From their original abode the Angiosperms covered the whole of the earth migrating to different countries along different routes. Moving northwards they covered the whole of Asia and following a coastal route reached the Bearing Straight and crossed over to America.
Mountains of Malaya and Burma brought them to the Himalayas by which route they had access to Europe. In N. Vietnam, Upper Burma, Assam, Yunnan and E. Himalayas a series of phylogenetic links do exist that connect the temperate forms of plants to the subtropical and tropical forms. Those plants that reached Antarctica travelled westwards and ultimately reached S. America.
Migration of plants depends on dispersal of seeds. Vegetative propagation is also responsible in few cases but this must have played an insignificant role at the early stages of plant migration. Seeds are carried over to wider and wider areas by different agents.
The plants that come out of the seeds that fall in new areas need to acclimatize themselves in the new situations for survival.
This depends on a few factors, viz.:
(a) Character of the seed or fruit that makes it easy or difficult for the dispersal of a particular plant;
(b) Natural barrier that stops further dispersal; and
(c) Ecological conditions that make it easy or difficult for the seed to germinate, for the seedling to survive for the new-comer to compete with the local plants.
Dispersal of seeds is a continuous process by which a species survives and spreads. Migration of plants as a result of dispersal of seeds to wider areas is also a constant phenomenon in the plant kingdom. The range of distribution of a species, genus or family may be continuous or discontinuous.
Discontinuous distribution means the presence of gaps in the range of distribution. Such gaps appear due to ecological conditions, failure of survival in competition with other pre-established plants, changing of climate and composition of the soil in geological periods, etc..
Examples of discontinuous distribution are: Ranalisma, Magnolia, etc. Ranalisma rostrata Stapf occurs in Malaya while R. humile (Ktz) Hutch is found in Africa. No species of the genus is found in between.
A few species of Magnolia occur in S.E. Asia while some are found in tropical America; and this is the distribution pattern of the family Magnoliaceae. Calycanthaceae is restricted in 2 areas widely apart, i.e. in, southern part of tropical N. America and Upper Burma and Yunnan.
Cochlospermaceae is confined to W. Tropical Africa and India, Malaysia to Australia, Schizandraceae has a Malaysian distribution but one species Schizandra glabra occurs in N. America. Illiciaceae is distributed in Atlantic N. America and in E. Asia including Japan.
Due to same reasons we find many groups of plants are restricted to smaller areas, e.g. Ruscus found only in the Mediterranean region.
Taxa with a very restricted distribution are called endemic taxa. Degeneriaceae a monotypic family is endemic in Fiji Islands. Amherstia with a single species is endemic in Burma. Calacanthus a monotypic genus is endemic in Western India. Many larger genera have some species distributed over wide areas and some are endemic and restricted to smaller areas.
Primula, Impatiens, Gentiana, Rhododendron, etc. include many endemic species as well as many distributed over wide areas. Gomphostemma an East Asiatic genus has a few species endemic in Assam. Rhododendron nilagiricum Zene. is endemic in the Nilgiri hills of S. India while quite a number of species of the genus occur in the Himalayas and a few of them are endemic.
Two factors are considered to be responsible for endemism. Areas which are widely separated from other areas by natural barriers, viz.: islands in the middle of wide oceans, alpine regions of the high mountains or oasis in vast deserts.
In these areas when new species are evolved fail to spread over other areas due to the natural barriers. The other factor is the change of ecological conditions for a particular species making it too difficult for the species to survive except in some niches and corners.
Examples of such endemic genera as mentioned by Ridley are Metasequoia and Ginkgo endemic in China and Japan. These are considered to have had an wide distribution in the past but due to change of conditions from favourable to unfavourable for their survival they perished in most of the areas. Unsuccessful competition with other species might also have played an important part in their elimination from different areas.
New species originate by natural hybridization between closely related species growing in a particular area. They spread over wide area in course of time unless the place of origin is not cut off by natural barriers.
Therefore percentage of endemic species in wide landmasses is much less than islands, high mountainous regions, etc. For example in the Hawaii Islands, the percentage of endemic species is as high as 82%.
According to Willis a very widely distributed species originated in the long past while an endemic species or a species with a restricted distribution is a recently evolved species. In other words the span of existence of a species on the face of the earth is proportional to its range of distribution.
This theory is known as the age and area theory of Willis. This theory however is not applicable in all cases. As is evident from the examples given by Ridley the genera like Metasequoia and Ginkgo having restricted distribution are not at all recent plants but are representatives of very primitive group of plants.
Therefore we can distinguish two types of endemic taxa viz: those which are newly evolved and those primitive taxa the distribution of which becomes restricted to small areas due to unfavourable ecological conditions or due to competition with other taxa. The former type is called Neoendemic while the other type is paleoendemic.
Along with endemic species we may also discuss about rare species and vicariads. A species is a rare species in an area if only few individuals of that species are found in that area. The range of its distribution may be wide but if the percentage of its occurrence in a particular locality is low it is said to be a rare species in that locality.
Similarly if a large number of individuals occur in a particular area it is said to be abundant species in that area. A rare species may be a newly evolved species or a remnant of a primitive species which is on its way to extinction. When a widely distributed species becomes rare in a particular area it may be due to unfavourable ecological condition or due to its incapability to compete successfully with other local plants.
When 2 or more species of a genus are considered to have originated from a common species i.e. a common ancestor then these species are called vicariads. Vicariads are new and distinct taxa produced in course of evolution by a single widely distributed species.
Ranunculus ambigens and R. lingua are cited as examples of vicariads as these are supposed to have originated from a common ancestor. The former is an American species while the latter is Eurasian.
Morphological and genetically differences are manifested in the case of vicariads after the individuals of ancestral species established themselves in their new abodes. When such differences make their appearance before the individuals spread in new areas the new taxa are called pseudovicariads.
In India the percentage of endemic species is very high, about 50%. These are concentrated in the Himalayan region and in S. India. In the Indo-Gangetic plain on the other hand comparatively few endemic species occur.
Preponderance of endemic species in the Himalayas and S. India is due to their separation from other areas by the altitude in the former case and by sea in latter. The dry Tibetan plateau on the north and the hot plain area on the south checked the spreading of the species evolved in the Himalayas.
Similar is the case with S. India bounded by sea and the northern alluvial plain. Due to the presence of high percentage of endemic species some authors consider that India has an original flora.
Hooker, Champion, etc. are of the opinion that the Indian flora is a derived flora composed of elements that migrated from different adjacent countries. The families having greater number of endemic species in India are, Leguminosae, Rubiaceae, Compositae, Euphorbiaceae, Acanthaceae, Primulaceae, Asclepediaceae, Rosaceae, Gesneriaceae, Balsaminaceae, etc.
None of these can however be called a typical Indian family. It further appears that the endemicity in case of Indian genera has not been properly assessed. For example it has been stated that in India the endemic species of Ranumculus is 367o; on a critical review however it is found to be not more than 16.5%.
Sapria himalayensis of Raflesiaceae was considered to be endemic in E. Himalaya but it was recently discovered in Manipur. Hooker’s view that India’s flora is a derived flora appears to be quite correct and therefore the statement that India has a flora of its own is nothing but a fallacy.