FOSSIL AND FOSSILIZATION in Plants
The study of fossils of plant life of the geologic past is called paleobotany. Paleobotany tells us the story of preserved vestiges of the plant life of the past. In simplified language paleobotany may be defined as a branch of botany which deals with the study of such plants which are living in past but extinct now.
The study of dead remains of the organism or their imprints preserved in the geological rocks is called fossil. Fossils are used to study organisms present in the past. Fossils is derived from a Latin word fotlere meaning “to dig”. In fact fossils are impressions or cast of the organelles (organism) living sometime back but now extinct, on. They are present in rocks. Now they are required to be dug out. Normally the plant dies and it degeneration starts immediately. This degeneration remains continue till the entire body is broken into chemical compounds. Thus no evidences of its former remain in existence. But sometimes under certain circumstances the degeneration of plant body stops. Thus parts of the body remain in visible form. These remains are known as fossils. These fossils are lifeless.
Plant fossils are normally present in rocks composed of sediments deposited in waters. These stratified rocks are super imposed upon one another in series. These rocks are built of sediments. These accumulate in bottoms of seas, lakes, swamps, flooded valleys, subsiding beaches etc. The rock formation is correlated with the geological approach in the subject paleobotany. In fact fossil are the makers of geologic time. The study of a botany as well as geology is essential for the study of fossil plants. The classification of fossil plants is very difficult as there are many chances for error.
Importance of Fossils
The study of fossil plants has great importance. Ills as follow:
1. They throw light on Phylogeny and evolution of plants. The extinct plants tell us some stages through which existing group have passed during course of their development.
2. Fossil plants give a historical approach to plant kingdom.
3. Fossils are tivipful in classification of plants.
4. Fossils plant can be used in the field of descriptive and comparative anatomy.
The process of formation of fossil in the rocks is called fossilization. It has following stages:
1. First of all the wind or stem borne plant come into quiet water and become saturated. Now it begins to sink.
2. The entire plant or some part of it lies in parallel to horizontal position. In presence of mud or sand plant it will sterile with sand.
3. Abundant sediment will cause rapid accumulation and the plants will be separated from one another. Thus rocks al . 707.ned in between which plant parts (mainly stem, leaves) remain present.
4. The weight of the accumulating sediments will flatten it. The sediments increase in thickness, compact. As a result, less resistant and more compressible plant part’s flattened to a new portion of its original thickness.
5. The plant parts become cylindrical. They are made up of hard tissues the weight overly of sediments will produce • lens shaped object. Thus the rocks have the compression of impression of plant. Thus rock having compression when split open on one surface usually bears the impressed outer part.
Types of Fossils plants
Some of the important type of plant fossils is as follows
1. Petrifactions or Mineralized plants: In this type of plants fossil the original cell of the plant tissue is retained by means of some minerals like, silica etc. These mineral has infiltrated the tissues. In this type of fossil sometimes the material of original plant may be preserved e.g. coal balls, Silcified wood etc.
2. Cast on incrustations: In this type of plant fossil, the form of plant if preserved as a cast. The cast result from the liking of a avity formed by decay of tissues of plant part. Here the internal
structures are destroyed and carbonaceous substances of the plant are totally gone e.g. stem leaf scers, larger seeds etc.
3. Compressions: In this type of plant fossil, the external form of plant modifies and leaves impressions on the sediment which over lie it
4. Compaction or Mummified plants: In this type of plant fossil, the plants or their parts get compressed by vertical pressure against one another. Mostly plant rudiments found in pea and coal as compactions. Coal or coal balls are the important sources of plant fossils. Coals are irregular or sub spherical mass of calcium or magnesium carbonates (or some other mineral matter).
5. Impressions: In this type of plants fossil, the roots, stems, leaves, fruits and seeds are preserved as impressions in such a fashion that they seem to be the actual dried specimens laid on the stone.
6. Amber: Coniferous plants exudates resinous substance. It drops on the floor of forests. It accumulates and hardened over ages. Insects, fragments of plants and other animals get preserved in it and become fossilized. It is called ambers.
