bryidae, Cavers and Campbell recognise four groups:

(1) Tetraphidales,

(2) Polytrichales,

(3) Buxbaumiales and

(4) Eubryales.

Of these, the Tetraphidales is con­sidered by them to have the simplest peristome but there is no evidence that this group, in spite of the apparent valvular dehiscence of its capsule simulating the Andreaeidae, forms a link between the Andreaeidae and the Bryidae, specially because there is no peris­tome in the former.

Cavers, however, considers that the Tetraphidales arose out of the Andreaeales as the ancestor of the Bryopsida (Fig. 497).

According to him, there were two lines of evolution out of the Tetraphidales: the Polytrichales arose in one line while the Buxbaumiales and the Eubryales arose in the other line. The Polytrichales have very ad­vanced characters but their relatively small number leads Campbell to suppose that they are more primitive than the general Eubryales.

The Buxbaumiales have some ano­malous characters but they may be related to the Polytrichales through Dawsonia. The very stable low basic chromosome number (n = 7) of the Polytrichiidae also points out to their primitiveness. The Eubryales is the predominant group but they are pro­bably the most recent in evolution like leptosporangiate ferns. Cavers and Dixon consider the Nematodonteae to be the more primitive.

The common mosses (i.e., the Eubryales) comprise the Arthrodonteae, where again, probably the Haplolepideae (one row of peristome teeth) came first and the Diplolepideae (two rows of peristome teeth) came last. Philibert, however, considered the Heterolepideae (Encalyptales) as the possible common origin of both the Haplolepideae and the Diplolepideae.

Within the Diplolepideae, again, the pleurocarpous foms are the latest in evolution, all the previous forms being acrocarpous. Fossil evidence also shows that the more primitive mosses (Nematodonteae or Arthrodonteae) were acrocarpous.

Fleischer, Brotherus and Reimers (Engler’s System), on the other hand, consider that the cohort Eubryinales (including Tetraphidales within the Eubryales-, Reimers has called this subclass Bryidae) is more primitive and the other two cohorts Buxbaumii­nales and Polytrichinales (called subclass Buxbaumiidae and Polytrichiidae by Reimers) are later developments.

So, here again the problem is whether the more advanced Polytrichales are the ancestors of the simpler Eubryales (or Bryidae) i.e., whether evolution was progressive or regressive. Most Bryologists agree that the Polytrichales might be very primitive but whether the Eubryales came out of them or arose from a common ancestor is still open to question.