In this article we will discuss about:- 1. History of Bryophytes 2. Origin of Bryophytes 3. Meaning 4. Characteristics 5. Vegetative Reproduction 6. Ecological Importance 7. Affinities 8. Life Cycle.
History of Bryophytes:
The fossil history of the bryophytes is very meagre due to the fragile nature of the plant body. The stems orthalli do not possess lignified vascular tissue and cutinised epidermis. In addition, the leaves of bryophytes are mostly one-layer in thickness.
Majority of the bryophytes are known as vegetative remains of the gametophyte. Fossil bryophytes have been reported as old as the Devonian. Numerous spores have been identified as bryophytes and recorded as “Sporae dispersae”.
Because, it is difficult to distinguish spores of bryophytes from that of vascular plants. Some of the “Sporae dispersae”, perhaps originating from bryophytes have been recorded even in the Ordovician and the Silurian deposits.
Origin of Bryophytes:
The fossil records of bryophytes are very meagre due to the soft and fragile nature of the plant body. Hence, the geological history of bryophytes does not throw any light on the origin and subsequent evolution of bryophytes. However, the comparative study of the plant groups has been helpful to trace the origin of the group.
Opposing Views about Origin of Bryophytes:
There are two opposing views about the origin of bryophytes: A. Pteridophytic origin i.e., the bryophytes are the descendants of pteridophytes, B. Algal origin i.e., the bryophytes have arisen from algae.
1. Pteridophytic Origin:
This theory suggests that the bryophytes are the descendants of pteridophytes and evolved by the progressive degeneration of pteridophytes. In other way, this theory suggests the regressive evolution of the simpler forms (liverworts) from the more complex ones (mosses).
This theory was supported by various authors like Wettstein (1903), Scott (1911), Lang (1917), Kidston and Lang (1917-1921), Kashyap (1919), Takhtajan (1953), Mehra (1953), Christensen (1954), Proskauer (1960), Zimmermann (1966), Udar (1970), etc.
The basis of this theory lies on the similarity between the earliest vascular plants, Rhyniopsida of the Upper Silurian to Lower Devonian age, and the sporophytes of certain mosses (Andreaea, Funaria, Sphagnum, etc.) and members of hornworts (Anthoceros, Dendroceros, etc.).
The characteristics of Rhyniopsida that are comparable with the sporophytes of bryophytes have been enumerated:
(a) The members of Rhyniopsida are rootless (bearing rhizoids), leafless and dichotomously branched shoots bear terminal sporangia.
(b) The sporangia of Horneophyton and Sporogonites (plant showing bryophytean affinity) are provided with columella.
(c) The capsules of some members of bryophytes (Funaria, Sphagnum and Anthoceros) comprised of photosynthetic tissue and are provided with stomata. These are the common features of pteridophytes.
(d) The cells of the outer layer of the columella and the lining layer of the jacket in Dendroceros cripsus (Anthocerotophyta) form incomplete spiral thickening. This lining layer of the jacket is found to be identical with the tapetum (a one-celled thick layer lining the jacket of sporangium) of Horneophyton (Rhyniopsida).
The similarities between the sporangia of pteridophytes and bryophytes have led many botanists to interprete that the Anthocerotophyta of bryophytes may have originated from the Horneophyton type of pteridophyte. Subsequently the simpler forms of hepatics might have evolved from the complex forms by the progressive reduction or simplification.
2. Algal Origin:
Many scientists believed that the bryophytes have originated from the algae, though they were not unanimous about whether the origin of bryophytes was monophyletic or polyphyletic.
Even, there is controversy regarding the type of ancestor alga that has given rise to bryophytes. The concept of the algal origin of bryophytes is based on the similarity in algae (specially green algae) and bryophytes.
The characteristics common for both the groups are:
(a) Presence of thalloid gametophyte,
(b) Amphibian nature,
(c) Similar photosynthetic pigments (chlorophyll a, b and carotenoids α, β),
(d) Cell wall made up of cellulose,
(e) Presence of filamentous protonema,
(f) Starch as reserve food,
(g) Presence of flagellated sperms, and
(h) Water essential for fertilisation.
