In this essay we will learn about Bryophytes. After reading this essay you will learn about:- 1. Meaning of Bryophytes 2. Distribution of Bryophytes 3. Habitat of Bryophytes 4. General Characters 5. Adaptations 6. Economic Importance.
Essay # Meaning of Bryophytes:
Bryophyta (Gr. Bryon = mass; phyton = plant), a division of kingdom Plantae comprises of mosses, Hornworts and Liverworts. They are groups of green plants which occupy a position between the thallophytes (Algae) and the vascular cryptogams (Pteridophytes).
Bryophytes produce embryos but lack seeds and vascular tissues. They are the most simple and primitive group of Embryophyta. They are said to be the first land plants or non-vascular land plants (Atracheata). Presence of swimming antherozoids is an evidence of their aquatic ancestory.
Essay # Distribution of Bryophytes:
Bryophytes are represented by 960 genera and 24,000 species. They are cosmopolitan in distribution and are found growing both in the temperate and tropical regions of the world at an altitude of 4000-8000 feet.
In India, Bryophytes are quite abundant in both Nilgiri hills and Himalayas; Kullu, Manali, Shimla, Darjeeling, Dalhousie and Garhwal are some of the hilly regions which also have a luxuriant growth of Bryophytes. Eastern Himalayas have the richest in bryophytic flora. A few species of Riccia, Marchantia and Funaria occur in the plains of U.P., M.P. Rajasthan, Gujarat and South India.
In hills they grow during the summer or rainy season. Winter is the rest period. In the plains the rest period is summer, whereas active growth takes place during the winter and the rainy season. Some Bryophytes have also been recorded from different geological eras e.g., Muscites yallourensis (Coenozoic era), Intia vermicularies, Marchantia spp. (Palaeozoic era) etc.
Essay # Habitat of Bryophytes:
Bryophytes grow densely in moist and shady places and form thick carpets or mats on damp soils, rocks, bark of trees especially during rainy season.
Majority of the species are terrestrial but a few species grow in fresh water (aquatic) e.g., Riccia fluitans, Ricciocarpos natans, Riella etc. Bryophytes are not found in sea but some mosses are found growing in the crevices of rocks and are being regularly bathed by sea water e.g., Grimmia maritima.
Some Bryophytes also grow in diverse habitats e.g., Sphagnum-grows in bogs, Dendroceros-epiphytic, Radula protensa. Crossomitrium -epiphyllous, Polytrichum juniperinum-xerophytic, Tortula muralis-on old walls. Tortula desertorum in deserts, Porella platyphylla-on dry rocks, Buxbaumia aphylla (moss), Cryptothallus mirabilis (liverwort) are saprophytic.
Essay # General Characters of Bryophytes:
1. Plant body is gametophytic, independent, dominant, autotrophic, either thalloid (i.e., thallus like, not differentiated into root, stem and leaves) or folise (Fig 1), containing a rootless leafy shoot.
2. Plant body is very small and ranges from a few mm. to many cm. Zoopsis is the smallest bryophyte (5 mm.) while the tallest bryophyte is Dawsonia (50-70 cms.).
3. Leaves and stems found in vascular plants are absent, Koch (1956) termed these ‘leaf’ and ‘stem’ like structures as ‘axis’ and ‘phylloid’ respectively.
4. Roots are absent. Functions of the roots are performed by rhizoids. Cells are also capable to absorb moisture directly from the ground or atmosphere. Therefore, Bryophytes can also survive on the moist soils.
5. Rhizoids may be unicellular, un-branched (e.g., Riccia, Marchantia, Anthoceros) or multicellular and branched (e.g., Sphagnum, Funaria).
6. In members of order Marchantiales (e.g., Riccia, Marchantia) scales are present. These are violet coloured, multicellular and single cell thick. They protect the growing point and help to retain the moisture.
7. Vascular tissue (xylem and phloem) is completely absent. Water and food material is transferred from cell to cell. However, in some Bryophytes (e.g., mosses) a few cells in groups of 2-3 are present for conduction of water and food (photo assimilate). These cells are known as hydroid (collectively hydrom) and leptoids respectively. Cuticle and stomata are absent.
Adaptations of Bryophytes to Land Habit:
Bryophytes are first land plants. Evidences support that Bryophytes are evolved from Algae. During the process of origin they developed to certain adaptations to land habit.
