Here is a compilation of essays on ‘Bryophytes’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Bryophytes’ especially written for school and college students.
Essay on Bryophytes
Essay Contents:
- Essay on the Definition of Bryophytes
- Essay on the Distribution of Bryophytes
- Essay on the Classification of Bryophytes
- Essay on the Plant Body of Bryophytes
- Essay on the Life Cycle of Bryophytes
- Essay on the Regeneration of Bryophytes
- Essay on the Relationship between Bryophytes and Pteridophytes
Essay # 1. Definition of Bryophytes:
Bryophyta, a division of plant kingdom, includes mosses and allied plants; commonly described as liverworts. Mosses are very small plants, at times microscopic, differentiated into stems and leaves. Liverworts are macroscopic plants, which are thalloid as well as leafy. A thallus is flattened (dorsiventral), dichotomously branched ribbon-like structure of variable size and shape. Bryophytes lack root system. Instead, there are rhizoids.
Bryophytes are essentially small plants; from a few millimeters to a few centimeters height. Their dependence on water for fertilization imposes restriction on their size—ciliate antherozoids have to swim in water drops to effect fertilization. For this, bryophytes are also described as ‘amphibians’ of plant kingdom.
Bryophytes are basically land plants; capable of growing on moist soil in small patches, often forming vivid greens on forest floor. At times, there are extensive mats described as peat. In dense forests bryophytes also grow on tree trunks, their branches, and even on leaves.
Lower plants (algae, fungi, bryophytes and pteridophytes) were earlier grouped together as cryptogams (hidden-wedded). This was mainly due to lack of knowledge about their sexuality. Higher plants were grouped as phanerogams (open-wedded). However, now with the knowledge of sexuality in lower plants the term cryptogam should be only of historical importance.
Algae and fungi are microscopic plants and hence, at times referred to as lower cryptogams. Contrary to it, bryophytes and pteridophytes are macroscopic and hence, referred to as higher cryptogams. Bryophytes, pteridophytes and gymnosperms share a common sex organ, archegonium, and hence are referred to as archegoniatae.
Essay # 2. Distribution of Bryophytes:
Liverworts (hepatics) as well as mosses (musci) are distributed worldwide and are able to grow luxuriantly in humid climate of temperate and tropical regions. Liverworts are richly represented in humid tropics and become rare in arctic environment.
Mosses can, however, survive in extreme conditions of arctic and alpine regions and can tolerate submergence in watery to semi-arid condition. Some of the mosses recover after years of dehydration and others can withstand extended periods of freezing.
The word moss, at times, is applied to plants that have no relevance to bryophytes. Reindeer moss is a lichen (Cladonia rangifera), club moss is a pteridophyte (Lycopodium), spanish moss is a flowering plant (Tillandsia usneoides) and Irish moss is an alga (Chondrus crispus).
The word ‘liverwort’ is a misnomer. Its origin can be traced to an ancient belief that the Creator shaped plants to indicate their curative significance. Some of the liverworts, particularly thalloid ones, resemble the human liver and hence the name liverwort. However, there is no evidence that these liverworts are of medicinal value. Moreover, many of liverworts are leafy.
Essay # 3. Classification of Bryophytes:
To begin with, two groups recognized in bryophytes were Hepaticeae (liverworts) and Musci (mosses). However, in recognition of anomalous characters of hornworts, liverworts with horn-like sporophytes, the current trend is to have three groups; Hepaticeae (liverworts), Anthocerotae (hornworts) and Musci (mosses).
As per recommendations of ICBN (International Code of Botanical Nomenclatures) these class names have been changed to Hepaticopsida (Hepaticeae; Liverworts), Anthocerotopsida (Anthocerotae, Hornworts) and Bryopsida (Musci, Mosses).
(a) Hepaticopsida (Liverworts):
Liverworts are a large group of about 330 genera and 8000 species. Earlier, two subclasses were recognized in this group-Jungermanniae and Marchantiae. These are based mainly on structure of the gametophyte. The sporophytes are essentially similar.
(b) Anthocerotopsida (Hornworts):
The hornworts are a small group – of about 7 genera and a single order Anthocerotales.
(c) Bryopsida (Mosses):
Mosses are the largest group, with over 700 genera and some 14000 species.
The classification of mosses is based on characters of gametophyte and sporophyte. The emphasis is, however, on characters of sporophyte; especially the peristome. In the past, mosses have been classified in two groups- acrocarps and pleurocarps. The former having sex organs and sporophytes on the main stem or a branch and the latter with archegonia and sporophytes on lateral branches.
Earlier taxonomists, on the basis of structure and features of a moss sporophyte, recognized three distinct evolutionary lines which were:
(a) Bryales:
The class Musci of Bryophyta includes mosses. Of the different evolutionary lines included in it, the most prominent is Bryales and hence the name of this subdivision Bryopsida.
A majority of the mosses (about 90%) belong to the order Bryales. In this account Funaria is taken as a representative.
(b) Andreaeales
(c) Sphagnales:
The Sphagnobrya—the bog moss—with a single genus Sphagnum, has characters common with Hepaticeae, Anthocerotaceae and Musci.
It differs from other mosses in having:
(a) Thallose protonema, that produces one gametophyte.
(b) Leaves without midrib, and made up of two types of cells.
(c) Axillary antheridia, that are with a distinctive differentiation of fertile portion.
(d) Archegonia, that are acrogynous.
(e) Sporogenous tissue of sporophyte, that is derived from amphithecium.
(f) The sporophyte, that has a pseudopodium.
In mosses, Bryales is the most extensive order comprising about 30 families.
