Here is a compilation of essays on ‘Pteridophytes’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Pteridophytes’ especially written for school and college students.
Essay on Pteridophytes
Essay Contents:
- Essay on the Introduction to Pteridophytes
- Essay on the Origin and Success of Land Plants
- Essay on the Classification of Pteridophytes
- Essay on the Relationship of Pteridophytes and Other Archegoniate Plants
- Essay on the Economic Importance of Pteridophytes
- Essay on the Life Cycle of Pteridophytes
Essay # 1. Introduction to Pteridophytes:
The land plants have complex internal organization and pteridophytes occupy a position intermediate between bryophytes and higher land plants (Fig. 6.1). The pteriodophytes resemble bryophytes, in having similar events and requirements of life cycle. The sexual generation (gametophyte) of bryophytes and pteridophytes is dependent on water for fertilization.
However, the asexual generation (sporophyte) of pteridophytes is increasingly able to cope with terrestrial environment. In pteridophytes, ‘for the first time’ one notes the differentiation of root. Bryophytes, the simplest of green land plants, lack root; instead there are rhizoids for absorption.
Pteridophytes resemble higher land plants (gymnosperms and angiosperms) in having complex internal organization (vascular elements) but differ from them in lacking seed habit. Some of the pteridophytes are heterosporous, they approach seed habit and pteridophytes of the past were seed-bearing plants.
Pteridophytes are unique in having free-living gametophytic as well as sporophyte generation.
Also, the following are noticeable on an evolutionary scale:
(a) Dominance to dependence of gametophyte generation,
(b) Dependence to dominance of sporophyte generation.
Essay # 2. Origin and Success of Land Plants:
Origin of land plants, one of the most significant events of this earth remains a mystery. This is mainly due to inadequacy of fossil record. Land flora was possible, when detrimental effects of UV light, a part of solar radiation, became ineffective due to the origin of ozone layer in the atmosphere.
Aquatic algae, because of their immersion in water, remained unaffected by UY light. With the passage of time and an increase in density of algae more oxygen became available. This increased the possibility of ozone layer. An advantage of sporophytic (asexual) generation of land plants lies in their diploid nature. It helps the sporophyte to guard itself against harmful and deleterious effect of radiation, i.e., mutation.
Success Story of Land Plants:
The salient steps in the success story of land plants, in general and of seed plants in particular, can be summarized in terms of the origin of:
1. Cell differentiation; photosynthetic and storage tissues,
2. Tissue differentiation, vascular system; for transport, conduction and support,
3. Cuticle; to prevent desiccation,
4. Stomata, for effective gaseous exchange,
5. Sporopollenin, in spore exine, to prevent microbial infection,
6. Independence of sporophyte (asexual); from the gametophyte (sexual) generation,
7. Root; anchoring and absorbing structure,
8. Continued growth of sporophyte,
9. Dominance of sporophyte generation,
10. Heterospory; for endosporic gametophytes, a secured place for success of sporophyte generation,
11. Seed, for effective multiplication.
Some of these advancements are seen in bryophytes and others in pteridophytes. Pteridophytes of the past were seed-bearing plants. Present-day pteridophytes are, however, short of seed-habit, but some of them are heterosporous. Heterospory is the unique evolutionary feature of this plant group.
Essay # 3. Classification of Pteridophytes:
A good number of botanists treat Pteridophyta as a single unit (a division of plant kingdom) with four major groups (subdivisions). According to the International Code of Botanical Nomenclature, as amended in 1950, the name of a division should end in a suffix phyta, and its subdivision should end in a suffix -opsida.
Accordingly, the four major groups of Pteridophyta are:
(i) Psilopsida (psilophytes):
Psilopsida is a group simple in organization. It is characterized by:
(a) Rootless sporophytes,
(b) Dichotomously branched rhizome and aerial axes,
(c) Aerial axes that are either naked (Gr. Psilos=bare) or have small spirally arranged appendages,
(d) A protostelic, vascular cylinder
(e) Thick-walled sporangia that either terminate branches or are borne laterally on them.
Psilopsida has been divided into two orders:
(i) Psilophytales (extinct fossil plants)
(ii) Psilotales (Living plants)
(ii) Lycopsida (lycopods, lycopsids):
Sporophytes differentiated into root,stem and leaves . Leaves are small (microphylls)with a single unbranched vein and leaf trace arises without causing a leaf gap in the vascular cylinder. Sporophylls with a single sporangium generally-form stobilii.
(iii) Sphenopsida (horsetails):
Equisetopsida is also referred to as Sphenospsida. It is after the abundant fossil form, Sphenophyllum of the group. Some authors prefer to call the group as arthropsida or articulate because of the jointed nature of their stems. Equisetum is the exclusive living representative of the group.