Several authors suggested various algal ancestors that have given rise to the bryophytes. Lignier (1903) proposed the hypothetical terrestrial plant, Protohepatics, which evolved from the aquatic algal ancestor. Subsequently, the Protohepatics have given rise to the bryophytes (Hepatics) on one hand and to the pteridophytes on the other hand.
Bower (1908) supported the algal origin of land plants (bryophytes and pteridophytes), possibly from the freshwater green algal (chloro- phycean) ancestor.
Fritsch (1916, 1945) hypothesised that the Chaetophoraceous ancestor have given rise both to the bryophytes and the pteridophytes. The Chaeotophorales (green algae) members showing heterotrichous habit, comprised of a prostrate and upright (erect) systems. The heterotrichous thallus has given rise to an erect land plant by elaborating the erect portion and by diminishing the prostrate portion.
An apical growing point is established as bas been observed in some members of Chaetophorales (Trentepohlia). The heterotrichous branching habit has been seen in mosses where filamentous protonema represents the prostrate system, while foliose gametophore represents the erect system.
Ultimately, the gametophytic plant body became much more elaborated by the development of the parenchymatous tissue and the sporophytic plant became a parasite on the gametophyte.
Church (1919) in his essay “Thallasiophyta and the subaerial Transmigration” supported the algal (marine) origin of land plants (both bryophytes and pteridophytes). He postulated that these marine algae were planktonic in nature and, subsequently, they became benthic in the shallow seas.
Further transmigration of these benthic forms to the subaerial condition led to the establishment of a large group of extinct seaweeds which are much more advanced than the present day thallophytes. He named this hypothetical group as “Thallasiophyta”.
According to Church, this transmigrant algae had:
(a) Metabolic efficiency of Chlorophyceae for the production of starch,
(b) Somatic equipment of the Phaeophyceae, like multiseptate organisation, differentiation into root-like holdfasts, stem-like stipe, leaf-like blades, conducting tissue, etc.,
(c) Reproductive scheme of life- history was more advanced than of Dictyota, such as isogamy, in situ fertilisation, alternation of haploid sexual and diploid asexual phases.
The both bryophytes and pteridophytes of today originated from this hypothetical “Thallasiophyta” group.
Smith (1955) strongly supported the algal origin of bryophytes. He supposed that bryophytes have originated from the green algal group, such as Ulotrichales of Chlorophyceae.
Mehra (1969) opined that both the bryophytes and pteridophytes have originated from some “Proto-archegoniatae” like ancestor.
In the earlier classifications, bryophytes were placed in the single phyllum Bryophyta, intermediate in position between the algae and the pteridophytes. Modern studies of cell ultra-structure and molecular biology, however, confirm that the bryophytes comprised of three separate evolutionary lineages, which are today recognised as liverworts (Phyllum Marchantiophyta or Hepatophyta), hornworts (Phyllum Anthocerotophyta) and mosses (Phyllum Bryophyta).
Bryophytes are not considered to have given rise to the vascular plants, but they probably were the earliest land plants (Qui and Palmer, 1999). Like the rest of the land plants, they evolved probably from green algal ancestor (Coleochaete), closely related to the Charophytes (For details see page 690).
Following a detailed analysis of land plant relationships, Kenrick and Crane (1998) proposed that the three phyla of bryophytes represent a grade or structural level in plant evolution. Phylogenetic analyses suggest that these phyla are housed among embryophytes and are paraphyletic with regard to each other (Mishler et al. 1994).
The mosses appear to be a sister group to the tracheophytes (vascular plants). The liverworts show their closest relationship with Charophytes, while horn- worts occupy an intermediate position between liverworts and mosses.
