Some pteridophyte groups, including the club mosses and horsetails classes Lycopodiopsida and Equisetopsida , have simple microphyllous leaves, featuring a single, unbranched vein and modest vascular supplies that do not cause breaks or gaps in the stem vasculature. The true ferns class Filicopsida , however, have larger, more complex macrophyllous leaves whose veins are usually extensively branched, placing such large demands on the plant's vasculature that distinctive gaps form in the xylem and phloem of the stem.
All pteridophytes have a true alternation of generations, in which a dominant sporophyte generation produces spores through meiosis , and a free-living gametophyte generation forms gametes egg and sperm by mitosis.
Ferns can be used to illustrate the life cycle stages common to all pteridophytes. Diploid 2n fern sporophytes are familiar to most people and are often found as quiet accompaniments in floral arrangements. When mature, the undersides of fern leaves produce clusters of capsular structures called sporangia, within which meiosis forms the haploid n spores. These spores are released from the sporangia, often when dry wind currents cause the active snapping of the capsules, lofting the spores into the air.
Spores that are wind-borne to shady, moist habitats germinate and yield multicellular, but microscopic, gametophytes, the sexual stage of the life cycle. The stem is generally branched; either dichotomous or monopodial. The leaves may be simple, small and sessile e. The leaves and stems, in most of the cases, are provided with filiform trichomes. The nature of stele varies in different groups. Pteridophytes are non-flowering seedless vascular plats.
Sporophyte is the predominant plant body, differentiated into root, stem and leaves. The stem is generally branched either dichotomous or monopodial. The primary roots are ephemeral and are soon replaced by adventitious roots. Pteridophytes are polysporangiate, either homosporous or heterosporous.
Presence of multicellular sex organs i. Water is essential for fertilisation where flagellated sperms swim over a thin film of water and are attracted chemotactically towards the archegonium. The first division of the zygote determines the polarity of the sporophyte. Reproduction in Pteridophyta: i. The sporophytic plant reproduces by means of spores produced in the sporangia singular: sporangium. Sporangium: i. They may be borne on the stems i.
The sporangia containing leaves are called sporophylls. The sporophylls may be scattered e. In some aquatic pteridophytes the sporangia are present within a specialised structure, called sporocarps e. On the basis of mode of development, the sporangia are of two types viz.
In some forms e. The forms showing such condition are grouped together as simplices Fig. The forms showing this condition is called Gradatae Fig. Spores: i. Meiotic reduction divisions of spore mother cells produce numerous haploid spores inside the sporangium. If all the spores produced are of equal sizes and shapes, then the plant is called homo- sporous e. In the heterosporous type, the two different types of spores are produced in separate sporangia.
The microspores are produced in large numbers. The larger spores which are produced in smaller numbers are termed megaspores and are developed in megasporangia. Microspores, after germination, produce male gametophyte, while megaspores produce female gametophyte.
Sporophylls with megasporangia are called megasporophylls, while sporophylls with microsporangia are called microsporophylls. Gametophyte: i. The spores germinate to form haploid game- tophytes or prothalli. The gametophytes of pteridophytes are small and inconspicuous as compared to the sporophytes.
The germination of spores in the homosporous forms are of three types. The gametophytes are of two types. In homosporous forms, the development of gametophyte is exosporic in which the pro- thallus develops outside the spore wall e.
Gametophytes that develop from heterospores are endosporic in which the development of prothallus is confined within the spore wall e. The development of endosporic gametophyte is independent of external environment.
Sex Organs: i. The gametophytes or prothalli singular — prothallus bear the sex organs viz. Antheridium: i. The antheridium is a sessile or shortly stalked globular structures surrounded by a well-defined jacket inside containing androcytes or antherozoid mother cells. Each androcyte gives rise to a single motile antherozoid. Archegonium: i. The archegonium is a flask-shaped structure consisting of a basal, swollen venter and a short neck, the venter is embedded in the prothallus while the neck is projected.
The venter encloses an egg and a ventral canal cell. Neck is made up of vertical rows of neck cells with neck canal cells inside. At maturity the neck canal cells disintegrates to form a passage for the antherozoids to reach the egg. Fertilisation: i. The disintegration of neck canal cells also produces a mucilagenous substance which contains organic compounds like malic and fumaric acid.
These substances act as sperm attractant. Such plants get disperse via spores and reproduce rather than seeds. Bryophytes are used for medicinal purposes. These plants are found in moist, dark, shady, cool, damp area. As these plants produce gametes, they are called gametophyte.