(iv) Pteropsida (ferns):
Sporophyte differentiated into root, stem and leaves.
Leaves are large, (megaphylls), compound, and are described as fronds. Stem is protostelic, siphonostelic or dictyostelic. Occasionally, it is polycyclic. Rarely, there is limited secondary growth. The siphonostelic cylinder differs from other pteridophytes (microphyllous) in having leaf gaps.
Sporangia are thick or thin-walled, homosporous or heterosporous, borne on unmodified foliage or on specialized fertile leaves. These are either terminal on leaf segments but more commonly are on the frond, marginally or superficially, and rarely in specialized structures, the sporocarps. The sporangia either are single or form sori or fuse to form synangia.
Antherozoids are multiflagellate.
Pteropsida is divided into following six orders:
(1) Cladoxylales
(2) Coenopteridales
(3) Protopteridales
(4) Marattiales
(5) Ophioglossales
(6) Filicales
The first three orders comprise only fossil forms and the last three, living as well as fossil forms.
Essay # 4. Relationship of Pteridophytes and Other Archegoniate Plants:
The presence of vascular elements in pteridophytes makes their grouping with gymnosperms and angiosperms as Trachaeophyta, and their reproduction by spores and similar events of life cycle place them among lower plants. Bryophytes, pteridophytes and gymnosperms are classified as Archegoniatae; due to the presence of a common reproductive organ, the archegonium.
The lower plants, algae, fungi, bryophytes and pteridophytes were earlier grouped together as cryptogams, since their mode of reproduction was unknown (Gr. kruptos=hidden, and gamos=wedded) and seed plants were classified as phanerogams (‘open wedded’). However, with the knowledge of sexuality in most lower plants the term cryptogam should now be of historical importance.
Essay # 5. Economic Importance of Pteridophytes:
Pteridophytes are most valued as horticultural plants. It is because of the beauty of their leaves. These are also valued as food plants, as delicacies, and food and feed substitutes during famines. Some of the pteridophytes are also dinitrogen fixers; these help to raise bumper food crop, serving as substitute for green manure.
Some of the pteridophytes are obnoxious weeds; difficult to eradicate. Pteridophytes are also valued for their insecticidal and medicinal properties.
(i) As Horticultural Plants:
A greenhouse is incomplete without a leafless land plant, that has been potted up and can be multiplied by its rhizome cuttings. The significance of Psilotum lies in not only being leafless but also a plant that is made up of green twigs of perfectly ordered dichotomies. It is nicknamed as ‘whisk-fern’. Anyone claiming for a complete greenhouse would like to have it at any cost – Interest in Psilotum has waxed and waned. There are Psilotum Societies of Amateur Botanists in Japan.
The lycopodiums are liked as ground pines; small pine-like plants with needle-like leaves. These are terrestrial plants which look like a moss but are quite big, i.e., club-mosses. Aesthetically more valued are pendulous epiphytes; Lycopodium spp. growing on tree trunks as hanging baskets in Botanical Garden, Ootacamund and in Bryant Park, Kodaikanal is a prized sight.
Selaginellas have been serving greenhouses as border plants. It is because of the beauty of their branches and foliage. An array of colour shades—dark to light green, bluish to iridescent is better to be seen than believed. The resurrection plant (S. lepidophylla) is sold as dried. It rejuvenates on coming in contact with water.
Aesthetic value of ferns Angiopteris and magnificience of Marattia remain unparallel in plant kingdom. These fern-like plants find their place in greenhouses because of their beautiful habit and elegance.
Fern leaves are known for their keeping quality and this makes them commercially important plants in preparing a bouquet and floral arrangement. For this a common source is Nephrolepis.
(ii) As Food and Feed Plants:
Young leaf tips of ferns, the croziers or fiddle-heads, have been a delicacy in the Indo- Malay region and adjacent Japan. It has been remarked that these simply taste like ‘soul of spring’. The croziers of ostrich fern Matteuccia struthiopters are regularly served as spring vegetable in Canada and United States.
For consumption, the croziers are either canned or frozen; freezing is described to improve the taste. Commercial plantations of Matteuccia and in vitro multiplication of this fern are fast coming up in the USA to cope with the increasing demand for croziers.
Marsilea in its quadrifid lamina resembles a clover and has in fact been used as a substitute for clover to feed the animals.
(iii) As Medicinal and Pesticidal Plants:
Since times immemorial Dryopteris filix-mas, the male-fern, has been valued as a medicinal plant, particularly for the treatment against tapeworm. It is also used for brewing of ale. The ferns are relatively immune to insect attack. This is possibly due to their chemical nature, which interferes with the normal moulting process of insects and makes them valuable as a source of pesticides.
