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It was in the Jurassic period when the Angiosperms first appeared on the globe. In the early Cretaceous the Angiosperms were confined to areas lying between latitudes 45°N and 45°S. The angiospermic flora was gradually replacing the non-flowering vegetation of the Jurassic and in the northern part of the globe Angiosperms established themselves only in late Cretaceous period.
Teslenko proved that no Angioperm existed in Siberia before the late Cretaceous and that Angiosperms migrated to Siberia from the south. Teslenko observed that the Angiosperms did not originate in the northern part of Angaria or the ancient Angara land.
In the present century many workers suggested a tropical origin of the Angiosperms. Hallier assumed that this group of plants originated in a continent in the south of Pacific Ocean. But no such continent ever existed.
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However, other workers, e.g. Diels Golenkin, Bailey, Axelrod, A.C. Smith, Thorne, etc. all agreed that the cradle of Angiosperms was situated in the S. Pacific, or western part of S. Pacific. It is an wide area stretching from Assam in the West and Australia and Fiji Isls. in the east including Burma, S. China, Japan.
Malaysia, New Zealand, New Caledonia, etc. The fossil evidences do not indicate the exact centre of origin but these established the fact that Angisperms arose in the tropical belt and spread to higher latitude areas.
In this wide area, i.e. S.W. Pacific Basin the primitive Angiosperms are found in abundance. Many primitive families are confined in this area and primitive representatives of many other families are to be found here.
Croizat points out that Malaysia is one of the most important phytogeographic zones of the world and all the five continents can easily be reached from its boundaries. It is however, a well-known fact that the continental S.E. Asia was connected to the Malayan Archipelago for a long time, the mountains forming prolonged link.
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The high lands of the Burma-Yunnan area ran as far as Antarctica through Malaysia, New Caledonia, N.E. Australia, Tasmania, etc. In the Jurassic and early Cretaceous these highlands served as the migratory route from Malaysia to Australia and Antarctica.
Van Steenis advanced a hypothesis that a land-bridge connection existed between S.E. Australia, Tasmania, New Zealand, Patagonia and the Antarctic continent. But Smith, Zimmerman, Axelrod are of the opinion that no such continuous land connection existed although there was insular chain and served as stepping stones for migration of plants.
The lowlands in the tropical regions were very damp throughout the year and were unsuitable to support Angiospermic vegetation. Axelrod therefore suggested that the Angiosperms originated not in the lowlands but in the mountains, under the most diverse and variable environmental conditions.
From the very beginning they had developed a high degree of adaptability to variations in atmospheric and soil moisture. The rapid ecological and morphological differentiations began in the very earliest stages of their evolution and as they migrated along different geographical and ecological pathways they underwent further transformation en-route.
Krassnov considered that temperate flora originated from tropical plants that migrated northwards and proved hardy to temperate climate. The periodicity of growth in temperate plants is an attribute which had already developed in the tropics.
Axelrod as well as Holttum said that the deciduous habit developed in plants in the tropics and permitted such trees to spread from the equatorial belt to regions having marked alternating seasons.
From the above discourses it is inferred that the Angiosperms originated in the mountains of S.W. Pacific basin lying between Assam and Fiji Isles., and from which area they spread to other tropical countries and in course of evolution gave rise to plants capable to survive in varied types of soil and climate.
They spread by land route and crossed the ocean-barrier by insular chains that existed in the Cretaceous period and earlier. The earliest primitive Angiosperms gave rise to different types of tropical plants as well as temperate and alpine. Thus we find that the extremely rich tropical flora of America is rather poor in primitive Angiosperms while the flora of tropical Africa has such Angiosperms.
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According to a second and older theory the Angiosperms originated in the north polar region. This theory was proposed by Heer in 1868. J.D. Hooker and A. Engler were among others who supported this theory. The theory of northern origin of Angiosperms has now been discarded as a result of investigations by the paleobotanists.