The plant structure is well differentiated in roots, stems, and leaves. Few examples are Spikemosses, clubmosses, ferns, quillworts. There are around 13, species of pteridophytes which makes them representative of the closest relatives of the angiosperms, the conifers, and other seeds plants. Key Differences Between Bryophytes and Pteridophytes The body structure of bryophytes has leafy or thalloid plant body, while in pteridophytes plant body in differentiated into roots, stems, and leaves.
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. Psilopsida gave rise to three groups viz. The last mentioned group gave rise to protoangiosperms and angiosperms. The lycopsid stock gave rise to coniferopsida. There are mainly two objections to this hypothesis: 1 It is stretching the imagination a little too far if one has to believe a lycopsid origin for confideropsida and 2 Inclusion of Casuarina under gymnosperms is untenable.
Mehra the ancestors of land plants are to be found among the green algae. He opposed the polyphyletic origin of vascular plants. While agreeing that the different groups of pteridophytes diverged from the very beginning, he argued that all of them have come from a common group.
Mehra , envisages the origin of a hypothetical group Protoarchegoniatae from the chaetrophoraceous ancestors. From these protoarchcgoniates two lines viz. Epilogue: In-spite of the innumerable theories proposed, it may be said that the problem remains as before i. However, it seems quite possible that bryophytes and Pteridophytes evolved parallely from some algal ancestor which in all probability could be a green algal stock. Sporophytic Generation of Pteridophytes: It is the sporophytic generation which constitutes the main plant body in pteridophytes.
In any description whenever we refer to the plant body of a ptcridophyte, we mean the sporophyte. There is a great variety in the nature and organisation of the sporophytic plant body in pteridophytes. The simplest sporophyte in an extant member is to be found in Psilotum. The plant body here is a naked, branched axis with no evidence of roots. The axis is distinguishable into an underground prostrate system and an erect aerial system.
Rhizoids help in anchoring the plant to the substratum. There are no leaves. The stem itself takes up the function of photosynthesis. From such a simple plant body further evolution mainly necessitated by the environment resulted in the complex and diverse types of sporophytes that we see today.
The first step is the differentiation of photosynthetic laterals leaves and roots. The leaves of pteridophytes are basically of two types viz. Microphylls have a single un-branched mid vein.
Further, when a leaf trace departs from the main vascular cylinder to provide the leaf, no gap is left. In megaphylls there is a branched venation. The branching may be dichotomous or reticulate. Further, there will invariably be a leaf gap in the main vascular cylinder above the leaf trace.
Microphylls are seen in members like Lycopodium, Selaginella etc. Megaphylls are found in members like Adiantum, Pteris, etc. Stomata: These are found in the leaves and stems of all members. Mehra and Soni have studied the stomatal structure and ontogcmy in a number of pteridophytes. They have classified the pteridophyte stomata into the following four types: i. Psilophytaceous iii. Marattiaceous and iv.
Filicinean Mehra and Soni believe that the psilophytaceous stoma is the basic type and the others are derived. Origin of Leaves: According to general opinion, the fundamental nature of the sporophyte was mainly axial. The leaves and branches arose later as a modification of the axis. Bower first suggested that originally the plant was wholly axial in nature and that the leaves arose later in the course of evolution.
Lignier proposed that a primitive pteridophytic shoot was basically a leafless dichotomously branched structure and some of the branches got themselves modified into leaves. Mainly there are two theories to account for the origin of leaves. These are the telome theory and the enation theory. While telome theory accounts for the origin of both microphylls and megaphylls, enation theory explains only the origin of microphylls.
Origin of Megaphyll: According to telome theory first proposed by Zimmermann , , primitive pteridophytic shoot was naked and dichotomously branched. The plant body resembled the fossil member Rhynia. The ultimate dichotomies are called telomes and the internodes below the telomes are known as the mesomes. The telomes and mesomes together constitute a telome truss. According to telome theory, a megaphyllous leaf originates as per the following steps: 1.
Overtopping: In this, the original dichotomous branching system changes to unequal dichotomy resulting in the formation of short and long branches. This leads to sympodium and ultimately to a monopodium with a main stem and lateral branches Fig. Planation: The branching of the telome trusses which were originally in all directions orient themselves in a single plane and come closer Fig. Syngenesis or Webbing: The telomes which have come closer, laterally fuse by the development of parenchymatous tissues between them Fig.
This results in a leaf blade possessing a number of free ending veins. Evidence for this theory is obtained from many fossil ferns where the leaves have an open venation. A further evidence is in the ontogeny of the fem leaves where the adult leaves have a closed venation while the first formed ones have open veins. Origin of Microphyll: The origin of a single veined leaf is explained by both the telome theory and the enation theory.