Meaning of Bryophytes:
The name Bryophytes came from the two Greek words “Bryon” (moss) and “phyton” (plant). Bryophytes are the simplest and primitive plants among the embryophytes (Embryophytes are the plants where the zygote forms an embryo). This is the first group of plants to invade the land, though they require water for their fertilisation.
Therefore, they are regarded as plant amphibians. Bryophytes are small and herbaceous plants. Unlike most of the higher plants, bryophytes are not found as single individuals but in groups that grow closely packed together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees, or as free-floating in water.
At present this group is represented by 1,237 genera and about 18,000 species. They flourish particularly well in moist, humid forests like the fog forests of the Pacific northwest or the mountain rain forests of the southern hemisphere. In India, they are mostly confined to the eastern and northern Himalayas and the Nilgiri and other hills.
All bryophytes are characterised by the ecologically persistent, haploid gametophyte generation. The sporophytes are very short-lived and are attached to and nutritionally dependent on their gametophytes.
The sporophyte is monosporangiate, consists of only a bulbous foot, with or without an unbranched stalk or seta, and a single, terminal sporangium. Hence, the sporophyte of bryophytes is called sporogonium. Bryophytes are non-vascular plants that never form xylem tissue.
Bryophytes have several ecological importance’s. They provide seed beds for the larger plants of the community. They capture and recycle nutrients that are washed with rainwater from the canopy. They bind the soil to keep it from eroding.
The moss Sphagnum has exceptional water-holding capacity and, when dried, forms a coal-like fuel called peat. Now-a-days peatlands are managed as a sustainable resource for both fuel consumption and horticultural uses.
General Characteristics of Bryophytes:
1. The life cycle of bryophytes is distinctly differentiated into gametophytic and sporophytic phases (heteromorphic).
2. The gametophytic phase is predominant and ecologically persistent, i.e., green, independent and long-lived.
3. The sporophytic phase is very short-lived, and completely dependent upon the gametophyte.
4. Unlike most of the higher plants, bryophytes are not found as single individuals but in groups of individuals which have characteristic features depending on their family, genus or species.
5. The gametophytic plant body is either thalloid or differentiated into the erect axis (stem) and lateral appendages (leaves).
6. Roots are absent in bryophytes. The rhizoids perform the function of roots. They are either unicellular and unbranched or multicellular and branched.
7. They never form xylem tissue, the special lignin-containing water-conducting tissue that is found in the sporophytes of all vascular plants. However, the plant body is made up of parenchymatous cells only.
8. They reproduce by vegetative and sexual methods. Asexual reproduction is completely absent in bryophytes.
9. Vegetative propagation takes place by some special structures, like gemmae, tubers, protonema, cladia, innovation, etc.
10. Sexual reproduction is only of oogamous type. They produce large, multicellular sex organs for reproduction. Bryophytes are unisexual, either homothallic (monoecious) or heterothallic (dioecious).
11. The male sex organs, called antheridia, are stalked, globose or ovoid with one celled thick jacket surrounding androgonial mother cells.
12. The female sex organs, called archegonia, are vase-shaped or flask-shaped structure having the basal swollen venter containing a ventral canal cell and an egg, and the upper elongated neck containing neck canal cells. Both the venter and neck are surrounded by the sterile jacket.
13. The sperms are motile and biflagellate having two whiplash flagellae.
14. Bryophytes require water for sperm dispersal and subsequent fertilisation.
15. The sperms move short distances in the water film and ultimately reach the open necks of the archegonia. The slimy mucilage secretions in the archegonial neck help to pull the sperm downward to the egg.
16. The zygote does not pass any resting phase. Embryonic growth of the sporophyte begins within the venter of the archegonium soon after fertilisation.
17. The embryo follows exoscopic mode of development. In this development, the zygote first divides transversely to form an outer epibasal cell and an inner hypobasal cell and the embryo develops from the epibasal cell. Thus, the shoot forming apical cell is directed outwards, i.e. towards the neck of the archegonium.
18. The sporophyte consists of only bulbous foot, with or without an unbranched seta and a single terminal sporangium Sometimes the sporophyte is represented only by a capsule (Riccia). The capsule has a protective covering called calyptra which is a part of gametophyte.
