[Biology] Bryophytes

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Introduction

  • Bryophytes belong to cryptograms which are nonvascular seedless plants.
  • They have approximately 20,000 known species and are widely distributed throughout the world.
  • Bryophytes are comparatively smaller than most seed-bearing plants.

Salient Features

  • Bryophytes inhibit humid and shaded places.
  • They can also be found in arid forests as well as in some rainforests.
  • They contain chlorophyll and are capable of synthesizing their own food through the process of photosynthesis.
  • Bryophytes do not have true roots, stems, and leaves, instead they possess analogous structures.
  • They are anchored to their substrate through rhizoids. Rhizoids are protuberances that extend from the lower end of bryophytes and are analogous to roots in terms of structure and function.
  • The rhizoid possesses long dead cells which are capable of water absorbency through apoplastic transport; these cells are called rhizoid cells.
  • The main body of the bryophytes is thallus like (erect) and is haploid.
  • The thallus is sometimes one cell layer thick through most of its width but in some species it may be many cell layers thick and comprise of a complex tissue organization,
  • Thallus branching can be forked, frequently frond like, digitate, or completely irregular.
  • They lack vascular tissues; however, they do exhibit specialized structures for water transportation.
  • These plants show a heteromorphic alternation of generations, between a green vegetative gametophyte and a sporophyte

GAMETOPHYTE

  • Gametophyte is the dominant part of plant body and is haploid.
  • The thalloid (resembling thallus) gametophyte is differentiated into rhizoids, leaves and axis.
  • The gametophyte is photosynthetic and bears multi-cellular sex organs namely;
    • Antheridium
    • archegonium
  • The antheridium produces male gametes-antherozoids, which consists of a pair of flagella (biflagellate).
  • The archegonium is flask shaped and produces female gamete-egg
  • The antherozoids from antheridium fuse with the egg from archegonium, giving rise to a diploid zygote.
  • The zygote undergoes development and give rise to multicellular sporophyte

SPOROPHYTE

  • The sporophyte is structurally divided into foot, seta and capsule
  • The sporophyte dependent on the gametophyte for its nutrition i.e. semi-parasitic.
  • sporophyte cells undergo meiosis to give rise to haploid gametes which eventually results in a gametophyte
  • The juvenile gametophyte is termed as protonema

CLASSIFICATION OF BRYOPHYTES

Bryophytes are mainly classified into three major groups:

  • Hepaticopsida (liverworts)
  • Bryopsida (mosses)
  • Anthocerotopsida (hornworts)

HEPATICOPSIDA (LIVERWORTS)

  • Liverworts are tiny, green, terrestrial plants that inhabit moist and damp environments and tend to be less resistant to desiccation than their relatives, the mosses.
  • Gametophyte of liverworts is either thalloid or foliose.
  • In foliose type, leaves lack a midrib and are dorsiventral
  • thalloid is a lobed structure, which is dorsiventral and display dichotomous branching
  • the plant body is anchored via rhizoids. Rhizoids are aseptate and branched.
  • Sex organs are dorsally located on the gametophyte.
  • Sporophyte has two variants in terms of structure; it may either consist of a capsule only (Ricca) or distinguished into root, seta and capsule (marchantia)
  • The capsule of sporophyte plant lacks collumela
  • Liverworts can reproduce both sexually and asexually.
  • Asexual reproduction occurs either by fragmentation or by the production of specialized structures known as gemmae.
  • Sexual reproduction occurs by the fusion of male and female gametes and zygote formation.
  • Male (antheridium) and female (archegonium) sex organs are present either on the same thallus or different thalli.
  • Development of zygote leads to the formation of diploid sporophyte.
  • The sporophyte bears miotically produced, haploid spores.
  • These spores germinate to produce haploid, free-living gametophytes.

BRYOPSIDA (MOSSES)

  • Mosses are green, clumpy plants that inhibit moist environments.
  • Two different developmental stages of the gametophyte exist; protonema and gametophore
  • Gametophyte body is differentiated into an axis (stem) and lateral appendages (leaves) that lack a midrib.
  • The Rhizoids in mosses are multicellular, filamentous, branched with oblique septa
  • The capsule of sporangium possesses columella but lacks elaters.
  • Their sex organs are produced in separate branches and immersed in a group of leaves.
  • Mosses are able to absorb a considerable amount of water and have historically been used for insulation, water absorption, and a source of peat.
  • Mosses can reproduce both by sexual and asexual means
  • Asexual reproduction occurs through the process of fragmentation and budding
  • Sexual reproduction occurs by the fusion of male and female gametes. The gametes are produced by haploid gametophyte. The gametes fuse to form zygote, which then gives rise to diploid sporophyte.

