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Term Paper on Pteridophytes
Term Paper Contents:
- Term Paper on the General Characters of Pteridophytes
- Term Paper on the Origin of Pteridophytes
- Term Paper on the Evolution among Pteridophytes
- Term Paper on the Economic Importance of Pteridophytes
Term Paper # 1.
General Characters of Pteridophytes:
They have a well-defined alternation of generations in their life cycles and the sporophyte is always an independent plant and free from the gametophyte at maturity. The gametophyte, on the other hand, is either wholly independent of the sporophyte or may be partially or wholly dependent on it for nutrition and is always smaller than the sporophyte.
The plant body is usually well differentiated into stem, root and leaves, and with an internal conducting system consisting of xylem and phloem. The asexual generation or the sporophyte may be homosporous (with spores of one kind) or heterosporous (with spores of two kinds, viz., small or microspores and large or megaspores).
The Pteridophytes differ from the Bryophytes in having an independent sporophyte with well-differentiated vascular structures. They also differ from the seed-bearing plants (Spermatophytes) in the liberation of the female gametophyte from the sporangium. The method of sexual reproduction resembles that of the Bryophytes.
Similar to all vascular plants, the shoot of pteridophytes consists of the cortex and a central cylinder, which is known as the stele. The stele is mainly made up of xylem and phloem, and is delimited from the cortex by the endodermis. The histological structure of the protostele varies widely.
A protostele with smooth core of xylem surrounded by phloem is termed a haplostele (e.g., Rhynia, Horneophyton, etc.). If the core of xylem of a protostele, in transactional view, is lobed or star-shaped, it is designated an actinostele (e.g. Asteroxylon, Psilotum, Lycopodium selago, etc.).
The specialized type of protostele, in transection seems to consist of separate plates of xylem between and around which occurs the phloem—this form is designated a plectostele (e.g. Lycopodium clavatum, Cladoxylon scoparium, etc.).
There are two principal types of steles – the protostele and the siphonostele.
The protostele is characterized by the absence of a central column of pith, and from the phylogenetic as well as ontogenetic standpoint, it is the most primitive type of stelar construction.
A stele with a central column of pith is regarded phylogenetically as an advance in anatomical development and is known as a siphonostele. The pith in a siphonostele may have resulted either by the gradual inclusion of areas of cortical parenchyma or by the phylogenetic reduction of the original tracheid’s to parenchyma.
A siphonostele may be amphiphloic with phloem both external and internal to xylem (e.g. Marsilea, Dicksonia, etc.) or it may be ectophloic with pholem restricted to the external face of the xylem only (e.g., Ophioglossum, Botrychium, etc.
The evolution of siphonostele has been accompanied by the formation of parenchymatous gaps (leaf gaps and branch gaps) in the vascular cylinder. Siphonosteles without an overlapping of gaps are known as solenosteles (e.g., Marsilea, Dennstaedtia, etc.,) and those with overlapping gaps are termed as dictyosteles (e.g., Polypodium, Thelypteris, etc.).
The intervening vascular bundle to overlapping gaps is known as meristele. Dictyosteles with medullary bundles and often with cortical bundles are designated as cyathean dictyosteles (e.g., Cyathea, Thyrsopteris, etc.).
Term Paper # 2.
Origin of Pteridophytes:
The question of the origin of the present-day pteridophytes is a highly debated one, as a considerable gap exists in between the pteridophytes and other plants placed lower in the scale of evolution. There are two schools of view concerning the origin of pteridophytes, one proposing an origin from algae and the other from bryophytes.
According to the advocates of an algal origin, the similarities noted between the bryophytes and the pteridophytes are not due to any phylogenetic relationship existing in between the two groups, but these are the results of a parallel evolution.
While Scott (1900), and Eames (1936) do not suggest any particular group of algae as the supposed ancestors, other workers like Church (1919); and Arnold (1947), are of opinion that the pteridophytes originated from some complex marine algae ‘transmigrating’ from the ocean to the land.
Obviously, their idea implies that the pteridophytes evolved from the Phaeophyceae. On the other hand, Boblin (1901), Lotsy (1909), and Fritsch (1916) believe that this origin has taken place from some Chaetophora-like filamentous green algae.
In a subsequent publication Fritsch (1945) expressed his idea that the hypothetical ancestor was a green alga with an erect parenchymatous body possessing an isomorphic type of alternation of generations in its life cycle.
Lignier (1903), Bower (1935), and Zimmermann (1930, 1938) believe that the bryophytes and the pteridophytes are phylogenetically connected but divergently developing evolutionary lines, which have separated early from very primitive archegonia-bearing hypothetical land plants. But Campbell (1895, 1899, and 1924) is of opinion that the pteridophytes have originated from the true bryophytes of the anthocerotean type.
This idea was opposed at first on the ground that wide differences exist between the sporophytes of the known pteridophytes and that of the anthocerotean type. But, when the rootless, non-leafy and dichotomously branched sporophytes of the psilophytes were discovered, this objection was rendered invalid.
Further, though nothing is known about the embryology of the members of the Psilophytales, but a surprisingly superficial resemblance exists in between the embryos of the living Psilotales and the Anthocerotes.
Term Paper # 3.
Evolution among Pteridophytes:
Among the pteridophytes, the PsiIophytales are very simple as well as very primitive, and they can be traced as far back as the Silurian. Their sporophytes were rootless and represented by either completely leafless branches or by branches having minute veinless leaves.
The sporangia were borne singly and terminally at the end of a branch. From this psilophytalean stock probably there appeared early in different divergent directions all the present-day pteridophytes.
It has, however, been suggested by many authors that the simplicity of the Psilophytales had resulted from reduction in response to some environmental stimulus, rather than being manifestation of genuine primitiveness.
The much supporting evidence for such a hypothesis is that the Silurian Baragwanathia, a lycopod, was already well-defined and strongly established prior to the appearance of most of the psilophytaceous plants, which are conceived as ancestral to all other forms.
Term Paper # 4. Economic Importance of Pteridophytes:
In comparison with the seed-bearing plants, the pteridophytes are less important from the economic standpoint of view. Lycopodium and Selaginella are chiefly grown as ornamental plants and are utilized in the preparation of Christmas wreathes. Spores and stems of Lycopodium have got some medicinal importance, the former also being used in making fireworks.
Species of Selaginella, e.g., S. caesea, and S. wildenovii, are used in decoration, because the leaves are remarkable for their metallic and many-hued tints, specially bronze and bluish colours, the latter being very uncommon among plants in general. S. serpens is well known for the periodic changes in the colour of its leaves S. lepidophyla and S. pilifera are sold as curios under the name ‘Resurrection plants’.
Ducks and other aquatic animals feed upon the corms of Isoetes. In early days people used Equisetum for polishing wood, floors and cooking utensils. The ferns are mostly ornamental plants of gardens and greenhouses. Some of them are used in the preparation of bouquets and are also placed in the buttonholes.
Some species possess so dense and prominent epidermal hairs, that they are sometimes employed as stuffing material. In some tropical countries stems and leaves of tree ferns are used for building purposes.
Some genera, like Pteris, Ceratopteris and Marsilea, are edible. The rhizomes and leaf bases of Dryopteris filix-mas are used in medicine as a taenifuge. Practically, all the members of the pteridophyta have contributed extensively to coal formation.
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