Chapter 1 Evolutionary Trends and Plant Groupings Evolutionary trends, Monophyletic and Polyphyletic origin, Premanera, Evolutionary groupings of living organisms, Five kingdom system of Classification, Origin and Evolution of different plant groups, Evolutionary relationships.

EVOLUTIONARY TRENDS There has been always a dispute among biologuts in tracing the possible origin of cells. Did early cells arise from a single first cell or did numerous first cells originale inde pendently, te. whether they evolved monophyletically, from a single common ancestral stock or polyphyletically, from several separate ancestral stocks Monophyletic versus Polyphyletic origin The proponents of monophyletism beiseve that cells are so complet that multiple origins of such complexities are statistically quite unlikely. They propose that all life started in a single place at a single time from a single cell which formed by lucky accident. They seek the support of homology argument that all cells are fundamentally so remarkably alike in chemical composition, in microscopic structure and in Function that a common origin from a single ancestry in clearly evident. The proponents of polyphyletism propose a probability argument in their own way They state that original cells could not have been so complex and structural complexitics must have evolved subsequently in gradual steps. Thus, cell formation was not simply a lucky accident and its one time occurrence would have a very remote probability. Further they extend support from an analogy argument and believe that all aggregates that actually became living cells must have shared the same broad structural and functional charac teristics. Le all living cells must have contained atleast water, various mineral subuances adenosine phosphates, carbohydrates, fats, proteins and nucleic acids. The aggregations which have had these structures would have been abl, to holotrophism. All these food gathering procedures collectively made the heterotroplus forms of nutrition which can not be endless. Under these circumstances, premoneruns would have been forced to evolve means of utilizing external energy in the production of organic compounds. And the two categories of such autotrophic nature came into existence the chemaronment and the photorynthesis It becomes apparent that sooner or later after the appearance of first cells, five proce dures for obtaining the food evolved, the parasitiam, the saprotropphim the kolotroph the chemosynthesis and the photosonthesis Out of these photosynthesis occurred to an ever-increasing extent and it brought about far reaching changes in the physical environ ment such as capability synthesising enormous food and changing the environment from anaerobic to acrobic by ynthesising free molecular oxygen as by-product of the process Comprehensively it brought profound oxygen evolution on earth which provided conduc tive environment for structural revolution. Advanced Botany EVOLUTIONARY GROUPINGS OF LIVING ORGANISMS German biologist E. Haeckel (1866) was perhaps the first to consider all living or ganisms for their collective classification Till then two separate kingdoms were recog nised, the plantar and the animalia. It is now believed that a division of living world merely into plants and animals is too simple. It does not take into account the gradual evolutionary development and it provides no place for primitive organisms which still are neither plant nor animal or which are both An attempt has been made to establish alternative clasifications that do reflect our present knowledge of evolution one such alternative scheme is to recognise nor two but four main categories, each of which has a taxonomic rank equivalent to a kingdom. These are Monera, Protista, Metaphyta and Metazoa. The Monerans are characterised by a uni que cell structure that does not include an internal membrane around the genes of or ganisms (procaryotes) and these include bacteria and blue green algae. Protista do posen internal membranes around the genes in their cells (eucaryotes. These are simple or ganisms that include algas, fungi, slime molds and protozoans. These are either unicellular showing both plant and animal like characters or multicellular without having cellular dif ferentiation Metaphyta includes plants having cellular differentiation and Metarna in cludes animals having cellular specialization or differentiation The four main categories e organisms and their probable evolutionary interrelations are shown in fig. 1.1 Monera Protista Metaphyta Metazoa Fig. 1.1. Origin and evolutionary patterns of four broad cargories of the organить With advancement in the knowledge of cell ultrastructure Stainer and Van Neil (1962) proposed that all living organisms on the earth are composed of one or other of the two types of cells, the prokuryotic and the eukaryotic. The differences between prokaryotic and eukaryotic cells are given in table 11 No. Table 1.1. Differences between Prokaryotic and Eukaryotic Cells