As the sporophyte of bryophytes is monosporangiate (containing a single sporangium), they are called sporogonium.
19. Bryophytes are homosporous — isospores (spores are identical morphologically and physiologically) are produced from the sporogenous cells of the capsule.
20. The spore after germination either produces a filamentous germ tube that gives rise to a young gametophyte (Riccia, Marchantia) or produces a protonema which bears leafy buds that will ultimately form the adult gametophytic shoot.
Vegetative Reproduction in Bryophytes:
The most common method of multiplication in bryophytes is vegetative reproduction. This mode of reproduction does not involve meiotic division and fusion of gametes. This is fundamentally a process of division and/or detachment of some parts of the plant body that subsequently develops into a complete plant.
In most of the dioecious species, sexual reproduction is almost inhibited and the propagation of the species is obviously taking place by vegetative means.
The following are the common methods of vegetative propagation noted among the bryophytes:
(a) By Fragmentation or progressive death and decay of the older parts,
(b) By tubers,
(c) By Gemmae,
(d) By the formation of adventitious branches,
(e) By cladia,
(f) By formation of innovations,
(g) By protonema, and
(h) By persistent apices.
1. Fragmentation or Progressive Death and Decay of the Older Parts:
Propagation through this process involves progressive death and decay of the older parts of the thallus and consequent separation of the younger parts at the point of dichotomy of the thallus (Fig. 6.65A-C).
These separated parts, called fragments, develop to form new mature thaIII. This is the common method of vegetative reproduction found in liverworts (e.g. Riccia, Marchantia, Plagiochasma), hornworts (e.g. Anthoceros) and in some mosses (e.g. Sphagnum).
2. By Tubers:
The tubers are round structures formed on the thaiIi under unfavourable conditions like drought (Fig. 6.66A-D). They remain unaffected and lie dormant while the rest of the thallus gets dried. On the return of favourable environmental conditions, the tubers resume growth and develop into new thalli.
Tubers are frequently produced in species of Riccia (R. discolor, R. vesicata, R. billardieri), Anthoceros (Anthoceros pearsoni, A. himalayen- sis, A. laevis), Asterella angustata, and Fossom- bronia himalayensis.
3. By Gemmae:
Gemmae are special propagative organ with definite form. They are quite abundant in the members of liverworts, but less common in hornworts and mosses. They are of different shapes, stalked or sessile and may develop on different parts of the parent plant.
A brief account of different types of gemmae of liverworts, hornworts and mosses are given:
(a) One- to Three-Celled Gemmae:
Gemmae are 1-3 celled and borne on the leaf surface of Marsupella emarginata, Diplophyllum albicans, etc. Such gemmae are also found on the stem apex of Cephalozia bicuspidate, Lophozia heterophylla, etc. (Fig. 6.67A, B).
(b) Two-Celled Endogenous Gemmae:
They are endogenously formed within any external cell. They are found in Riccardia sinuata, R. pinguis and R. palmata.
(c) Discoid Multicellular Gemmae:
Discoid multicellular gemmae are produced on leaves in Radula complanata, R. germana, Porella rotundifolia etc. (Fig. 6.67C). Similar gemmae are also produced on special gemmiferous branches of Metzgeria uncigera (Fig. 6.67D).
(d) Multicellular Stalked Gemmae:
In this case, the gemmae are multicellular, short-stalked, green and discoid and are produced in gemma cups on the dorsal surface of the thallus (Fig. 6.67F, G). They are found in species of Marchantia, Lunularia, Neohodgsonia and Cavicularia.
(e) Sub-Spherical Gammae:
These gemmae are produced in flask-shaped gemma receptacle in some species of Blasia.
(f) Star-Shaped Gemmae:
Star-shaped gemmae are produced on the dorsal surface of the thallus of Blasia sp.
