APOSPORY IN PTERIDOPHYTES PDF

Herbaceous or woody and branched. Leaves: Leaves may be microphyllous e. Lycopodium, Selaginella, and Isoetes or megaphyllous. The leaves bearing sporangia are called sporophylls. In some cases sporophylls for compact structures called cones or strobili e. Selaginella, Equisetum 5.

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Introduction to Pteridophytes 2. Origin of Pteridophytes 3. Sporophytic Generation 4. Gametophytic Generation 5. Classification 6.

Economic Importance 8. Sexuality 9. Indian Work. Introduction to Pteridophytes: Pteridophytes constitute a significant and important group in the plant kingdom. As the first true land plants, they offer a very favourable material for the study of various adaptations that have made the colonization of land possible for the plants.

Pteridophytes have a long geological history on our planet. They are known from as far back as million years. Fossils of pteridophytes have been obtained from rock strata belonging to Silurian and Devonian periods of the Palaeozoic era.

In terms of duration in the life cycle the gametophyte is insignificant. In this respect pteridophytes are totally different from bryophytes. The reasons for the predominance of the sporophyte in the life cycle are not far to seek. Being a more robust body a sporophyte can effectively meet the challenges of a terrestrial environment better than the gametophyte. As a group, the pteridophytes lie between bryophytes and spermatophytes. They share the characters of both these groups in addition to their own unique features.

Origin of Pteridophytes: Problems concerning the origin and evolution of any plant group have not been solved to date, to the satisfaction of all. Problems of origin and evolution by their very nature give ample scope for speculation and imaginative interpretation.

Hence, it should surprise none, if the views concerning the origin and evolution of a group are as varied as the morphologists themselves. The problem of the origin and evolution of vascular cryptogams is no exception to this general rule. There have been various interpretations and perhaps many more are yet to come. A detailed analysis of all these theories is beyond the scope of this book. Basic outlines of a few of the more important theories concerning the origin of pteridophytes is given here.

According to one, pteridophytic ancestry is to be sought among bryophytes. According to the other, pteridophytes arose from some algal stock and evolved parallel with bryophytes. While the latter experimented with the gametophyte on a terrestrial habitat with little success, the former experimented with the sporophyte and achieved phenomenal success paving way for other terrestrial plants. Bryophytic Origin: Even among the proponents of bryophytic ancestry there is no unanimity as to the precise group which constituted the ancestral stock or the mode of origin.

There are four theories. Anthocerotean Theory: Campbell and Smith are the chief supporters of this theory. They believe that the sporophyte of Anthoceros has all the attributes of becoming the ancestor of land plants. Of all the members in bryophytes, according to Campbell , Anthoceros is the most advanced, in having a mechanism for indefinite growth, limited tissue for spore production etc.

Campbell , has reported that in one of the Californian species; A. Campbell compares the sporophyte of Anthoceros to the plant body of Rhynia and points out many similarities between the two.

The Green naked plant body, possession of a meristem assuring indefinite growth, presence of a columella remotely resembling the vasculature of land plants are some of the characters in the sporophyte of Anthoceros which support its claim of being the ancestral stock of land plants. Smith has further elaborated the anthocerotean hypothesis.

He traces the origin of Rhynia, from an Anthocerotean type stock, with some modifications Fig. He suggests if an anthocerotean type of sporophyte had a shifting of the meristematic region from the base to apex, this would make possible an initiation of dichotomous Fig. Smith extended his observations to the sex organs and opined that the sex organs of pteridophytic gametophytes have much in common with those of Anthoceroies.

The points of resemblance in the sex organs are as follows: Antheridia Fig. The primitive type of antheridium seen in eusporangiate forms like Lycopodium, Equisetum, etc. Smith compares the fertile cells of this eusporangiate type antheridium to the entire antheridium of Anthoceros and observes that the jacket of the antheridium is comparable to the roof of the antheridial chamber in Anthoceros. Evolution of the pteridophytic archegonium is the result of further reduction in antogeny. Since the archegonial initial in pteridophytes Fig ,c directly forms the axial cell, the entire archegonium is comparable to the axial row and cover cells of the anthocerotean archegonium.

Most of the botanists however do not favour an anthocerotean origin for land plants. According to him in a life cycle without alternation of generations, the zygote immediately undergoes reduction division producing haploid cells spores, zoospores, etc.

Hence the entire zygote is fertile i. Bower postulated the origin of a sporophyte antithetic origin from such a zygote by the postponement of reduction division to a later stage. Meanwhile the zygote divides mitotically to produce a diploid tissue which forms the sporophyte. Hence originally in a sporophyte all the tissues are devoted to spore production i. Subsequently by the progressive sterilization of the potentially sporogenetic tissue, a well differentiated sporophyte arose. Such sterilization is necessary, because for a free living sporophyte reproduction is not the only function.

