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of vegetation at goon, , scales., 10.5 DISPERSAL OF PLANTS, , al of plants refers to migration, and spreading of plants from the places of their, origin due to a host of factors. The present-day, distribution of world vegetation communities 1s, the result of gradual dispersal of plants through, ages of geological history. The study of, dispersal of plants includes the consideration of, nature and factors of dispersal, patterns of, dispersal in historical perspective, role of man, in plant dispersal ete. These aspects are, eleaborated below :, , Dispers, , 1. Nature and Factors of Dispersal, , The present-day distribution of plants, over the globe is the result of migration of plants, over time into ecologically suitable areas. The, spread or migration of plants takes place mainly, intwo ways e.g. (i) through vegetative propagation,, and (ii) through seed dispersal. Since plants do, not have mobility like animals, and hence plants, spread or dispersal through vegetative propagation, is exceedingly slow process and the new shoots, , * which come out from the parent stocks do not, acquire the genetic variability of seeds. Thus, the plants coming out from the shoots of the, parent stocks are not able to adapt to the, variations in habitat where they reach through, the gradual process of spreading., , ‘Thus the dispersal of plants becomes, effective through external factors mainly through, the dispersal of seeds, The following factors, determine and control the nature of seed, , di :, —— we therefore dispersal and migration, , (1) Properties of seeds,, , (2) Agents of seed transportation,, (3) Speed and distance of dispersal,, (4) Geographical barriers,, , (5) Tolerance factor,, , Scanned with CamScanner
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PLANT SYSTEM, , (6) Continental drift, and, (7) Anthropogenic factor (role of man)., , (1) Properties of Seeds, , The properties of seeds which help in, the process of seed dispersal include the, following :, , > The properties of seeds to stay up in the, air while carried away from one place to, another,, , > The properties of seeds of sticking to the, bodies of carriers (animals),, , > The properties of sticking to the feathers, and feet of birds,, , Property of floating in the waters,, , The ability of seeds to germinate and, establish themselves as plants successfully, in new habitats having different, environmental conditions,, , > Number and size of seeds and, frequency of their reproduction etc., , the, , (2) Agents of Seed Transportation, , ‘The main agents of seed-transportation, are wind, ocean currents and animals including, man who transport seeds of various sorts in, varying amounts in different ways. Wind, transports seeds mainly in suspension from one, place to another place.Smaller and lighter seeds, are more efficiently transported by wind but, such seeds are susceptible to high rate of, Mortality. Birds carry such seeds which stick to, different parts of their bodies. Birds are very, effective carriers of seeds because they can, transport seeds to great distances. Besides,, Seeds which are kept in the stomach of birds are, also dispersed when the birds release them as, Wastes. There are certain limitations of seed, “spersion by birds i.e. only smaller seeds are, , ‘tried away by birds, there is high rate of, Mtality of seeds carried by birds because if, Coven U& lansported to such areas on, , “ting long distances where environmen, Ons are not favourable for the germination, , 145, , of newely brought seeds they may die. Other, animals transport seeds through their bodies, when seeds are stuck to different parts of their, bodies. The nature of seed dispersion by, animals depends on the nature of their mobility,, slow or rapid, daily, seasonal or annual etc., Water transports seeds and disperse them through, its various types of movements e.g. sea waves,, tidal currents and oceanic currents. Man has, now emerged as the most potent and effective, carrier of seeds because he is capable of, increasing the speed and range of seed dispersal, to the greatest extent., , (3) Limiting Factors of Seed Dispersal, , There are certain factors which limit and, restrict the dispersal of seeds e.g. distance factor,, geogrphical barriers, speeed factor etc., , > Smaller and lighter (in weight) seeds are, more efficiently transported to greater, distances but they are susceptible to high, rate of mortality,, , > Larger seeds are difficult to be dispersed, but these have low rate of mortality, because they have greater chance of, survival for long time due to greater food, reserve contained by them;, , > The size and number of seeds produced, by different plants also affect and control, seed dispersal. For example, annual plants, produce large quantity of small seeds, which can be easily transported to greater, distances while many perennial plants, produce few seeds but of large size. Such, seeds are transported and dispersed as, elaborated above., , > Oceans and seas are the most formidable, limiting factors of seed dispersal because, these are so extensive in size and become, so enormous barriers that these are not, likely to be crossed over by any means of, seed dispersal. The most significant, carriers of seeds through the oceans are, the ocean currents which take very long, time to transport them from one end of the, , Scanned with CamScanner