(iv) Obnoxious Weeds:
Pteridium aquilinum, the cosmopolitan bracken, has been of devastating effect on human affairs. It can rapidly colonize the open forest land to the exclusion of other plants and is hard to destroy because of its perennial rhizome. In Scotland alone about 400,000 acres of potential range land is infested with bracken. The plant is toxic to cattle, producing bizarre symptoms and at times sudden death. Potent carcinogens have been isolated from this fern.
Salvinia, the water fern, with its free-floating fragile rhizome is able to secure quick vegetative propagation. It can occupy the entire water surface in lakes and irrigation reservoirs. It is a troublesome weed that can block boating and free flow of water in irrigation canals. Being an aquatic plant it has defied chemical eradication or biological control. At times, it has been described to occupy the entire river delta, blocking navigation.
(v) Dinitrogen Fixation and Agronomic Plants:
Azolla, a water fern is referred to as goddess of fertility and there are pagodas named after this fern in Buddhist parts of the world. A rice farmer is sure of a couple of good crops whenever Azolla in full bloom occupies the rice fields. This plant with fragile free-floating rhizome is able to secure quick vegetative propagation.
On a plant of pin-head size there are hundreds of microscopic leaves each harbouring live colonies of dinitrogen fixer, cyanobiont, Anabaena. Thus on the basis of surface-to-volume ratio, it is the most effective symbiotic relationship between a nitrogen fixer and a fast-growing plant, which has served as a green manure since time immemorial.
Essay # 6. Life Cycle of Pteridophytes:
Conceptually, the life cycle of an organism is the organism itself. But occasionally, the organism represents one of the developmental phases only. For instance, a fern plant has a typical foliage and growth habit. However, in this form it represents only one of its developmental phases (sporophyte). Since an organism is the biological unit of evolution, its life cycle should include all stages of its development.
The life cycle of a typical pteridophyte consists of a regular alternation of sporophytic (asexual) and gametophytic (sexual) generations.
In bryophytes, the gametophytic phase is dominant in life cycle, and the sporophyte is dependent on the gametophyte. By contrast, in gymnosperms and angiosperms the gametophytic generation is reduced and is dependent on the sporophyte.
Pteridophytes, with an intermediate position, are characterized by free-living gametophytic and sporophytic generations (Fig. 6.1). Nevertheless, the sporophyte is the dominant generation; it soon becomes independent of the gametophyte and attains a much greater size.
Accompanied with this increase in size of sporophytic generation is the attainment of external and internal complexity. The sporophyte is organized into root, stem, and leaves, and internally it shows the differentiation of vascular elements.
All these elaborations confer on the sporophyte the potentialities to exist under a wide range of environmental conditions. However, in many of the pteridophytes these potentialities cannot be realized and the sporophyte is impaired by the inability of the gametophyte to grow under variable environmental conditions.
The majority of the pteridophytes produce only one kind of spores, i.e. are homosporous; their gametophytes are exosporic thin plate of cells and have to be confined to humid conditions. Some pteridophytes, however, produce two kinds of spores; the larger megaspores and smaller microspores, forming two kinds of gametophytes.
The gametophytes of these heterosporous forms initiate within the spore coats, i.e. are endosporic, and in some the female gametophytes during their development are retained on the parent sporophyte for variable periods.
The development of a gametophyte within the resistant spore wall and its subsequent retention on the sporophyte are better starting points for the gametophytic generation. Endosporic gametophyte ensures embryonic development more than the exosporic gametophyte which is vulnerable to environmental conditions and has to be self-sufficient in its food supply.
A diagrammatic representation of life cycle, of a typical homosporous or a heterosporous pteridophyte, is possible (Fig. 6.2 and 6.3).
The sporophyte which represents the diploid generation produces the haploid non-motile spores by meiosis. The spores germinate to give rise to haploid gametophytes which are monoecious (Fig. 6.2) in the homosporous form and dioecious (Fig. 6.3) in heterosporous form.
The male gametes (antherozoids) produced by male reproductive organs (antheridia) are flagellated and able to swim in water. The female gametes (eggs) are non-motile and enclosed in female reproductive organs, the archegonia. The fusion between two gametes, the egg and the antherozoid, results in the formation of a zygote which is diploid. The zygote develops mitotically into a sporophyte.
During normal life cycle there is an alternation between these two morphological forms of haploid and diploid generations. The gametophyte on gametic union gives rise to a sporophyte and the sporophyte after meiosis gives rise to haploid spores, which form haploid gametophytes.
However, in an alternate way a gametophyte can directly give rise to sporophyte without syngamy i.e., vegetatively. This phenomenon is known as apogamy. Similarly, the sporophyte can give rise to gametophyte without the formation of spores. This phenomenon is known as apospory. In these alternate pathways, there is an alternation of morphologies but not of the different chromosome numbers.