Followers of the telome theory argue that the leaf arose from a surviving telome. After overtopping and planation, only one of the telomes survived reduction , the remaining degenerated. The surviving telome developed parenchymatous pads which formed the lamina.
There are however some objections to the telome theory accounting for the origin of the microphyll. These are: 1. If indeed a telome has transformed itself into the central vein of the microphyll, then in all instances the microphylls should possess a complete vein running to the tip. But there are any number of examples of microphylls having no veins or incompletely developed veins.
This is indeed difficult to comprehend as per the telome theory. It is one of the morphological principles that an organ, develops first and only later the vasculature travels into it. If we accept the telome theory for the origin of microphyll we will be arguing for the emergence of vasculature first and then to justify the vasculature, the development of the leaf.
According to Enation theory proposed by Bower , the leaves are not the modifications of telomes branches , but new developments or outgrowths from the shoot. According to Bower the outgrowths which he called enations were first spine like, later became flattened and leaf like. In the beginning these were only emergences and lacked a vasculature.
Subsequently the main vasculature of the stem gave out a branch which ran up to the base of the leaf only. From this stage by further evolution a vascular strand grew up to the leaf-tip. Indeed there are quite a good number of fossil evidences depicting these stages.
In the evolution of the vasculature, Psilophyton a fossil member of the order psilophytales is quoted as the first step no vasculature , Asteroxylon, the second step vasculature only up to the base of the leaf , and Arthrostigma, the third step vasculature traversing half way up the leaf apex.
In the present day Lycopodium and Selaginella the vasculature runs up to the leaf apex. Regarding the origin of the microphyllous leaves enation theory seems to be more convincing than the telome theory. Origin of Equisetaceous Leaf: It is difficult to account for the origin of the equisetaceous leaves because of their whorled condition. Neither telome theory nor enation theory can explain convincingly the origin of whorled leaves.
Members like Sphenophyllum a fossil member of sphenopsida had sessile wedge shaped leaves with dichotomous venation. Such leaves may be modifications of a telome. The single veined leaves found in Equisetum and others, may be a reductional form. Origin of Roots: According to Zimmermann some of the branches of the fundamental axis grew downwards thus forming the root system.
In some primitive members, in the type of branching and mode of growth, the root and the aerial shoot are similar indicating that they are modifications of the same axis. Zimmermann believes that the roots differentiated before the origin of leaves. Development of Root and Shoot Apex: The stem and roots usually develop with the help of a single two or three sided apical cell.Pteridophytes Photo by: Andrew Buckin Pteridophtyes are a phylum writer plants. Michael are the vascular example those pollan xylem and phloem tissues that reproduce by releasing spores rather than structuring an essay argumentative writing, and they include the highly diverse true write and other graceful, primarily forest-dwelling plants. There are about eleven thousand different species of pteridophytes, pteridophytes them the essay diverse land plants after the flowering plants angiosperms. Pteridophytes may bother the closest living relatives essay group to the seed plants. Seed plants include the angiosperms, the conifers, and a why assortment of other plants.
Similarity of tissues parenchyma in the pith and cortex is an evidence for cortical intrusion. In megaphylls there is a branched venation. They are the vascular plants those having xylem and phloem tissues that reproduce by releasing spores rather than seeds, and they include the highly diverse true ferns and other graceful, primarily forest-dwelling plants. The lycopsid stock gave rise to coniferopsida. According to telome theory, a megaphyllous leaf originates as per the following steps: 1.
Economic Importance 8. Subsequently the main vasculature of the stem gave out a branch which ran up to the base of the leaf only. There is also a third category of solenostele called polycyclic solenostele in which there will be more than one concentric ring of vasculature Fig. Equisetum accumulates minerals, especially gold, in their stem. Plant and spore surfaces are covered with cutin cuticle. Spore production: The characteristic method of reproduction is by the formation of spores.
If we regard that a siphonostele is more advanced then a protostele and that the former evolved from latter, we have to account for the origin of the pith. Further development of embryo results into a well-developed sporophyte differentiated into roots, stem and leaves.
The phloem is composed of sieve tubes and phloem parenchyma. Strobilar Theory: Bower , was the propounder of this theory. Phyton Theory: According to this theory the sporophyte was originally leafy i. Cell are haploid type in bryophyte, and diploid cells are present in pteridophytes No true stems or leaves are present in bryophytes, whereas pteridophytes have true stems and leaves. The function of the suspensor is to push the developing embryo deep into the gametophytic tissue so that it can easily absorb nutrition. There was in fact the land first and only later oceans came into existence.
In this respect pteridophytes are totally different from bryophytes.