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Diversity of Microbes and Cryptogams

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Diversity of Microbes and Cryptogams Diversity of Microbes and Cryptogams Pteridophyta Prof. S.P.Khullar (Retd.) Department of Botany, Panjab University Chandigarh - 160014 Significant Keywords: Pteridophytes, fossils, stellar system, classification, Psilopsida, Rhynia, Psilotopsida, Psilotum, Sphnopsida, Equisetum, Lycopsida, Lycopodium, Selaginella, Pteropsida, Pteris,Marsilea, Apogamy, Apospory, Reproductive Biology, Polyploidy, Gene-block hypothesis. Legends to FIGURES Fig. 1. The Geological periods of the Earth. Fig. 2. The stele in Pteridophytes, A. Protostele, B.Actinostele, C. Plectostele, D. Siphonostele, E. Solenostele, F. Polycyclic Solenostele, G. Polycyclic stele, H. Dictyostele. Fig. 3. Reconstruction of Rhynia gwynne-vaughani ( A. after Kidston & Lang; B. after Edwards; C. after Banks). D. T.S.Stem. Fig 4. Reconstruction of Agalophyton (Rhynia) major. A after Kidston & Lang.; B after Edwards); C. Longitudinal section of sporangium; D, E. Lynophyton rhyniensis, the supposed gametophyte of Agalophyton major (after Remy & Remy, 1980). Fig 5. Psilotun nudum Fig. 6. Psilotum nudum. A-C Transverse sections of : A. Rhizome; B. Stem; C. Distal portion of stem. D. Gametophyte. Plate 7. Psilotum nudum. A-F. Development of Archegonia. G. Antheridium. H-J. Embryogensis. Fig. 8. A. Equisetum diffusum; B. E. arvense; C. E. ramosissimum; D. A field of E. arvense. Fig. 9. Equisetum. A. Young cone; B. Mature cone; C. Spore; D. Sketch of a spore to show the elaters; E. Sporangiophore; F. Gametophyte. Fig. 10. Equisetum. A. T.S.of aerial internode of Equisetum ramosissimum showing an outer and an inner endodermis; B. internode, C-D. Endodermis in E. debile. E. Path of the vascular supply in the stem Fig. 11. The stele in various species of Lycopodium. A. L. selago Type; B. L. clavatum Type; C. L. cernum Type. D. L. squarrosum Type. Fig. 12. A. Lycopodium setaceum; B. L. hamiltonii, C. L. cernuum D. L clavatum. Fig. 13. Lycopodium: Sporophylls in various species. A. L. selago; B. L. inundatum; C. L. complanatum; D. L. clavatum Fig 14. Gametophytes in various species of Lycopodium. A,B L. selago; C.L. cernuum; D. L. clavatum Fig. 15. Lycopodium. A-D. Embryogensis; E. Protocorm. Fig. 16. Selaginella chrysocaulos. Fig. 17. The various types of stele in Selaginella. A. S. spinulosa Type (rhizome); B. Stem; C. S. chrysocaulos; D. S. uncinata Type; E. S. willdenovii Type; H-J. S. lyalli Type. Fig. 18. Selaginella. A. Microspore; B-I. Development of the male gametophyte; J. Megaspore (photograph kind courtesy Prof R. Mukhopadhya, Burdwan, India); K-Q. Development of the female gametophyte; R-W. Embryogenesis. Fig. 19. Pteris vittata Fig. 20. Pteris vittata A, B. Close up to show the marginal sori; C. Mature Prothallus D.Gametophyte loaded with only antheridia Fig. 21. Pteris vittata: development of the Antheridium. Fig. 22. Marsilea minuta; A . Plant growing on land; B.Plant with Sorocarp. Fig. 23 Marsilea A. Sporocarp; B Path of Vascular supply; C Receptacle with two lateral microsporangia and a terminal megasporangium; D-H Development of male gametophyte; I.A developing megaspore; J-M Stages in the development of the sterile tissue and archegonium of a megaspore. 2 Pteridophytes (Greek pteris – pteron = a feather; phyton = a plant; i. e. plants with a feather-like appearance). It is now generally believed and accepted that life has existed on earth for over 3000 million years. For most of this time it was restricted to water where it flourished and evolved. But nothing is permanent and nature always desires a change. Thus from the very favorable and luxuriant environs of water, for reasons unknown, life decided to invade land. It was only 400 million years ago that the first multicellular plants left water to invade the much more hostile environments on land. Here these first land invaders had to face severe and harsh conditions like extremes of temperature, strong winds and dust storms and erosion of the land surface. There was a general shortage and scarcity of water, resulting in less or its complete non availability. Further periods of dry drought alternated with wet monsoons ones, and wet-lands with arid regions. In spite of these unfavorable conditions, life not only managed to survive out of water, but evolved and rapidly spread over the barren land in the absence of competition. To overcome the hardships on land, plant life developed some necessary adaptations to survive here. Some of these basic features were: i) A system of anchorage or fixation (to the substratum) to prevent being blown away, but more importantly, to absorb water and other nutrients from the soil. ii) A transportation system- the vascular system composed of specialized cells (to transport food, water and nutrients from where they were available to where they were not available) and also to enable the plants to withstand the strong winds and be able to stand erect. iii) A covering of desiccation resistant cuticle, to prevent evaporation and loss of water from the surface but perforated