1. Development of compact plant body covered with epidermis.
2. Development of organs for attachment and absorption of water e.g., rhizoids.
3. Absorption of carbon dioxide from atmosphere for photosynthesis e.g., airpores.
4. Protection of reproductive cells from drying and mechanical injury i.e., jacketed sex organs.
5. Retention of zygote within the archegonium.
6. Production of large number of thick walled spores.
7. Dissemination of spores by wind.
Bryophytes: Amphibians of Plant Kingdom:
Bryophytes are also known as amphibians of plant kingdom because water is needed to complete the life cycle. In animal kingdom class Amphibia (Gr. Amphi = two or both; bios = life) includes those vertebrates which are amphibians in nature i.e., they can live on land as well as in water. Similarly, majority of the bryophytes are terrestrial but they are incompletely adapted to the land conditions.
They are unable to grow during dry season and require sufficient amount of water; for their vegetative growth. Water is absolutely essential for the maturity of sex organs ar. fertilization. Without water they are unable to complete their life cycle. On account of their complex dependence on external water for completing their life cycle, Bryophytes along with Pteridophyte are regarded as amphibians of plant kingdom.
Alternation of Generation in Bryophytes:
Bryophytes show a distinct and sharply defined heteromorphic alternation of generation. In the life cycle of these plants, there exist two distinct phases. One is haploid (X) or gametophytic phase (produces gametes). It is the dominant and independent phase of the life cycle. It produces the male and female sex organs i.e., antheridia and archegonia respectively.
Haploid gametes i.e. antherozoids and eggs are produced inside the sex organs. Antherozoids are produced in antheridia and eggs are produced in archegonia. The gametes fuse to form a diploid (2x) zygote. The zygote is the starting point of the next phase of the life cycle.
On germination the zygote forms the second diploid adult of the life cycle called sporophyte or sporogonium. Sporogonium produces spore mother cells in the capsule region, which undergo meiosis and form the haploid spores called meiospores. The zygote, embryo, sporogonium and spore mother cells together constitute the sporophytic generation.
This generation is dependent completely or partially on the gametophytic generation for its nutrition. Each meiospore germinates and produces a gametophytic plant which again bears the sex organs. In this way the life cycle goes on. Because the two generations (gametophytic and sporophytic) appear alternately in the life cycles, Bryophytes show alternation of generation.
Since the generations differ completely in their morphology i.e., gametophyte is either thalloid or foliose, and the sporophyte usually consists of foot, seta and capsule, it is called heteromorphic alternation of generation.
Apogamy and Apospory:
Bryophytes are endowed with a remarkable regeneration capacity. Parts of the plant or any living cell of the thallus are capable of regenerating the entire plant. The sporophytic cells regenerate to form a protonema on which appear gametophytes. This regeneration of diploid gametophyte from a sporophyte without the formation of spores is called apospory.
Conversely a gametophyte may form a mass of cells which may regenerate a sporophyte. This regeneration of a diploid sporophyte from a gametophyte, without the formation of gametes is called apogamy. Aposory and apogamy are rarely found in life cycle of Bryophytes.
Rhizoids and Scales in Bryophytes:
In Bryophytes roots are absent and the functions of the root i.e., anchorage and absorption is performed by the filamentous structures known as rhizoids.
Rhizoids may be unicellular, un-branched (Fig. 3B-D) in thallose forms of Hepaticopsida and Anthocerotopsida (e.g., Riccia, Marchantia, Anthoceros) or multicellular and branched in foliose forms of Bryopsida (Fig. 3 E) (e.g., Funaria, Polytrichum) Multicellular rhizoids possess oblique cross walls.
Unicellular rhizoids are of two types smooth-walled and tuberculated (Fig. 3 B-D). The members of order Marchantiales (e.g., Riccia, Marchantia) possess both types of rhizoids while Anthocerotales (e.g., Anthoceros) possess only smooth walled rhizoids.
In thalloid forms rhizoids are borne on the ventral surface (Fig. 3 A) along the mid rib, however, in foliose forms rhizoids arise from the base of the ‘stem’. In aquatic Bryophytes (e.g., Riccia fluitans, Ricciocarpus natans) rhizoids are absent.
Scales are present only in the members of order Marchantiales and absent in all Bryophytes. The scales are multicellular, violet coloured and single cell thick. They are violet in colour due to the presence of the pigment anthocyanin. Scales develop on the ventral surface of the thallus (Fig 3A).