FOSSIL OF PTERIDOPHYTES
Many of the early vascular belonged to the divisions of plant kingdom that are now extinct. The members of all these divisions , were present during the Devonian period. These include Rhynitiophyta, Zosterophyllophyta, Trimerophytophyta, Microphyllophyta, Arthrophyta, and Pteridophyta.
1. Division: Rhyniophyta
The members of the division show a simple construction. Their plant body consist of dichotomously branches axes. The leaves and roots are absent. The sporangia are present at the tips of branches. These are thick-walled and homosporous. There is a solid vascular bundle present throughout its axis. The most common genera are:
a) Rhynia: They have tufts of unicellular rhizoids.
b) Horneophyton: The basal part of the plant consisted of a number of swollen, bulbul structures. They give rise to upright dichotomously branching aerial shoots arise),
c) Cooksonia: These are morphologically similar to the living genus Psilotum.
2. Division Zosterophyllophyta
These are extremely primitive group of vascular plants. They coexisted with Rhyniophyta. But each group evolved in different directions. The members of this division are considered to be ancestors of Microphyllophyta. The members possess spine–like projections. It resembles the minute leaves. A modification of these spines result in structures closely resembling leaves of certain Lycopodium sp. The sporangia are borne laterally. These are associated with small leaves. The most commonly found genera are:
a) Zosteropbyllum: The basal part consists of axis. It gives two branches. One grows in air and other downward branching.
b) Koulangiophyton: It resembles creeping species of tycopodium,
c) Crenaticaulis: They possesses structures resembling rhizophores of Selaginel la
3. Division Trimerophytophyta
The members of the division are leafless and rootless. They bear terminal sporangia like Rhyniophyta. But they have different branching pattern which is lateral. The laterals are dichotomously branched. The best known genus of the division is Psilophyton. Its leafless axis is covered with spine like projections. The sporangia are terminal, paired and pendulous.
4. Division Microphyllophyta
The members of the division originated in the Devonian period. It includes plants with features like living Lycopodium and Selaginelia. These are believed to be derived from Zosterophyllophyta. The leaves are small, simple and closely spaced. The sporangia are lateral. These are associated with leaves, either attached directly on the adaxial surface or situated in the axil. In most cases the sporangia are present terminally. These form strobili. Both herbaceous and arborescent species are included. The most common being: Protolepidodendron, Asteroxylon, Baragwanathia, Lepidodendron, Lycopodite.c, Selaginellites, etc.
5. Division Arthrophyta
The history is similar to Microphyllophyta. The members of the division are characterized by whorled branches and ribbed stem. Their sporangia present on reduced branches called sporangiophores. The ancestors of Arthrophyta are believed to be plants like lbyka ampitikoma. The important genera are:
a) Pseudobornia: The have an upright tree with jointed stem. A whorl of branches arise from this.
b) Calamites: These are giant horsetaiil. They flourished from the
upper Devonain though the Permian. Their upright portion arose from horizontal underground rhizome as in Equisetum. Their stem hollow with pith. Their sterile appendages borne in compact strobili. Sporangiophores just like those of Equisetum are present.
c) Sphenophylium: They have young stems ribbed. A whorl of leaves is present at each node. Their strobili are borne at tips of certain branches.
d) Equisetites: They are like herbaceous plant except that the leaves were more conspicuous)
6. Division Pteriodphyta
The division includes ferns. They all are megaphylious vascular plants. Their sporangia present on leaves. The most likely ancestor of ferns is Trimerophytophyta. The lateral branch system of these plants gives rise to megaphyll. The ferns and fernlike plants are in great abundance in Carboniferous rock. Thus time period is called age of ferns. They have trunk like stem. It bears a crown of branches at its ape>.. Some branches are sterile and called fronds. Other fertile bearing sporangia, basal branches bear roots. Its important genera are:
a) Stauropteris: Their appendage borne in pairs and sporangia borne terminally.
b) Botryopteris: Their fronds contain vascular traces. Their sporangia are borne on frond in closely packed clusters.
c) Psaronius: It is a fern tree with a crown of large pinnate fronds. Their sporangia borne on the adaxial surface of pinnules).
d) Thmnoppteris: It is an osmunda.like fern.