Essay # 4. Plant Body of Bryophytes:
The bryophyte plant body is a gametophyte. It is an independent and autotrophic plant. The gametophyte is either a thallus (dorsiventral), freely branched ribbon-like structure (thalloid liverwort) or a leafy structure differentiated into the stem and leaves, but without root (leafy liverwort). This is also true of a moss.
Arising on a gametophyte, and dependent on it for nutrition and physical support, is the sporophyte. It is differentiated into foot, seta and capsule. The foot is positioned within the gametophyte and is meant for anchorage and absorption. Seta is an elongate, stalk like structure meant to project the capsule; the spore-bearing structure. The capsule is also described as sporogonium.
Essay # 5. Life Cycle of Bryophytes:
An alternation of generations; between gametophyte (gamete-producing generation) and sporophyte (spore-producing generation) completes the life cycle of a bryophyte (Fig. 1.1). The gametophyte is haploid and represents the sexual generation; it bears sex organs — antheridia and archegonia — and produces gametes; antherozoids and egg.
Fertilization of an egg with an antherozoid (syngamy) results in the formation of an embryo — the diploid or sporophyte generation. The sporophyte undergoes meiosis to form haploid spores. One of the features of bryophyte life cycle is heteromorphic alternation of generations. The two generations — gametophyte and sporophyte — are not only different in function but also different in their morphology.
A characteristic of the group is the independent and autotrophic gametophyte generation to which is always attached the sporophyte generation. The sporophyte is always dependent on the gametophyte.
The spore is the first or germ-cell of the gametophyte generation. It germinates to produce a filamentous protonema and rhizoid; the sporeling. The protonema is an ephemeral structure in thalloid liver worts, it is relatively long-lived in leafy liverworts and is an extensive filamentous and branched structure in mosses.
At times it is ovoid or plate-like in shape. On the protonema differentiate the gametophytes which ultimately bear sex organs. The sex organs, the antheridia (male reproductive structures) and archegonia (female reproductive structures) are borne at different positions on the gametophyte.
These sex organs are not exposed. Hence, these plants are described as cryptogams (Gr. kruptos = hidden, gamos = wedded). Both sex organs are characterized by a jacket of sterile cells, forming a distinct covering. An antheridium encloses numerous spermatocytes which differentiate into biciliate spermatozoids, whereas in an archegonium is an axial row of cells — the neck canal cells, the venter canal cell and an egg.
Fertilization is possible in the presence of water. In a mature archegonium the neck canal cells disorganize, their cell-contents exude from an opening at the apex in the form of proteinaceous mass and get mixed up with the water drop that bathes the archegonia. Similarly, from a mature antheridium the sperm-cells which are released, gradually get mixed up with the surface-film of water that bathes the antheridia, and metamorphose into biflagellate sperms.
When sperms encounter the diffused fluid contents of an archegonium they swim towards the highest concentration of the fluid, down to the archegonial neck and finally to the egg. Many sperms reach an egg but only one is effective in fertilization – union of sperm nucleus and egg nucleus—to form a zygote.
The zygote is the first- or germ-cell of sporophyte generation. It divides to form a multicellular embryo. The embryo is retained within archegonium, the basal cells of which enlarge and divide to form a protective cover; the calyptra.
The embryo forms spore-producing structure, the capsule. It may be a simple spherical structure embedded in gametophyte or differentiated into a capsule that projects out of gametophyte by means of a stalk-like structure, the seta.
In either case the sporophyte derives its nutrition from the gametophyte by means of specialized haustorial cells of foot. The capsule encloses archesporium, the fertile tissue. Each of the cells of this tissue differentiates to form either a spore-mother-cell or an elater-mother-cell. Each spore-mother-cell undergoes reduction division to form four haploid spores. All spores are alike; bryophytes are homosporous.
A mother-cell gives rise to non- fertile cells – elaters; which are hygroscopic and help in the dispersal of spores. Elaters are particularly present in liverworts and are absent in mosses. For effective dispersal of spores, moss capsule has a specialized structure, the peristome. Elaters and peristome are unique structures of liverworts and mosses, respectively.
Essay # 6. Regeneration of Bryophytes:
Bryophytes are endowed with a remarkable regeneration capacity. The gametophytes are brittle, especially when dry, and isolated fragments readily regenerate to form entire plants. For effective vegetative reproduction, specialized structures; (gemmae) are produced. A gemma is a cluster of a few to many cells, which give rise to a new gametophyte on separation from the parent plant.
Cells of sporophyte are also capable of regeneration but are relatively refractory. The sporophytic cells regenerate to form a protonema on which appear gametophytes. The regeneration of a gametophyte from a sporophyte without the formation of spores is apospory.
Conversely, a gametophyte may form a mass of cells from which may regenerate a sporophyte, apogamy. Regeneration of a sporophyte from a gametophyte, without the formation of gametes is apogamy. Apospory and apogamy are rare events in regeneration of bryophytes.
Essay # 7. Relationship between Bryophytes and Pteridophytes:
In an evolutionary consideration, bryophytes occupy an intermediate position between algae and vascular cryptogams – the pteridophytes.
Features in which bryophytes and pteridophytes are similar (but differ from algae) are:
(a) Multicellular sex organs, with an outer layer of sterile cells,
(b) Permanent retention of zygote within the archegonium,
(c) Retention of embryonic sporophyte within the archegonium, and
(d) Dependence of sporophyte on the gametophyte.
Features in which bryophytes differ from pteridophytes are:
(a) The sporophyte of a bryophyte is not an independent plant,
(b) Sporophyte is not capable of unlimited growth,
(c) Conducting system in bryophytes is not differentiated into distinct xylem and phloem.