In many species of Anthoceros (A. glandu- losus, A. propaguliferous and A. formosae) the gemmae are produced along the margin and dorsal surface of the thallus.
a. Multicellular Stalked Gemmae:
In Tetraphis pellucida, the stalked, multicellular green and lenticular gemmae are produced at the tip of the shoot (Fig. 6.67H). Such gemmae are surrounded by a cup-shaped structure formed by the widening of the leaves. In Aulacomnium androgynum stalked, fusiform gemmae are produced at the end of leafless terminal stalk (Fig. 6.67E). In Bryum rubens and B. erythrocarpum globular, multicellular gemmae are produced at the base of the stem.
b. Multicellular Articulate Gemmae:
Multicellular articulate gemmae are produced on the leaves of Tortula papillosa, Ulota phyllantha (Fig. 6.67I). Multicellular gemmae are also produced on the rhizoids of Pogonatum sp., Polytrichum sp., Ceratodon pur- pureum, Bryum erythrocarpum, Tortula stanfordensis, Barbula convulata, etc.
4. By the Formation of Adventitious Branches:
In some thailoid liverworts and hornworts adventitious branches develop usually from the underside of the midrib of the thallus (Fig. 6.68). These adventitious branches on detachment from the parent plants develop into independent plants (e.g., Riccia fluitans, Marchantia, Targionia, Reboulia, Asterella, Sphaerocarpos, Anthoceros, etc.). This process, in fact, achieves the luxuriant growth of Riccia fluitans rapidly covering the whole pond surface.
5. By Cladia:
Cladia are small detachable branches which serve the purpose of vegetative reproduction. These may develop either from the individual cells of leaves or stems and are known as leaf cladia (Bryopteris fruticulosa, Frullania fragilifolia) or stem cladia (e.g., Leptolejeunia sp., Drepanolejeunea sp., etc.) as per their mode of origin.
6. By the Formation of Innovations:
The formation of innovations is frequently noticed in Sphagnum (Fig. 6.69). In this case, one of the divergent branches in each node develops more strongly and becomes dominant and erect like the main axis and is called innovation.
During humification process innovation is separated from the main axis due to the progressive death and decay of the lower basal parts of the main axis and eventually establishes as an independent plant body.
7. By Protonema:
The green, filamentous protonema which develops from the germination of spore is known as primary protonema. The primary protonema may break into small pieces — either accidentally or due to the death of some cells in between. Each of these fragments bearing buds is capable of forming a new plant (e.g., Funaria).
The protonema that develops from any part of the plant other than the spores are called secondary protonema. They may develop from the rhizoids of gametophore (e.g., Funaria sp.), from leafy gametophore (e.g., Funaria, Sphagnum), from primary protonema (Sphagnum), from wounded portion of the leafy shoot (e.g., Funaria).
Additional buds develop via formation of secondary protonema, thus increasing the number of buds derived from a single spore (Fig. 6.48).
8. Persistent Apices:
In many thalloid bryophytes (e.g., Anthoceros, Cyathodium, Athalamia etc.) and in some creeping mosses, all parts of the thallus except the growing apex becomes dry during the dry summer season. These apical parts remain dormant during dry season. These apices, on return of favourable environmental condition (rainy season), become active and form new plants.
Ecological Importance of Bryophytes:
Bryophytes are important constituents of the ecosystem in temperate and tropical forests that have significant ecological importance. Bryophytes are important stabiliser of substrata that later become suitable for higher plants colonisation. Extensive bryophyte mats are significant in the water balance of the forest.
They are capable of absorbing water and nutrients directly through the surface. They prevent soil erosion as they have trample-resistant structure and high regenerative capacity. Some bryophytes provide suitable substrata for the biological fixation of N2 in association with blue green algae.
The recent increase in atmospheric pollution has revealed the bryophytes as “bioindicators” of pollution and accumulators of heavy metals.
The ability of bryophytes to grow on open and nutrient poor areas and their tolerance to desiccation can be exploited in successful stabilisation of soil on road sides and open areas. Bryophytes also harbour a number of invertebrates and provide them shelter, food and a place for deposition of eggs.
