CLASSES OF FOSSIL GYMNOSPERMS

RAMANDEEP KAUR

ASST.PROF, IN BOTANY

FOSSIL GYMNOSPERMS: CLASS # 1.�CYCADOPSIDA: ORDER-PTERIDOSPERMALES (CYCADOFILICALES�

• The Ptieridospermae or the Cycadofilicales are fossilized plants that first appeared in the upper Devonian. They were very common throughout the Carboniferous and extended upto Permian strata. After the Permian they began to decline and disappeared altogether.
• It is in the early Devonian rocks that the members of some very primitive type of land plants disappeared. These plants were probably the simplest pteridophytes.

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• They had no roots, some without leaves and some had small tiny outgrowths which represented the leaves. This primitive group of land plants was different from pteridophytes. Their stem had well formed xylem and phloem tissue and the tips of the branches bore sporangia on them.

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It appears that very extensive forests got submerged and it is the submerged forests from which we get the extinct coal supply of present day. During this Carboniferous which has been sometimes described as age of ferns, very large sized Lycopods and Equisetales were common.

Along with these fossil pteridophytes, there were also present seeded plants which developed seeds of Cycadian type on them. Those plants which bore seeds were formerly described as Cycadofilicales. English workers (Oliver and Scott) described them as Pteridosperms while American workers call them Cycadofilicales.

These seeds were known to previous workers also but their association with fern like leaves was recognized in 1903. It was during this year that some of these Carboniferous seeds were found in organic continuity with the leaves of fern like plants. These seeded fossil plants have definite relation with gymnosperms of which American workers think they form an extinct class

• One of the best known plants is Lyginopteris which is best known of all Carboniferous plants. Before 1903 different parts of the plants were known, their stems, roots, leaves and also the seeds. On the whole surface of seeds capitate glands were present and it was the presence of these capitate glands that gave rise to idea that all these different parts of plants might be of one or same plant.
• Later on the entire plant was found in one place in organic continuity from roots, stems and leaves.
• A number of stem genera have been described from time to time. These stem, genera may be grouped under a number of families.

• Gymnosperms
• Class: Cycadopsida
• Order: Pteridospermales
• ADVERTISEMENTS:
• Family: Lyginopteridaceae
• Genus: Lyginopteris (Calymatotheca)
• The genus Lyginopteris also known as Lyginodendron

• The stem Lyginopteris was slender and covered with large scaly leaves. Near the base of the plant adventitious roots developed. The plant seems to have been a climber.
• Lyginopteris oldhamia also known as Calymatotheca hoeninghausi was described in detail by Williamson, Scott, Brongniart, Binney, Potonie, and Oliver and Scott. It was found abundantly in the coal ball horizon of Lancashire and Yorkshire

Anatomical Features:

The Primary structure was an ectophloic siphonostele with large pith round a number of primary mesarch bundles. Older plants showed normal secondary growth. In some specimens, however, the xylem portion of primary vascular bundles was in a continuous ring. In some there was an abnormal type of secondary growth.

This abnormality was of two forms, either there was an inner ring of secondary phloem developed or it was that the cambium appeared in strips found separately in vascular bundles giving rise to a polystelic appearance.

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Reproductive Structure:

Some of these Palaeozoic leaves bore microsporangia on them. The fertile pinnules were more or less peltate in form and on their underside they bore usually six sporangia. These sporangia are usually bilocular. Such a type has been described as Crossotheca type. The microspores seem to have formed a male prothallus. The sperms seem to have been of like those of present-day cycads.

Seed:

The best known seed has been described under the name Lagenostoma. These seeds were small in size, only about 1/4 but they were highly organized. It was barrel shaped and whole seed enclosed in cupule. This cupule opened out when seed was mature.

