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  1. EVOLUTION IN FLOWERING PLANT

    Saturday, January 14, 2012

















    This is just a biology essay as my assignment in higher plant subject, hehe.
    hope it can be usefull for all.


    I believe that everybody like flowers, except some people who already have tendency to get allergy with that kind of powder or pollen. generally, people like the flowers with colorfull petal, and good smell. They will use it to decorate their house, or give the flowers to their girlfriend. But, have you ever thinked about what exactly make flower so distinct? What do they have that no other plant have? How was their shape in over-past period? This is very interesting to know what happened with this flowering plant. How was their history and evolutionary from past period to present.
    Actually, the flowering plants or angiosperms emerged in the Cretaceous period, some 130 million years ago. Angiosperm means "contained seeds" and unlike the previous seed plants examined, the ovules are sealed within the carpel and the seeds sealed within a fruit. Flowering plants (angiosperms) comprise about 90 percent of the Kingdom Plantae. The total number of described species exceeds 250,000, and many tropical species are as yet unnamed. Virtually all flowering plants produce some type of functional floral organ, although in some families such as the Lemnaceae, the flowers are microscopic and are seldom seen by the casual observer. Certain grasses and specialized cultivars apparently do not produce flowers, although they may still have rudimentary flowers (vestigial floral parts). The common connecting link between all flowering plants is that they produce sexual reproductive organs called flowers. A flower is composed of 4 major parts: petals, sepals, stamens and one or more pistils . The number, shape, size, and arrangement of these floral parts varies considerably with different plant families, and is reflected in the tremendous diversity of flowering plants.

    The perianth of angiosperms has undergone principally four different kinds of change. Such as reduction, particularly the suppression of the corolla and sometimes also the calyx . Differentiation of calyx and corolla. Intercalary concrescence,producing “fusion” of sepals or petals and “adnation” of stamen filaments to the corolla tube. and change of symmetry, from radial to bilateral, producing zygomorphy or “irregular flowers”. As has already been mentioned, each of these trends has occurred independently in many different evolutionary lines. The first stage of reduction in the perianth is from indefinite numbers of parts, spirally arrange to definite number in distinct whorls. This stage occurred very early in the evolution in the angiosperm. The second stage of reduction, suppression of the corolla or of the entire perianth,has taken place chiefly in association with the shift from insect to wind pollination. More instructive are examples of families in which genera exist, some of which posses a corolla whereas others are apetalous. For examples are family of Ericales, the loss of petals is associated with the shift from insect to wind pollination accompanying the entrance of the group into a region with harsh climate, where insects are scarce and high wind prevail. 
    A perianth that is differentiated into “outer” sepals and “inner” petals has probably originated by still another pathway in at least one family. Most genera of Portulacaceae have a perianth consisting of two sepals and five petals. The “fusion” of sepals to form a synsepalous calyx and that of petals to form a sympetalous corolla have both occurred repeteadly in many different evolutionary lines. On the basis of any hypothesis that postulates some internally controlled tendency toward union of parts, or some factors that cause different parts to be come more specialized in association with one another so that the more specialized condition are necessarily correlated. Actually, this combination is difficult to understand. On the other hand, all condition that exist in above mentioned families migh be explained on the basis of adaptive modification along the lines of least resistance. Selective pressure on the calyx would be expected to be very different from those acting on the corolla, because of their very different function. The function of calyx is protection of young floral organs agaimst drought and cold as well as against the attack of predatory insect, and the function of corolla is both attraction of animal pollen vectors and the efficient accomplishment of pollination. Functionally, cup or wheel shaped flowers having the petals united only at their bases, as in Ericaceae, Gentianaceae, and Primulaceae, are little if any different from flowers having similar shapes but with the petals completely separated. This condition may have arisen simply by the chance fixation of genes favoring an intercalary basal meristem in corolla.
    The intercalary concrescence, therefore might be simply the result of gene acting to promote growth and greater adult size, and exerting their effects upon a floral meristem in which localization of growth substances had been largely suppressed because of its small size. This kind of pleiotropic action of genes, although it has not yet been demonstrated to exist, is mentioned because experiment could be devised to find out wheter or not in present. 
    For the origin of zygomorphy, the shift from radial to bilateral symmetry of flower has taken place independently in an even larger number of separated evolutionary lines than have other changes in perianth structure. The predominant selection pressure that favors the origin of zygomorphy has already been discussed before. In a plant that bears flowers in terminal racemes or other elongate inflorescens that have most of individual flowers placed laterally, adaptation to specialized pollinators usually involve placing the lower petals or corolla lobes in such a position that they form a “platform” upon which the pollinator can land, and often the upper ones in such position that they are associated with the stamens and stigma, orienting these structures in a way that ensures efficient cross-pollinator. Terminal racemes that bear laterally positioned but radially symmetrical flowers are rather frequent. On the other hand, zygomorphic flowers not always occure in families having other genera that bear zygomorphic and laterally placed flower for examples Mimulus primuloides, and Limosella aquatica. These species have probably been derived from ancestor that had zygomorphic flower position. 

    In the evolution in stamen, the chief trends in androecioum of flower have been 1. Elongation of stamen filaments through increased activity of individual intercakary meristem. 2. Change in stamen number 3. Sterilization of stamen through suppression of the differentiation of archesporial tissue. 4. Intercalary concrescence of filaments to produce stamina columns 5. Adnation of stamen to the corolla. All of these trends have occurred independently in several different evolutionary lines. Trends toward reduction in stamen number are far more common than trends toward increase. This is an expected result of floral evolution based upon interaction between flowers and their pollinators. 
    For pollen grains, on account of their excellent fossils record, pollen grains have recently received much attention from botanist interested in angiosperm evolution. The original angiosperms probably had pollen grains containing a single furrow in the middle of which was a single pore persist in monocotyledons, but in the dycotyledons it is confined to the primitive Magnoliales and few other primitive or archaic families. The shift from one furrowed (monocolpate) to the three furrowed (tricolpate) condition cannot be recognized within the limits of any modern family. Possibly, the presence of three germ pores enables the grain to germinate promptly no matter in what position it may lie when deposited on the stigma. If the stigma were large or deeply grooved, the single pore of a monocolpate grain would be placed in position favourable for germination .In considering adaptive shift of pollen grain structure , one must recognize that selection is operating upon genetic different level than with respect to most other characters. 
    Evolution in gynoecium, the most signivicant trends of evolution in gynoecium have been union of carpels, shift in the nature of placentation of ovules, and the evolutionary of epigyny or the inferior ovary through adnation of the gynoecial walss to the calyx, the receptacle, or both of these structure. In the great majority of angiosperms the gynoecioum, through either its multicelullar structure, the preence of two or more styles stigma, or its vascular anatomy , shows that it has been derived from ancestral condition in which several separate carpels were present. This ancestral apocarpous condition survives in most of the Magnoliidae, Dilleniales, Rosales, and few other isolated groups of monocotyledons. Nevertheless, careful comparison between members of these families that resemble them with respect to a large number of diverse characteristic indicates that they have followed the evolutionary oathway of carpelllary union followed by reduction. Carpellary union has occurred independently in many different evolutionary lines. 
    For the placentation evolution, it have been well reviewed from anatomical view point by Puri (1952). That in apocarpous forms marginal placentation was the primitive condition, and that following the union of conduplicate carpels this led to primitive sutural placentation in compound gynoecia, from which all other forms of placentation have been derived. 

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