They may be arranged in one row (e.g., young thallus of Riccia) or in two rows on each side of the mid rib (e.g., Targionia) or in two to four rows on each side of the mid rib (e.g., Marchantia) or irregularly distributed over the entire ventral surface (e.g., Corsinia).
In Riccia the scales are ligulate (Fig. 3G) while in Marchantia the scales are of two types-ligulate and appendiculate (divided by a narrow constriction into two parts—body and appendage, Fig. 3F). Scales protect the growing point by covering their delicate cells and secreting slime to keep them moist. The scales are absent in some aquatic members of order Marchantiales e.g., Riccia fluitans.
The archesporium is the first cell generation of the sporogenous tissue. It divides and re-divides to form a mass of cells. It is a solid tissue and also called sporogenous tissue. The cells of the last cell generation of solid tissue separate from each other and are known as spore mother cells. The origin, position and fate of archesporium varies in different members of Bryophytes.
It is as follows:
Economic Importance of Bryophytes:
The Bryophytes are not considered to be of much economic value because except a few Bryophytes none of these are of direct use to man. However, they play an important role in the economy of the nature.
We can study their economic importance under the following heads:
1. Ecological Importance.
Bryophytes are of great ecological importance due to following reasons:
(a) Pioneer of the land plants. Bryophytes are pioneer of the land plants because they are the first plants to grow and colonize the barren rocks and lands.
(b) Soil erosion. Bryophytes prevent soil erosion. They usually grow densely and hence act as soil binders. Mosses grow in dense strands forming mat or carpet like structure.
They prevent soil erosion by:
(i) Bearing the impact of falling rain drops
(ii) Holding much of the falling water and reducing the amount of run-off water.
(c) Formation of soil. Mosses and lichens are slow but efficient soil formers. The acid secreted by the lichens and progressive death and decay of mosses help in the formation of soil.
(d) Bog succession. Peat mosses change the banks of lakes or shallow bodies of water into solid soil which supports vegetation e.g., Sphagnum.
(e) Rock builders. Some mosses in association with some green algae (e.g., Chara) grow in water of streams and lakes which contain large amount of calcium bicarbonate. These mosses bring about decomposition of bi-carbonic ions by abstracting free carbon dioxide. The insoluble calcium carbonate precipitates and on exposure hardens, forming calcareous (lime) rock like deposits.
2. Formation of Peat:
Peat is a brown or dark colour substance formed by the gradual compression and carbonization of the partially decomposed pieces of dead vegetative matter in the bogs. Sphagnum is an aquatic moss. While growing in water it secretes certain acids in the water body.
This acid makes conditions unfavorable for the growth of decomposing organisms like bacteria and fungi. Absence of oxygen and decomposing microorganisms slows down the decaying process of dead material and a large amount of dead material is added year by year. It is called peat (that is why Sphagnum is called peat moss).
Various Uses of Peat are:
(a) Used as fuel in Ireland, Scotland and Northern Europe.
(b) In production of various products like ethyl alcohol, ammonium sulphate, peat, tar, ammonia, paraffin, dye, tannin materials etc.
(c) In horticulture to improve the soil texture.
(d) In surgical dressings.
3. As Packing Material:
Dried mosses and Bryophytes have great ability to hold water. Due to this ability the Bryophytes are used as packing material for shipment of cut flowers, vegetables, perishable fruits, bulbs, tubers etc.
4. As Bedding Stock:
Because of great ability of holding and absorbing water, in nurseries beds are covered with thalli of Bryophytes.
5. In Medicines:
Some Bryophytes are used medicinally in various diseases for e.g.,
(a) Pulmonary tuberculosis and affliction of liver—Marchantia spp.
(c) Acute hemorrhage and diseases of eye—Decoction of Sphagnum.
(d) Stone of kidney and gall bladder—Polytrichum commune.
(e) Antiseptic properties and healing of wounds—Sphagnum leaves and extracts of some Bryophytes for e.g., Conocephalum conicum, Dumortiera, Sphagnum protoricense, S. strictum show antiseptic properties.
6. In Experimental Botany:
The liverworts and mosses play an important role as research tools in various fields of Botany such as genetics. For the first time in a liverwort, Sphaerocarpos, the mechanism of sex determination in plants was discovered.
7. As Food:
Some Bryophytes e.g., mosses are used as food by chicks, birds and Alaskan reindeer etc.