The details are as:
1. Bryophytes and Plant Succession:
Among the bryophytes, the mosses are considered to be the most potent forms in successional process. They colonise over the nutrient-poor sites where no other plant can survive. After death and decay, they form humus, in other way increasing soil fertility.
Thus, the accumulated organic matters become suitable for the microorganisms. The microorganism increases the nutrient availability and makes the site suitable for growth of higher plants. The important species under this category are Cephalozia media, Isopterygium elegans, Lepidozia septans, Pellia epiphylia and Tetrapis pellucida.
2. Bryophytes and Animal Association:
Bryophytes possess several attributes viz., inconspicuous forms, relative abundance in the community, ability to survive in extreme environmental conditions and water absorbing and retention capacity, which affect the distribution and abundance of dependent animals and microorganisms.
(i) Bryophytes and Animal Succession:
While bryophytes participate in the early stages of plant succession,’ their associated animals form similar stages of faunal succession. For example, moss cushions developing on rock faces are first colonised by rhizopods, rotifers, nematodes and ciliates.
As dead material form under the cushions, rotifers and tardigrades become abundant and arthropods begin to appear. As a thicker decomposition layer is formed, the composition of the fauna becomes similar to that of the soil fauna.
Water retention is a unique feature that makes bryophyte community an attractive habitat for many invertebrates. Bryophytes provide food and nesting materials for small mammals and invertebrates. Indirectly, they serve as a matrix for a variety of interactions between organisms. Insects are the most richly represented on bryophytes.
Many protozoa, rotifera, nematodes, earthworms, molluscs, arthropods like spiders, millipeds, centipeds and various crustaceans are found in bryophyte communities. Large pores of Sphagnum leaves facilitate the entrance of water and allow unicellular animals to enter the leaf cells and live inside them.
Many invertebrates feed on bryophytes. Orthopterans, beetles, moth and caterpillars bite and chew whereas bugs, aphids and mites suck out the contents of moss cells.
(iv) Ovipositor and Pupation:
The animals which feed on bryophytes also ovideposit their eggs there. Snails and slugs are frequently depositing their eggs upon the gametophores. Water beetles appear to live preferentially among mosses and spend their dormancy period.
Many insects associated with bryophytes deposit their eggs there, and -the larval stages often browse on the gametophores. Pupation of the water beetle takes place within a small cell.
Some insects have morphologies, surface patterns or appendages that permit them to blend in with their bryophyte habitat. A few insects paste the parts of gametophores on their wings and thus camouflage themselves against predation.
Camouflage may be used by larvae that construct their cases from blades of Fontinalis, Hygrophynum, Anomobryum and Plagiochila.
3. Bryophytes and Cyanobacteria:
In natural association, cyanobacteria typically grow in association with bryophytes. Nitrogen is often a limiting nutrient for plant growth. Even small contributions from biological nitrogen fixation may, therefore, be important to the ecosystem.
Some mosses, hornworts and liverworts provide suitable habitats for the biological fixation of nitrogen in association with cyanobacteria (e.g., Nostoc). The ability of cyanobacteria to fix atmospheric nitrogen allow a few bryophytes to grow in areas that are naturally low in nitrogen and serve as fertiliser to soil.
4. Bryophytes as Ion-Exchanger:
The cell walls of Sphagnum function as ion-exchanger. They rapidly absorb cations, such as calcium and magnesium, supplied by rain water, and in exchange^ release hydrogen ions into the water. Rydrogeri ions make the soil acidic (pH 3-4). Therefore, Sphagnum creates as well as maintains a nutrient-poor, acidic environment that fosters their own growth, but is mostly intolerable to other plants.
5. Bryophytes Maintain Water Balance in the Forests:
In forests, especially in the montane tropics, bryophytes (especially Sphagnum) absorb huge quantities of water and maintain humidity over dry periods, thus preventing rapid run-off and flooding. Without bryophytes, rainforest would be merely wet and mountain rocks would be barren.