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• Each seed was borne at the tip of stalk. The cupule rose from the base of the seed but not fused with it.
• The cupule was in three main lobes. These lobes were divided in the upper parts of the seeds.
• The seed or ovule was orthrotropus and of cycadian type. It was radially symmetrical.
• The cupule was separated from the seed along its entire length. The seed itself has an integument which surrounded the nucellus.
• The integument and nucellus were fused except at the top. The integument formed nine projections. In each of these projections there was a vascular bundle present. These projections surrounded the nucellar beak.
• The pollen grains then came to lie in pollen chamber formed by disorganization of some of nucellar tissue round the base of nucellar beak.
• In some of preparations the megaspore membrane is very well seen. In the centre of the seed there was a tissue but so far neither any archegonium nor any embryo has been found in these Palaeozoic seeds.

• The seeds were borne at the tips of the stalks. They were not organized to form cover. In addition to this Lagenostoma there are many other Palaeozoic seeds which have been described some simpler other more complex than Lagenostoma.
• A peculiar feature of all Palaeozoic seeds which so for has not been explained is the absence of any embryo in them. Pollen grains have been found in pollen chambers.
• Some of them had even showed their germ tube, but so far no seed is discovered in which embryo was developed.
• It is possible that all the seeds described might have not preserved or they preserved before embryo formation.

In the Mesozoic times, however, we came across fossils plants which had cones and were definitely related to gymnosperms. So in Mesozoic the Cycadofilicales were replaced by true gymnosperms which formed strobili, and the seeds had a naked dicotyledonous embryo in them. The ovule or the seed was never enclosed in closed carpel.

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The Mesozoic gymnosperms can be placed into two separate groups:

1. Cycadeoidale (Bennettitales) and

2. Cycadales

The Cycadeoideales (Bennettitales) first appeared in the Permian they reached their highest range during the Jurassic period, after which they disappeared altogether.

The second group Cycadales had a world-wide distribution during the Mesozoic period Majority of them had altogether disappeared; only a few types have been left which are confined to special parts of the East.

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The present day cycads are only the remnants of very large dyeing out group, i.e., they are sometimes described as living fossils, because they are on their way to extinction.

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The Cycadeoideales (Bennettitales) were very much like the cycads in their general appearance, and as the Mesozoic had these two prominent groups of gymnosperms, so that period sometimes described as age of cycads.

• These Cycadeoideales are closely related to the Cycadofilicales on one side and to cycads on the other but they have their own characteristic features which distinguish them from all other gymnosperms except the Gnetales.
• The important feature which separates the Cycadeoideales from other gymnosperms is the presence of bisporangiate strobili.
• The plants of this group were diversified in their habit. Some types had short columnar stems like most of the living cycads. The short columnar stem was usually un-branched and at the apex of the plant there was a terminal crown of leaves which in most cases pinnate.
• Some other forms had branched stems with multiple crown. In present day cycads we know that young leaves and megasporophylls are covered up by unicellular hairy outgrowths known as ramenta.
• In Cycadeoideales (Bennettitales) these ramenta were not unicellular; they were scale like, flattened and were several cells in breadth. Like cycads the plants had well organized strobili or cones, but in cycads they are monosporangiate whereas in Cycadeoideales they were usually bisporangiate and they were either terminal or axiarlly in position.
• Majority of Cycadeoideales (Bennettitales) seem to have flowered only once in their life and after flowering the plant died out as we find in some of present day angiosperms.

Genus: Cycadeoidea (Bennettites)

Systematic Position:

Gymnosperms

Class: Cycadopsida

Order: Cycadeoideales

Family: Cycadeoideaceae

Genus: Cycadeoidea

Morphological Features:

In Cycadeoidea the stem was un-branched with a single crown of pinnate leaves at the tops, but some species had branched stem with a multiple crown. In some the stem was tuberous. In all cases the stem was covered up by persistent leaf bases as we find in Cycas.

Anatomical Features:

• In structure the stem usually had large pith and thin vascular cylinder in which the protoxylem was endarch, thick cortex with a number of gum canals in it.
• There was small amount of secondary growth. Growth rings were only in few cases where the cambium persisted and was more active, so on the whole the stem anatomy was like those of present day cycads i.e., with large pith, broad cortex and narrow vascular cylinder.
• In some few cases, however, the vascular cylinder was sufficiently broad. In the stem there were no traces of mesarch vascular bundles which is a common feature of leaf traces of present day cycads.