ANTHOCEROTOPHYTA (HORNWORTS)

  • Hornworts are named because of the characteristic long horn-like sporophyte.
  • The Gametophyte plant body however, is simple (without internal differentiation), thalloid and flat.
  • Scales are absent in the thallus.
  • Each cell of the thallus comprises of a single large chloroplast with a pyrenoid.
  • Sporophyte body is cylindrical and partly dependent upon gametophyte for its nourishment.
  • Due to the presence of the meristematic zone, their sporophyte shows indeterminate growth i.e., it continues to grow indefinitely.
  • Sporophyte body is differentiated into bulbous foot and cylindrical capsule.
  • Endothecium forms the sterile central columella in the in the capsule.
  • They can reproduce both sexually and asexually.
  • Asexual reproduction in hornworts takes place by;
    • fragmentation of thallus
    • Tubers
  • Sexual reproduction occurs by the fusion of male and female gametes and zygote formation.

REPRODUCTION IN BRYOPHYTES

Bryophytes may reproduce both sexually and asexually.

ASEXUAL REPRODUCTION

  • Bryophytes can also reproduce asexually in enormous ways i.e. fragmentation, budding, gemmae formation etc.
  • The most common asexual mode of reproduction in bryophytes is fragmentation.
  • In this process, a segment breaks off from a gametophyte and then lands in a suitable habitat. There, it grows into a new gametophyte
  • The fragmentation may be accidental, such as animal trampling or caused by the erosion of an existing bryophyte colony.
  • However, many bryophytes have weakness zones which cause the breakage of parts of the gametophyte, such as whole branches, branch tips or even just parts of leaves.
  • Fragmentation is much more frequent in the leafy bryophytes than in the thallus ones.

SEXUAL REPRODUCTION

  • Sexual reproduction occurs by the fusion of male and female gametes and zygote formation.
  • Specialized sex organs are present on the gametophyte thallus.
  • The male sex organ is antheridium and is responsible for the formation of antherozoids (male gamete). The antherozoid possess a pair of flagella for movement towards the female gamete.
  • the female sex organ is known archegonium. It is a flask-shaped organ which produces a single egg (female gamete).
  • When the antherozoids are released into the environment (usually water), they interact with the archegonium.
  • Fusion of gametes occurs and a zygote is produced. The zygote stays in the archegonium for some time. It then undergoes multiple mitotic cell divisions and produces an embryonic sporophyte.
  • This growing embryo gives rise to mature diploid sporophyte.
  • Sporophyte bears diploid spores which eventually give rise to the gametophyte generation (alternation of generation)

LIFE CYCLE OF BRYOPHYTES

  • The lifecycle of bryophytes comprises of alternation of generations between the haploid gametophyte and the diploid sporophyte.
  • During the gametophyte stage, haploid gametes are formed in the specialized sex organs: the antheridia (male) and archegonia (female).
  • The male gamete is a flagellated sperm, which swim through water or is transported by insect species towards the egg (female gamete).
  • The two haploid gametes fuse together and form a diploid zygote.
  • The zygote of bryophytes grows inside the archegonia and eventually becomes a diploid sporophyte.
  • Mature sporophytes remain attached to the gametophyte form and generate haploid spores via meiosis inside the sporangium.
  • These spores are dispersed under favorable environmental conditions and give rise to new gametophytes.
  • The sporophyte is dependent on the gametophyte for the sake of nutrients and the gametophore for water and minerals.

ECONOMIC IMPORTANCE

  • Bryophytes are known as pioneers of the land plants because they are the first plants to grow and colonize the barren rocks and lands.
  • They prevent soil erosion by bearing the effect of falling rain drops.
  • Because of their great capacity of holding and absorbing water, the beds in plant nurseries are covered with thalli of Bryophytes.
  • Some Bryophytes are used medicinally in various diseases e.g.,
    • Pulmonary tuberculosis and affliction of liver—Marchantia spp.
    • Acute hemorrhage and diseases of eye—Decoction of Sphagnum

REFERENCES

  • Arihant’s handbook of biology. Plant Kingdom. Page no: 24-42.
  • NCERT biology; textbook for class 11. Animal Kingdom. Page no: 46-62.
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