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land to the other end. On the other sie, ‘it is very likely that seeds will Bene, during their long journey of water, because there ae very few seeds of land or, fresh water plants which can withstand, the long period of their immersion In the, oceanic saline waters. Similarly, extensive, high mountains and extensive deserts are, effective formidable barriers for the, dispersal of seeds., , (4) Tolerance Factor, , The tolerance factor plays more important, role in the dispersal and migration of plants than, the rate of seed production and the efficiency of, transporting agents of seeds because ultimate, success of seed dispersion depends not only on, the ability of seeds to germinate in new, environment of the habitat to which they have, been transported but also on their ability to, establish in new habitat and to adapt to new, environment. The more the tolerance limit of, plants to adapt and adjust to new environmental, conditions of new habitat, the more Is the, chance of their survival and greater opportunities, and possibilities of their distribution and, expansion in extensive areas., , (5) Continental Drift, , Continental drifts are also helpful in the, dispersal of plants at global scale. The distribution, and dispersal of flowering plants (phanerogams), developed during Cretaceous period can be, explained in the light of continental drift. The, presence of glossopteris vegetation in South, America, Africa, Australia etc. could be possible, due to continental drift. This aspect will be, detaield out in the succeeding section (10.6)., , (6) Role of Man in the Dispersal of Plants, , Man has changed and diverted the direction, of evolution of plants and animals, has accelerated, the rate of reproduction, breeding and evolution, and development of different populations of, organic communities. He has intentionally, created and evolved new species of plants and, , ENVIRONMENTAL GEOGRAPHY, , animals throu gh the process of ‘artificial selection’,, He has introduced new plant species through, <cross-pollination’ among different types of, plants and has encouraged ‘vegetative propagation’,, Man has developed new species by creating, habitats of new environments. i, Man has affected the dispersal, distribution,, and redistribution of plants and animals and, vegetative propagation through. the following, methods :, > Through his destructive activities e.g., deforestation for domestic, argicultural, and industrial purposes, forests fires etc., > Through the discovery and development, of new types of vegetation and their, establishment in suitable habitats., , > Through the removal of plants from their, native places and their transplantation in, , other areas., , , , , , v, , Through agricultural practices., > Through changes in the habitats., , It may be pointed out that the dispersal, and distribution of plants by man is carried out, in two ways e.g. (i) Intentional dispersion- when, man intentionally brings with him the seeds of, desired plants or even plants to those areas, where he migrates to settle down or a country, officially imports the seeds developed in other, countries, (ii) Unintentional dispersion-some seeds, are unintentionally transported by man during, his migration from one place to another place, but he does not know that he is transporting, seeds with him. Such type of unintentional, transport of seeds by man may be possible ina, variety of way e.g. the seeds may be attached to, oe packings, to the means of transport (vehicles,, Lr siegs eee ery oe, aesiniies oie may be carried to the desired, germinate (if se aoe the eo gee ae, places are Broads whee cone ons of oa, sa eve a least to minimum level), Sisen ihe eal: others may perish. There has, and anigal | increase in the number of plant, , Species due to colonization of New, Zealand a, and Hawaiiland by men because the, , Scanned with CamScanner
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PLANT SYSTEM, , native flora and fauna before the mi gration of men, to these areas were limited in number., , A few plants introduced in new areas by, , man through intentional transportand dispersion, have now become problems for human society,, Forexample, since the time of first introduction, of prickly pear cactus as fodder for animals in, Australia from South America ithas so enor mously, multiplied due to rapid rate of increase of its, pastures that it has adversely affected and, damaged the natural source of animal fodder., Similarly, kudzu (a type of vine creepers) was, introduced in the U.S.A. from Japan to check, soil and gully erosion but its growth has been so, phenomenal that it has covered many areas of, the U.S.A. and now it has-become very difficult, to eradicate this problematic plant from the, U.S.A. The growth of Ipomoea Cornea, brought, in India from North America, has been so rapid, and enormous that it has spread in almost all, parts of the country. This plant has become so, problematic that there is need of movement, from the government side as well as from the, public side to eradicate this problem plant, otherwise it will cover many of the agricultural, fields., , 10.6 DISPERSAL OF FLOWERING PLANTS, , Figure 10.3 depicts the situations of the, continents during (A) Carboniferous period,, (B) late Cretaceous period, and (C) early, Tertiary period wherein the directions of plant, dispersal have been shown by arrows. There are, contrasting