Of the predominating families of plants in the flora of India Gramineae outnumbers all the other families with over 1100 species in India. There are meadowlands in the plains as well as on the hills, the savannahs of the tarai and dooars and the rolling downs of Ootacamund and other hills. Bamboo groves are common in warm and humid parts.
Next to Graminae in magnitude is Fabaceae (Papilionaceae) with about 900 species. Plants of this family are found all over the country in different ecological conditions and are often predominant in dry situations with herbaceous or shrubby species while the trees and climbers are met with more in humid forests.
Compositae has about 800 Indian species and are almost all herbaceous or shrubby. These are conspicuous in open situations on the hills and occur as road side weeds in the plains. Eupatorium odoratum an exotic herb is gregarious in many parts of eastern India. Orchidaceae is another large family with over 700 species. This family is concentrated mostly in E. Himalaya and Assam and in Malabar.
These herbs with showy flowers lend a colour to the forests during the summar and monsoon months. Acanthaceae with over 500 species in India is more abundant in the Deccan peninsula.
This is predominantly a herbaceous family, some species being gregarious in Nilgiri and Malabar hills. Labiatae is another herbaceous family with over 400 species mostly confined to the hills. The family Rubiaceae has about 550 species in India, the majority being in southern India.
Euphorbiaceae also has over 400 species of trees shrubs and herbs, some of which are succulents inhabiting warm and dry localities where some species are gregarious. Scrophulariaceae has about 300 herbs mostly confined to the hills.
Rosaceae with about 250 species of trees and shrubs are conspicuous in the temperate parts where Ericaceae, Lauraceae, Betulaceae and Fagaceae predominate. The Pines play an important role in the composition of temperate forests in the Himalayas. In the plains Meliaceae, Anacardiaceae, Sapindaceae, Dipterocarpaceae, Myrtaceae, Combretaceae being predominantly trees, help in climax forest formation.
The family Palmae is represented in India with a comparatively few species. But the tall unbrached palms are a conspicuous feature of the landscape in many parts of the country. The Palmyra palm — Borassus flabelifer is the most widely distributed palm in India.
The Coconut — Cocos nucifera and Betlenut — Areca catechu are much cultivated in the states near the sea. In the Sunderbans and other mangrove areas Phoenix paludosa a dwarf palm is very common and often form pure associations. Nypa fruticans is another palm of mangrove forest but is represented by a few individuals in the Sunderbans within Indian Union.
Nannorhops ritchieana a desert palm of tufted habit having short or often prostrate stem occurs in Rajasthan. The climbing palms or the Rattans are found mainly in the humid parts of the country and often form impenitrable thickets.
Succulents in India include a few shrubs of Euphorbiaceae and herbs like Portulaca, Sedum, kalanchoe, Suaeda, etc. Cactaceae, although represented in India by a single introduced species, viz. Opuntia monacantha requires a special mention as this species is widely distributed all over the country in the plains usually in drier areas.
The Pteridophytes are also well-represented in Indian Flora and there are about 500 species of Ferns and Fern-allies in India. There are a few species of Cyathea which attain tree-size and look like palms. These are common in some parts of E. Himalaya, Assam and Western Ghats.
A study of the vegetation of different areas of India and of the adjacent countries reveals that it has close affinities with the vegetation of these countries and is composed of elements which are:
(1) Gondowana-relic
(2) Malaysian,
(3) African & Oriental,
(4) Eurasian and
(5) Sino Himalayan.
The peninsular India is one of the oldest known land masses of the earth. In this area there are many trees and shrubs of the rain forest type which form part of a flora that extends from Malaysia through India to N.E. Africa and in Eastern S. America.
This flora originated in the Oligocene period on Gondowanaland and disappeared in the thickly populated and long-cultivated regions of India and Central Africa which gradually turned to deserts or arid lands. Relies of the old flora occasionally persist in distant mountainous localities unaffected by man as examples of discontinuous distribution.
A large number of genera common to Malaya, India, Africa and Eastern S. America may be mentioned which formed the components of this flora, e.g. Tetracera, Eriodendron, Sterculia, Erythroxylon, Bauhinia, Cynometra,Crudia, Parkia, Acacia, Casearia, Homalium, Ixora, Ardisia, Rauvolfia, Buddleia, Strychnos, Piper, Croton, Acalypha, Alchornea and many others.
Anaxagorea is found in Malaya. Andaman Isls., Ceylon and in West Indies, but not in peninsular India or Africa. Klugia occurs in India and S. America. There are more such examples of discontinuous distribution which prove the existence in the past of same type of vegetation in India, Africa, and S. America.
The Malaysian element is represented in the Indian flora by Dipterocarpaceae, Myrtaceae, Guttiferae, Dilleniaceae, Meliaceae, Connaraceae, Tectona, Schleichera etc. and by a few mangrove species. Most of these evolved in the coral islands and travelled westwards by sea as well as by land.
After the closing of the east end of the Mediterranean a desert flora came from Africa and Arabia via Buluchistan to Sind and Rajasthan, and penetrated as far south as Travancore. Azima, Salvadora, Prosopis, Dodonea, Heliotropium, Indigofera, many species of Zygophyllaceae, Capparidaceae, Farsetia, Malcolmia, Zoysia, Andropogon etc. are characteristic of this xerophytic invasion.
A few families developed chiefly in the Mediterranean region and these are Cruciferae, Caryophyllaceae, Labiatae. Polygonaceae, Liliaceae etc. These have entered India from the northrwest. Along with these came species of Juniperus, Cedrus, Crataegus, Astragalus, many Compositae and Euphorabiaceae. These constitute the Oriental element in Indian Flora.
During the Pleistocene ice age the circumpolar flora was pushed fanwise southwards. The part of the flora which followed the direct route through Central Asia, which might have been partly desert, had to cross the Tian Shan Mts. and Tibet plateau to reach the Himalayas which were also glaciated.
These artic plants therefore did not survive. Those which followed other routes entered the Himalayas from the north-west and the east during interglacial periods and after.
The alpine plants of the Himalayas which are common to the Artic regions are considered to have come by the north western route and form the Eurasian element. These include Saxifragaceae, Thalictrum, Corydalis, Aconitum, etc.
The Sino-Himalayan element is represented by a great number of genera, viz. Camellia, Saurauja, Schima, Altingia, Rhododendron, Engelhardtia, Tsuga, Primula, Androsace, Meconopsis, Gentiana, Pedicularis, Stachyurus, Mahonia, Rubus, Cotoneaster, Photinia, Eurya, Gordonia, Podophyllum, Actinidia, etc.
These genera followed the coast line of China while migrating towards south and reached the highlands of S. China Then as the ice receded they eventually turned north-westwards towards the Himalayas. This happened several times and the resultant Sino-Himalayan flora is a composite flora, a mixture of floras that originated in different parts of northern and eastern Asia in different geological times.