It has to sustain itself first and only then comes the perpetuation of the race. Bower gave a series of examples from both liverworts and early pteridophytes to prove his point. Further, some of the fossils of early pteridophytes were quite complex and not simple strobiloid plants as Bower liked them to be. Phyton Theory: According to this theory the sporophyte was originally leafy i.

The axis arose later due to the fusion of leaf bases. The phyton theory proposed by Celekovsky is related to the strobilar theory in that at the stage where the sporophyte is like a strobilus a cluster of leaves — sporophylls, attached to a central core the plant is a cluster of leaves and there is no axis as such.

Subsequently the leaf bases fuse resulting in the formation of an axis. The pericaulome and leaf-skin theories are related to the phyton theory in that both these theories assume that a considerable portion of the axis is built up by the leaf bases. According to Eames , in the face of overwhelming evidence in support of the axial nature of the sporophyte, the phyton theory seems to be irrelevant. He states that the cauloid theory, an old theory which assumes the derivation of the modern sporophyte from a primeval cauloid, an axis seems best to fit the facts of history.

Protocorm Theory: This theory was put forward by Treub who regarded that basically a primitive pteridophytic sporophyte was an undifferentiated mass very much like a gametophyte.

He cites the example of the protocorm in some species of Lycopodium as an evidence to the protocormous origin of the sporophyte. He regards the protocorm to be a vestige of an archaic sporophyte and a transitional stage recalling an earlier step in the evolution of the sporophyte.

Another evidence cited by Treub , is the similarity between the protocorm and the adult plant body of Phylloglossum, which according to him is a permanent protocorm. But it has to be conceded that the so called protocorm is present not in the primitive species, but in the more advanced species of Lycopodium.

Detailed investigations conducted on Phylloglossum have revealed that its simplicity is not due to primitiveness but due to reduction. Algal origin: The current trend seems to favour a direct algal origin for vascular plants. But even here, there is no consensus as to which group among algae could be regarded as the ancestral stock. Below is given a few hypotheses of some prominent botanists. He believed in a polyphyletic origin. According to him a hypothetical group of marine algae called thallassiophyta brown algae formed the ancestral stock for all land plants.

The main points of his theory are as follows: 1. The surface of the earth in the remote past was completely enveloped by a common ocean. There were many kinds of marine plants which were mostly planktonic free floating. Due to geological changes there was upheaval of the ocean bottom. Due to the emergence of the land, planktonic forms changed to benthic fixed forms. The new environment terrestrial gradually introduced all the adaptations like roots, leaves, vasculature etc. There was in fact the land first and only later oceans came into existence.

Another demerit of this theory is the non-homology of marine algae and early land plants in their pigmentation. If land plants indeed arose from brown algae they should have at-least had some traces of the brown pigments. Scott , supports Church and states the discovery of Rhynie plants goes to show that they evolved from a fairly highly evolved group of algae.

He derived mosses from chlorophyceae and liverworts from the phaeophyceae. Similarly Rhynia and Horneophyton were derived from chlorophyceae while Psilotum and Temesepteris were derived from phaeophyceae. All these based only on the branching pattern without any regard to the phylogenetic relationships.

Occurrence of such plants made Andrews to believe that several algal groups independently attempted the invasion of the land. These groups gave rise to different groups of vascular plants. He believes that the morphological diversity exhibited by psilophyta, lycophyta etc. According to her, land plants must have had their origin somewhere during the Precambrian era. This assumption is supported by the discovery of fossil spores probably of land plants in the rock strata belonging to ordovician and cambrian periods.

She considers the simple psilophytes like Rhynia as the descendants of complex race that existed prior to the Devonian period. Axelrod supported the polyphyletic origin of Leclercq and elaborated it by his own palaeopalynological discoveries. Merker also believes in the algal origin. He recognises five main evolutionary lines: 1 Rhyniaceae, Psilotaceae and ophioglossaceae, 2 Bryophytes, 4 Lycopsida and 5 Pteropsida including pteridosperms.

He also agrees with Leclercq that Rhyniaceae are simple due to reduction and not due to primitiveness. The two independent lines are psilopsida and lycopsida which arose from Thallophyta independently somewhere during the Cambrian period.

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In botanyapomixis was defined pteridohpytes Hans Winkler as replacement of the normal sexual reproduction by asexual reproductionwithout fertilization. The cause of apogamy include ageing of the prothallus, failure of sex organ formation and normal fertilization, prothallus growing under bright light and high temperature etc. Apospory is the development of unusual diploid gametophyte from the vegetative parts of sporophyte and not from the spores. The sporophytes of plants of these groups may also have the ability to form a plant that looks like a gametophyte but with the ploidy level of the sporophyte, a phenomenon known as apospory. Apospory and apogamy in A nitida. Biological Apowpory of the Linnean Society.

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