Formation of Vascular Land Plant and Seed Fossils and their Types

You may raise a question as to how can one know “Where, when, and from what ancestral group did the first vascular land plant and seed-like structure evolve?” To find the answer to these questions we have to depend on fossils.

Let us first try to define a fossil and the ways in which fossils came to be formed. We will also try to know the extent to which they may be expected to provide information useful to the morphologist.

What are Fossils?

Fossils are the remains and/or impressions of organisms that lived in the past. In its correct sense fossils include the remains of organisms or their parts and also anything connected with an organism proving its existence, i.e., anything which gives evidence that an organism once lived.

How are Fossils Formed?

The actual nature of fossilization depends on the environmental conditions in which it takes place.

Dead plant remains are liable to get disintegrated and it is only rarely that they get fossilized.

Chances of fossilization are better for organisms having stiff tissues / skeletons. The details of fossilization process are discussed below.

Fossilization Process

The process of formation of fossils is going on ever since the sedimentary rocks began to deposit and it is going on in nature even now.

In some cases, plants may be deposited on the site where they grow (in situ), such as swamps and small inland lakes. Due to low oxygen content and the presence of a toxic substance in the water, microbial growth is inhibited, so the plants do not decay.

This results in the preservation of the plant remain until they were covered by layers of sediments. European and Enugu coal forests are examples of this type of fossilization.

In other cases, plant parts are carried down by flowing water and finally sink to the bottom of a lake or estuarine water where they are less susceptible to decay by microbes.

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During fossilization, the protoplasmic contents and softer parenchymatous cells disappear first, while the harder wood and other sclerenchymatous or cutinized tissues resist to the last.

The growing pressure of the heavy sedimentary rocks above, first reduces the vacant spaces inside the cells and forces the liquid substances out. Some organic substances may also escape as marsh gas.

Naturally, all fossils get highly compressed and the final result depends on how far the conditions were favourable for good fossilization. In spite of all hazards sometimes fossils are formed, which retain their cellular structure beautifully and sometimes even some of the cell contents.

Types of Fossils

According to the nature of fossilization, fossils may be of the following types:

1. Petrifaction

It is the best type of fossilization. In this type of buried plant material gets decayed with the passage of time and gets replaced, molecule for molecule by mineral solutions. The impregnation of silica, calcium carbonate, magnesium carbonate, and iron sulphide takes place within the tissues.

Most of the plant material may get decayed but at least some original cell wall components remain. After fossilization the whole structure becomes stone-like and it can be cut into the fine sections.

The structure of the tissues may be observed by examining the section under the microscope. The anatomical structures of ancient plants are beautifully obtained from such petrifactions. Silicified and calcified pieces of wood are quite common.

2. Castor incrustation

This type of fossilization is also quite common. The plant parts get covered up by sand or mud. After some time the plant material inside degenerates leaving a cavity known as mold.

This cavity again gets filled up by some rock-forming material which in course of time solidifies into an exact cast of the plant material, showing all its surface features.

A cast fossil does not actually contain any part of the original plant but it is of great use as the cast correctly shows the original features of plant part.

3. Impression

These are found when a leaf or any other part of the plant falls on and leave an impression on the surface of semisolid clay.

In course of time, this impression becomes permanent when the clay turns into stone. Such impressions often very clearly show details of external features and structures like stomata are clearly seen in good preparation.

4. Compression

In compression, the organic remains of the plant part actually remain in the fossils but in a highly compressed state. During fossilization, the great pressure of sediments above causes the flattening of plant parts.

In the fossil usually, a carbonaceous film remains which represents the surface features. However, in good compressions, it has been possible to swell out the organ by some chemical treatments so that plant some details become visible.

A good type of compressed fossil is the “clay nodule”. In this the plant material gets encased in a ball of clay, gets compressed and the clay ball turns into stone.

Nomenclature of Fossils

Therefore, in the meantime, each fragment of fossil plant is described under a separate generic name and such genera are known as “Form genera”. In naming such form genera we usually add suffixes, signifying which part of the plant it came from.

Following are a few examples:

• Leaf – phyllum
• Fern-like or frond – pteris
• Tree trunk – dendron
• Woody part – xylon
• Seed-like structure – spermum, – carpon, – carpus, – storms
• Microsporangium – theca
• Cone – strobilus, – strobes

It is the work of palaeobotanists to collect bits of such fossils, i.e, form general, and to reconstruct the form, structure and mode of life of the plant from which they came. Success has been achieved in reconstructing a few fossil plants.

Morphology

As you have noticed in the earlier units on Algae, Fungi and Bryophytes, each of these major plant groups are classified into smaller groups on the basis of distinguishable characteristics. You may recall the following major divisions of extant and extinct pteridophytes.

Lycopodium

Classification:

• Division – Lycopodiophyta
• Class – Lycopsida
• Order – Lycopodiales

Lycopodium, popularly known as club moss, is a large genus with about 180 species of which approximately 33 species are found in India.

They are distributed worldwide in tropical, sub-tropical forests, and in temperate regions. Some species are abundant in hills at comparatively high altitudes. They grow in cool climates on moist humus-rich soil.

The adult sporophyte is herbaceous and with a wide range of habits. Generally in tropics they are pendulous epiphytes, whereas in temperate regions they are prostrate or erect.

They usually grow about 30 to 60 cm in length. The stem may be unbranched or dichotomously branched which later becomes monopodial. It is covered with microophylls which in most species are spirally arranged.