opinions about the exact place of the, origin of the flowering plants, painly the, angiosperms during Cretaceous period. Some, Scientists are of the view that the flowering, Plants were originated in the areas of high, latitudes mainly in the Arctic and Antarctic, teas but DJ. Axelord (1961) has strongly, , ted the aforesaid concept and has maintained, that the flowering plants were originated in _, “opical and subtropical areas. According to the, , nd P.A. Raven, aaeut views of D.I. Axelord a aoa, , the flowering plants are believ', beenorginatedinthe westempartof Gondwanaland, , hk, , (figure 10.3B) (during Cretaceous period), which now represents modern Africa and South, America. Axelord and Raven have further, maintained that there were different habitats, having various combinations of the arid and, semi-arid climatic conditions. The combinations, of arid and semi-arid environmental conditions, different types of soils and varying topographic, characteristics provided favourable environmental, Conditions for the maximum and rapid rate of, Vegetative growth of the flowering plants in the, tropical areas during Cretaceous period., , From this original nucleus (western part, of Gondwanaland) of the origin of the flowering, plants, the dispersal of plants was radiated in all, directions, which continued from late Cretaceous, toearly Tertiary periods (fig. 10.3 B and). The, dispersal of plants from the said nucleus of, origin was directed towards north into north, Africa and Eurasia during late Cretaceous, period. These flowering plants were redispersed, from Europe westward into North America, because North America was at that time (late, Cretaceous and early Tertiary periods) united, with Eurasia in the form of Laurasia. From the, nucleus of South America the flowering plants, were dispersed southward into South America, and then eastward into Antarctica and Australia, (fig. 10.3B). The flowering plants were also, dispersed eastward from the African nucleus, into India through Madagascar as these were, united together. It is obvious that most of, tropical and subtropical flowering plants were, developed in India and Madagascar due to their, dispersal from Africa during middle Cretaceous, period. The dispersal of the flowering plants, continued further eastward from Africa through, Madagascar and India to South-East, Asia., , The disruption of Gondwanaland began, some 100 million years ago due to sea-floor, spreading and the opening of South Atlantic, ocean began with the westward drifting of, South America. About 75-70 million years ago, Indian subcontinent lost contact with, Gondwanaland due to northward movement of, Indian plate which resulted in the formation of, , Scanned with CamScanner
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148, , Indian Ocean and widening of gap between, Indian subcontinent and Australia. About 55-50, years ago (during middle Miocene period), Indian plate was subducted below Asiatic plate, which resulted in the formation of the Himalayas., Due to this northward drift of the Indian, subcontinent many plant and animal groups of, the southern location were also drifted northward, with the drifting of the Indian subcontinent to, new locations but many groups of plants and, animals of India having tropical origin and, austral connections were lost during this northward, movement of the landmass because the plant, and animal groups had to move through, different climatic zones. The formation of the, Himalayas during Tertiary period presented a, formidable geogrpahical barrier in the dispersal, of plants and animals between India and, Asia., , It has been already pointed out that the, dispersal of plants and animals of tropical and, subtropical origin to North America took place, during Cretaceous and early Tertiary periods, from western part of Gondwanaland (the, nucleus of the flowering plants) via Africa and, Europe when North America and Eurasia were, united together as Laurasia. With the opening of, the Atlantic ocean due to sea-floor spreading, during early Tertiary period the water barrier, prevented dispersal of plants and animals from, Europe but still direct dispersal and migration, of biota of cooler environmental. conditions, continued through the landmasses of higher, latitudes because the landmasses of North, America and Eurasia in the higher latitude were, still connected together (fig. 10.3C). Some, dispersal and migration also became possible, through the volcanic islands associated with the, mid-Atlantic ridge., , At present many groups of plants and, animals such as nothofagus, southern beech,, marsupials (such animals who carry their, , in the pouch attached to their, , stomachs, like kangaroo) are found in the, nperate areas of the southern continents, the, member of previous Gondwanaland, e.g. South, America and Australia including New Zealand,, , , , ENVIRONMENTAL GEUGKATHY, , , , ~-~-- Mid oceanic ridges, , ——> Direction of dispersal, Fig. 10.2: Relationship between continental drift and, plant dispersal. A-The position of col, nents during Carboniferous period, B-P0sition of continents during late Cretaceous, period and the nucleus of the origin of, Slowering plants (shown by oblique lines), C-Position of continents during early Tel”, ary period and direction of plant dispers, , ___isbelieved that these plants and animals, were first originated during early and mide, , Scanned with CamScanner
