The most well-known plants of this group are ferns which we generally view as houseplants, in parks and as well in house landscapes all along with other ornamental plants. Ferns are instead small plants with graceful, frequently fragile compound leaves. Because of their prettiness and complexity in propagation, they are considered very expensive plants.
Pteridophytes are vascular plants and they have stem, root and leaves. All the vascular plants have water and food conducting pipelines build up of xylem and phloem tissues, correspondingly. In Pteridophytes, a natural gradation in vascular tissues from simple to complex forms is viewed.
Characteristics of Pteridophytes:
1) They reproduce by forming spores. Dissimilar to other plants, they don't produce seeds. Antheridia and archegonia are the male and female organ correspondingly. Eggs in the archegonia are fertilized through the sperms generated by the antheridia to produce the zygote.
2) They grow in a broad range of habitats like rock crevices, deserts, mountains and in moist and shady regions.
3) They contain roots, leaves and stem however they don't have flowers. They have specialized leaves termed as sporophylls that produce spores and encompass fibrous roots.
4) They are vascular plants that are; they have xylem and phloem which aids in the transportation of nutrients and water.
5) They encompass a complex life cycle which comprises alternation of generations. Their life cycle comprises a haploid stage followed through diploid phase.
Life cycle of Pteridophytes:
Similar to bryophytes, Pteridophytes as well encompass two different phases in the life cycle: gametophyte and sporophyte which follow one other in regular succession. Since the two generations look different, they are termed Heteromorphic. Beneath normal situations, gametophyte generates motile male gametes (that is, sperms) and non-motile female gametes (that is, eggs).
Fusion among the egg cell and male gamete outcomes in the formation of a zygote that is diploid. The zygote splits by mitotic divisions and makes the sporophyte. On sporophyte a number of haploid, non-motile spores are generated by meiosis. The life cycle is then done when a spore germinates and generates haploid gametophytes through mitotic divisions.
In bryophytes, the dominant stage in the life cycle is the gametophyte and the sporophyte is either partially or fully dependent on it for nutrition. However in Pteridophytes the sporophyte very soon becomes independent of the gametophyte and is main generation.
The sporophyte exhibits greater degree of complexity in the structural organization. It is ordered into stem, root and leaves, apart in the most ancient fossil Pteridophytes and in the most primitive living member. The vascular tissues (that is, xylem and phloem) are built up only in the sporophytes.
Moreover, the aerial parts are covered by a layer of cuticle. On the epidermis there are stomata for the exchange of gases. Such anatomical complexities of the sporophyte helped in inhabiting a much broader range of ecological conditions than the gametophyte could.
What are Fossils?
Fossils are the remains and/or impressions of organisms which lived in the past. In its exact sense fossils comprise the remains of organisms or their parts and as well anything joined with an organism proving its existence, that is, anything that provides evidence which an organism once lived.
The method of formation of fossils is going on ever as the sedimentary rocks start to deposit and it is going on in nature still now.
In some situations plants might be deposited on the site where they grow (in situ), like swamps and small inland lakes. Due to the low oxygen content and presence of toxic substance in the water, microbial development is inhibited; therefore the plants do not decay. This outcome in the preservation of the plant remains till they were covered via layers of sediments.
Type of fossilization:
In other situations, plant parts are carried down by flowing water and at last sink to the bottom of a lake or estuarine water where they are less vulnerable to decay through microbes.
Throughout fossilizations the protoplasmic contents and softer parenchymatous cells disappear first, whereas the harder wood and other sclerenchymatous or cutinized tissues resist to the last. The growing pressure of the heavy sedimentary rocks above, fist decreases the vacant spaces within the cells and forces the liquids substances out. A few organic substances might as well escape as marsh gas. Naturally, the entire fossils get highly compressed and the final outcome based on how far the conditions were favorable for good fossilization. Despite of all hazards at times fossils are formed, which retain their cellular structure beautifully and at times even some of the cell contents.
Types of fossils:
According to the nature of fossilization, fossils might be of the given types:
It is the best kind of fossilization. In this kind buried plant material gets decayed by the passage of time and gets substituted, molecule for molecule by mineral solutions. The impregnation of silica, magnesium carbonate, calcium carbonate, iron sulphide takes place in the tissues. Most of the plant material might get decayed however at least some original cell wall components remain. After fossilization the entire structure becomes stone-like and it can be cut into fine part.
2) Cast or incrustation:
This kind of fossilization is as well quite common. The plant portions get covered up by sand or mud. After sometime the plant material within degenerate leave a cavity termed as mold.
These are found if a leaf or any other portion of the plant falls on and leave an impression on the surface of semisolid clay. In course of time this impression becomes permanent if the clay turns into stone.
In a compression the organic remains of the plant portion in reality remain in the fossils however in a highly compressed state. All through fossilization the great pressure of sediments above causes flattening of the parts of plant. In the fossil generally a carbonaceous film remains that represents the surface characteristics.
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