However, in some species leaves are arranged in a whorled or decussate manner decussate x-shaped, with pair of opposite leaves each at right angles to the pair below:

Selaginella

Classification;

• Division – Lycopodiophyta
• Class – Isoetatae
• Order – Selaginellales.

Most of the species of Selaginella are restricted to damp areas of the tropical and subtropical regions of the world.

A few species are markedly xerophytic and inhabit desert regions. These are sometimes called “resurrection plants” because of their extraordinary power of recovery after a prolonged drought. The plant may be prostrate, erect, or sub-erect. Only a few are epiphytic.

Some form delicate green mossy cushions, others are vine-like, with stems growing to a height of several meters, while many have creeping axes, from which arise leafy branch systems that bear a striking superficial resemblance to a frond of a fern.

Branching in Selaginella is characteristic, terminal, and unequal, forming weaker and stronger branches. At each dichotomy, there are one or two meristems on either side. These angle-meristems develop into cylindrical outgrowths known as “rhinophores”.

In most species, only the ventral angle-meristem develops into the rhizosphere, while the other remains as a dormant papilla. The rhizophores grow downwards into groups and give rise to a small tuft of adventitious roots at their tips.

The morphological nature of rhizophore has been controversial. It has been held to be a (a) root, (b) a branch of stem (c) a structure sui generis (falling in neither of the categories). Earlier investigators reported a unique combination of characters of rhizophore:

i. Exogenous origin from the stem at the time of branching, ii lack of root cap, production of roots endogenously behind the tip, and ability, in some instance, to be converted into leafy shoots. Since these features are not typical of root these outgrowths are called rhizophores.

The features suggestive of their root nature are;

i. Positive geotropism,

ii. In some species when these structure re less than 1 mm the root cap develops, in S. martensil cap differentiates when it nears the soil.

Using labeled auxin (C¹⁴ IAA) ‘it has shown that auxin transport in rhizophores of Selaginella is acropetalous as in case of angiosperms root, whereas it is basipetalous in stems. Therefore, now the term “rhizophore” as well as the arguments regarding its nature are of historical significance.

In Selaginella the leaves are sessile with a single unbranched vei . Leaves of Selaginella are ligulate. The ligule is present in or near the axil of each leaf as a laminate outgrowth.

It differentiates and matures very early in the ontogeny of leaf. A mature ligule is tongue-to-fan-shaped. Its basal region is made up of tubular, hyaline cells forming the sheath is a hemispherical region of thin and greatly vacuolated cells referred to as glossopodium.

The remaining cells are isodiametric. The apical region is one cell thick and is made up of elongated cells with scanty contents.

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Pteris

• Classification;
• Division – Pterophyta (= Filicophyta)
• Class – Polypodiatae
• Order – Filicales

Pteris is a widely distributed genus with about 250 species. It grows abundantly in cool, damp and shady places in tropical and subtropical regions of the world. In all there are 19 species recorded from Ind.

Pteris vitata is a low level fern which brings out new leaves throughout the year. It is very common along mountain walls and grows up to 1200 metres above sea level. Pteris quadriauriata grows abundantly along roadsides and in the valley throughout North-Western Himalayas.

Another species, Pteris cretica grows well from 1200 to 2400 metres above sea level. Some species are also found in the rainforest but of South-West South-East and South- South parts of Nigeria for instance.

All the species of Pteris are terrestrial, perennial herbs with either creeping or semi-erect rhizome covered by scales. Roots arise either from the lower surface or all over the surface of rhizome.

You may have noticed that the most conspicuous part of a fern plant is its leaves which are called fronds.

The leaves are compound in most species but a few have simple leaves, for example Pteris cretica. The stalk of leaf continues as rachis and bears leaflets called pinnae.

In Pteris vittata the pinnae present near the base and tip are smaller than those in the middle. The leaf apex is occupied by an odd pinna. Every pinna is transverse by a central midrib which gives off lateral veins that bifurcate.

The pinnae are sessile and broader at the base gradually decreasing in width towards the apex. The leaves are bipinnate in P. biauriata. The pinnules are rough in texture. The young leaves show typical incurving known as circinate vernation.

The leaves bear spore-producing structures on the underside of the leaflets. They appear as rows of brown dots (sori, sing, sorus). Each sorus is a cluster of sporangia.

In summary, Pteridophytes are vascular plants (with xylem and phloem) that produce neither flowers nor seeds. They are therefore called vascular cryptogams.

Pteridophytes include horsetails, ferns and club mosses. Specific generic members include Cooksonia, Psilotum, Lycopodium, Selaginella, Equisetum, Pteris, Dryopteris felixma, (Nigerian fern) and Marsilea. Some members are fossils. They are used for medicinal purposes and as ornamentals.

Pteridophytes are primitive, vascular, non-flowering land plants. Like bryophytes, they show distinct alternation of generations, but instead of gametophyte, sporophyte is the dominant phase of life cycle.

Fossils provide evidence for extinct member plants. They are of four types: Petrifaction, Cast, Impression and Compression,

Lycopodium stem is densely covered with microphylls. It is also protostellic. Roots are from pericycle and are diarchy.

In Selaginella the main stem may be prostrate, semi-erect or erect, branched or unbranched. It possess microphylls which are spirally arranged on the stem and are ligulate.

Pteris has a creeping rhizome which bears scales or branched hairs. The plant is characterized by prominent pinnately compound or digitate leaves. The sporangia are generally grouped together in sori.

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