The huge bryophyte mats in the forest floor slow down and delay run-off during rain. It has been predicted that the excessive flooding in India is at least partly due to loss of bryophyte covers.
6. Bryophytes Conserve Soil and Prevent Soil Erosion:
On bare and disturbed soil bryophytes are primary pioneers and they have the ability to stabilise soils. The soils in semi-arid regions are held in place by crusts predominantly composed of bryophytes, thus preventing the soil from blowing away. They also prevent soil erosion by slowing down and delaying run-off during rain.
When clay-rich soil has been laid bare due to landslides or road making, the first colonisation and subsequent stabilisation are substantially by bryophytes. The soil surface rapidly becomes bound together by rhizoid production followed by rapid branching of prostrate stems, thus preventing further soil erosion.
7. Bryophytes as Pollution Indicator:
The investigations with bryophytes in relation to different pollutants prove their potential as bioindicators of pollution. Due to their habitat diversity, structural simplicity, totipotency and rapid rate of multiplication bryophytes appear to be ideal organisms for pollution studies both under field and laboratory conditions.
Phyto- sociological and eco-physiological studies indicate that the decline and absence of mosses — especially epiphytic ones — in urban-industrial areas is a phenomenon primarily induced by air pollution caused by different gaseous and particulate pollutants. These plants can be reliable indicators and also monitor the air pollution.
Some bryophytes are very sensitive to pollution and show visible symptoms of injury even in the presence of minute quantities of pollutants. Such plants serve as good bio-indicators of the nature and degree of pollution. Some bryophytes have the capacity to absorb and retain pollutants in quantities much higher than those absorbed by other plant groups present in the same habitat.
Their efficient absorbing capacity is due to the absence of cuticle, presence of single cell thick lamina and larger surface area as compared to the volume. These plants, therefore, act as effective sink of pollutants and prevent their recycling for a considerable period of time.
Affinities of Bryophytes:
Bryophytes are a group of simple land plants, well-adapted to moist habitats. They perhaps evolved from green algal ancestors, closely related to the Charophytes.
The Bryophytes have traditionally been viewed as a distinct lineage from other land plants, though they show affinities with other groups of plants.
A. Affinities with Algae:
1. Gametophytic phase is dominant in the life cycle.
3. Chloroplasts contain similar pigments viz. chlorophyll a, b; α- and β-carotene.
4. Cell wall mainly composed of cellulose.
5. Store starch as reserve food.
6. Sperms motile and flagellate containing whiplash flagella.
7. Water requires for sperm dispersal and subsequent fertilization.
8. Absence of vascular tissue.
B. Affinities with Pteridophytes:
1. Presence of heteromorphic life cycle.
2. Gametophytes are parenchymatous, not filamentous.
3. Presence of multicellular sex organs, male called antheridium and female archegonium. Hence, the gametes are enclosed by a sterile jacket of cells.
4. Normally sperms are motile and flagellated.
5. Water is necessary for sperm dispersal and subsequent fertilisation.
6. They retain the zygote within the female sex organ and allow it to develop into an embryo there.
7. The cutin (a cuticle) is present on the plant and spores.
Life Cycle of Bryophytes:
The morphologically distinct gametophytic and sporophytic phases are present in the life-cycle of bryophytes (Fig. 6.62). Hence the life-cycle of bryophytes shows a heteromorphic alternation of generations where two phases follow one another in a regular alternate succession. The gametophytic phase is independent, autotrophic, haploid and bears gametes.
This phase initiates from the haploid spores (n) produced by the sporophyte and ultimately ends with the production of gametes within the multicellular sex organs. The sporophyte of bryophytes is called sporogonium as it contains a single sporangium.
The sporophyte is completely dependent on gametophyte. The sporophytic phase begins with the diploid (2n) zygote produced from the syngamy (fertilisation), and the spore mother cell represents the last stage of sporophytic generation (Fig. 6.62).