with small holes (stomata) to allow for the exchange of gases (oxygen and carbon dioxide) between plants and air. iv) The change from a haploid gametophytic dominant phase to a diploid sporophytic dominant phase. The haploid gametophytes are generally delicate tiny little plants which apparently have little possibility of survival under the conditions prevailing on land. Therefore, for successful colonization on land, during the course of evolution, a diploid sporophytic constitution must have been necessitated under Natural Selection. This situation had further advantage and survival value in throwing up various recombination of characters for Natural Selection to operate. These features enabled plant life not only to survive out of water but to spread on land. Plants that had these essential features were tiny plants, at first only a few centimeters tall, without leaves and roots but only a stem system that was forked or simple and terminating in structures (sporangia) that produced spores not seeds (a spore differs from a seed in not having an embryo) for reproduction and survival (to overcome the unfavorable weather conditions like drought or very low or very high temperatures). Such simple land plants were essentially Pteridophytes. They spread rapidly along the shores and river banks. Without any visual competition for new habitats and with their simple genetic make up, a very rapid evolution was stimulated and witnessed amongst these invaders of land. So fast was the rate of evolution that within 100 million years or a little more, these simple plants had evolved to such an extent and at such a rate that some of the major groups of vascular plants like ferns, equisetums, lycopods, amongst the pteridophytes and cycads, maiden-hair trees and conifers amongst gymnosperms had evolved. The maximum growth of such plants was during the Carboniferous and Permian periods, when the forests of the world were composed mainly of Pteridophytes- Lycopods, giant Equisetum and Gymnosperms (mainly 3 Cycads).The fossil record suggest that early land plants (bryophytes and vascular plants) established in terrestrial environments many times. How and when the necessary steps involved in the transition for an algal life in water to that of land took place will continue to intrigue and be an enigma clothed in mystery. The main features of the Pteridiphytes (always follow this sequence) are: i) These plants have an independent gametophyte and an independent sporophyte. (This is in sharp contrast to the bryophytes where the sporophyte is a parasite on the gametophyte; and the Gymnosperms and Angiosperms where the gametophyte is a parasite on the sporophyte). ii) The dominant phase in the life-cycle is the sporophyte. iii) This was the first group of vascular plants to invade the land. iv) This was the first group to have a vascular system (containing xylem and phloem). v) The xylem is mainly composed of tracheids. True fibers and vessels absent. vi) Secondary growth is absent in the living pteridophytes but was present in the extinct forms. vii) They do not produce seeds but instead have spores. Pteridophytes have had a long history on the earth. They probably had their maximum development during the carboniferous and started dwindling in numbers and luxuriance thereafter, till the present times when other than the ferns, only seven living genera are now available. These are: Psilotum, Tmesipteris, Equisetum, Lycopodium (in the conservative sense), Phylloglossum, Selaginella and Isoetes). The rest are extinct and represented by fossils. Fossils are the preserved remains of organisms that were once living on the earth but are now extinct. The study of plant fossils is known as Paleo botany. Fossils are present in different strata of the earth and their age is calculated by either counting the number of strata or through other means. Formation of fossils: Organic substances rot and decay due to the action of micro- organisms. If this activity can be prevented the organism can be preserved. Fossils may be formed by any of the following methods: i) Impressions: When an organic substance comes in between soil layers, it decays there but leaves behind its impression. Nothing of the original tissue of the organism is left behind. ii) Casts or molds: Any hollow portion may get filled up by organic substances and later soil gets in there. The organic outer portion decays and perishes but an impression or cast of it is left behind. iii) Compressions or Mummifications: Organic substances get carbonized, become dull and slowly turn to coal (under anaerobic conditions), or at times they may get carbonized, become dull and get mummified (distortions in size etc. may occur and correct results may not be obtained through such fossils). iv) Incrustations: (In all the above cases nothing of the original tissue of the organism is left behind). Minerals which happen to be present in a super saturation condition, in lakes, ponds, springs etc., get deposited layer by layer on any organic substance which happens to fall in such water, thus preserving it as an incrustation.
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