Another distinction from cycads was that the leaf traces were direct and no girdles while in present day cycads the girdling of leaf traces is quite common. The xylem had scalariform thickenings; pitted thickenings rather rare.

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• This is an unusual feature because in the xylem of Cycadofilicales pitted thickening was very common and the group is much older than Cycadeoideales (Bennettitales). The feature can only be explained that it was case of reversion

Leaves:

The leaves in Cycadeoidea (Bennettites) were large pinnate and showed xerophytic features. The vascular bundles in petiole and leaflets were mesarch with a strong sheath of sclerenchyma around it. Bipinnate leaves were rarely found in Cycadeoidea so the form and structure of leaf is practically like that of living cycads

Fructifications:

The fructification in Cycadeoidea was bisporangiate. The strobili were developed in the upper part of the plant in large numbers. In some cases each leaf seems to have an axillary strobilus.

The whole of the strobilus and the bases of leaves were covered up by large sized scales which were several cells in breadth and sometimes more than one cell in thickness; strobili so were axillary and borne at the tip of axillary stalk or peduncle and therefore, these strobili can be described as dwarf branches.

Each strobilus was made up of a number of heavy imbricate reduced leaves or bracts. These bracts completely surrounded the strobilus when it was the earliest stage of development where the strobilus developed these imbricate bracts separated and the inner part of the strobilus exposed.

The second whorl was made up of a number of leaf like microsporophylls, all of which were united at the base to form a cup-shaped structure round the central part of strobilus. The third central portion was hemispherical or domeshaped in appearance. The central part was made up of a number of ovulate sporophylls. These megasporophylls were simply stalked.

At the tip of the stalk was developed an ovule. Some stalks were sterile and the tips of sterile stalks were flattened. The central stalks stood up vertically upward and they were longer in length.

When the ovules matured the stamens were shed, so in the strobilus in which the ovules were ripe the microsporophylls were absent but in young strobilus both were present.

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The microsporophylls or the stamens were 10 or 20 in number. These microsporophylls or stamens were all united at the base and each stamen was pinnate in form and on each stamen there were about twenty slender pinnae on either side, under the pinnae were developed two rows of fused sporangia or synangium had a short stalk and two pollen sacs in it; so each stamen was pinnate in form and was very much like the Marattious ferns in which we know the sporangia fuse to form synangia.

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When stamens were very young they rolled downwards; so on the whole we can say that the stamens of Cycadeoidea (Bennettites) were very much like those of ferns, while in living gymnosperms they have lost their resemblance with the ordinary ferns.

• The central portion was dome-shaped in form and this part was made up of a number of slender stalks, the central ones were long and stood vertically upwards, the lateral ones short and they diverged outwards. On the whole the female part was oblong in shape. Some of the stalks bore ovules while the others were sterile and their tips were expanded.
• The male and female parts of strobilus were separated by the presence of some sterile bracts or scales. The ovule was orthrotropous and terminal. It was small in size and was surrounded by three-layered testa like that of Cycas.
• The nucellus was spearate from integuments only in the upper part the integument projects forward to form a long micropyle.
• At the base of the micropyle the nucellus projected to form nucellus beak and round the base of nucellus beak there was a depression, the pollen chamber.
• On the outside of the ovule there was a small basal cup which suggests the cupule of Lagenostoma, but it was much reduced in size and never surrounded the whole of ovule or seed at any time.

• The ovule seems to fertilize by swimming sperms. After fertilization the ovule developed into a dicotyledonous embryo which was non-endospermic.
• When the seeds were developed the whole of the female part of strobilus became fleshy and formed a fruit.
• In ripe fruit holes were present on the surface and at the base of these holes was dicot embryo while the fleshy portion of fruit was formed by stalk and interspersed sterile scales.
• During the development of seed it appears that there was no suspensor developed. It might have been developed in the earlier stages of development of seed, but it is doubtful, so Cycadeoideales (Bennettitales) differ from other gymnosperms in these two important respects: 1. The non-formation of suspensor and 2. The presence of a non-albuminous dicot embryo.

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