There are two theories in connection with the alternation of generations and the origin of the sporophytes:
(a) antithetic or interpolation or intercalation theory, and
(b) homologous or transformation theory.
(a) Antithetic or Interpolation or Intercalation Theory:
This theory was first proposed by Celakovsky in 1874 and subsequently developed by Bower (1890), Strasburger (1894), Cavers (1910), Chamberlain (1935) and Campbell (1940). According to the antithetic theory, the gametophytic generation is the original generation, while the sporophytic generation is a new phase which has been gradually evolved afterwards by the progressive elaboration of the transient sporophyte (zygote) of the green algal ancestor.
Thus, a structurally different sporophyte is intercalated or interpolated into the lite-cycle between the syngamy (sexual fusion) and meiosis in course of evolution.
The stages of progressive evolution of sporophytes from the zygote of green algae have been enumerated:
In the earliest hypothetical land plants, the sporophytic stage in the life cycle would be represented by a unicellular diploid zygote as in filamentous green algae which passes through a resting period and subsequently develops four haploid zoospores by meiotic division.
In this form, the sporophyte (zygote) would be very much similar to Coleochaete which increases greatly in size and develops four haploid cells by reduction division. These haploid cells later develop 16-32 zoospores.
In this stage, zygote first divided by mitotic division instead of meiotic division to form a diploid multicellular body retaining within the archegonium. Thus, a structurally different embryo was initiated. The ultimate cells of the multicellular sporophyte became spore mother cells, each of which then divided by meiotic division to form four haploid spores.
Phase four (final phase):
The zygote divided to form a diploid multicellular, spherical structure (the embryonic phase) as in the third phase. The outer layer of the spherical sporophyte became sterile to form a jacket, while the inner cell mass remained sporogenous.
Each cell of the inner cell mass then divided by reduction division to form four haploid spores. Thus, a simple, globose sporophyte containing a large number of spores with a single-layered sterile jacket is actually observed in Riccia.
Hence, the simple and dormant unicellular sporophyte (zygote) by progressive elaboration has gradually been modified and later, a complex multicellular sporophyte evolved which at the same time, was different in structure from the gametophyte.
(b) Homologous or Transformation Theory:
The homologous theory was put forward by Pringsheim in 1876 and then supported by Scott (1896), Church (1919), Zimmermann (1930), Evans (1939), Fritsch (1945) and Bold (1948). According to this theory, the gametophytic and sporophytic generations are fundamentally similar in nature. The sporophyte is not a new phase, but a direct modification of the gametophyte.
Thus, during the migration to land, the two generations evolved differently and one became the independent gametophyte, while the other specialised as a sporophyte that became a parasite on the gametophyte.
The various evidences in support of homologous theory has been summarised:
(a) Isomorphic alternation of generations:
A large number of algae (viz., Cladophorales, Ulvaceae, Ectocarpales, Dictyotales, etc.) shows two alternating isomorphic forms, one a haploid gametophyte, reproducing sexually by gametes and the other a diploid sporophyte, reproducing asexually by asexual spores.
(b) Nutrition of sporophyte:
The presence of photosynthetic tissue in the sporophytes of liverworts, hornworts and mosses suggests the fundamental similarity between the gametophytic and sporophytic phases with respect to self- nutrition.
(c) Apogamy and apospory:
Apogamy is a phenomenon where a haploid sporophyte is produced from a gametophyte directly, without syngamy or sexual fusion. While in apospory a diploid gametophyte is produced from a sporophyte directly, without the formation of spores. These two phenomena have been observed in a large number of pteridophytes and bryophytes, which support the homologous theory.
(d) Similarity between primitive gametophytes and sporophytes of pteridophytes:
The gametophytes of primitive pteridophytes like Psilotum, Tmesipteris, Ophioglossum are cylindrical, dichotomously branched with continued apical growth. These gametophytes, showing structural similarity with the sporophytes of primitive pteridophytes like Psilotum and members of Rhyniopsida, strongly support the homologous theory.