Plant diversity and angiosperms in india

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INTRODUCTION

Angiosperms or flowering plants (also called Angiospermae, Magnoliophyta, or Anthophyta) are the most diverse group of the plant kingdom, comprising of about 2,50,000 species in 350 families (Kenrick, 1999). Flowering plants are by far the most numerous, diverse, and “successful” extant plant group, containing well over 95% of all land plant species alive today (Simpson, 2006). Angiosperms are characterized by (i) seeds produced within a carpel with a stigmatic surface for pollen germination, (ii) a much reduced female gametophyte, consisting in most cases of just eight nuclei in seven cells; and (3) double fertilization, leading to the formation of a typically triploid nutritive tissue called endosperm (Judd et al., 2002).

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Several apomorphies distinguish the angiosperms from all other land plants: (1) the flower, usually with an associated perianth, (2) stamens with two lateral thecae, each composed of two microsporangia, (3) a reduced, 3-nucleate male gametophyte, (4) carpels and fruit formation, (5) ovules with two integuments, (6) a reduced, 8-nucleate female gametophyte, (7) endosperm formation and (8) sieve tube members (Simpson, 2006). Some of these apomorphic features, which represent the product of a unique evolutionary event, have become further modified in particular lineages of angiosperms. Almost all angiosperms produce vessels in the xylem tissue, though this feature probably evolved within the group. Angiosperm phloem differs from that of all other plants in having sieve tube elements accompanied by one or more “companion cells” that are derived from the same mother cell.

Flowering plants grow in virtually every habitable region and are dominant in some aquatic and most terrestrial ecosystems, the notable exception to the latter being coniferous forests. Angiosperms comprise the great bulk of our economically important plants, including our most valuable food crops.

India with a geographical area of about 32, 87,263 sq km is the seventh largest and tenth industrialized country of the world. It is situated between 804′ N to 3706′ N latitude and 6807′ E to 97025′ E longitude. The longitudinal variation divides Indian subcontinent into four climatological zones, viz., equatorial, tropical, subtropical and warm temperate. The forest cover of the country have been estimated to be 6, 37,293 km2 (19.39% of the geographic area of the country) and includes dense forest (3, 77,358 km2), open forest (2, 55,064 km2) and mangrove (4,871 km2).

India represents about 11% of world’s flora in just about 2.4% of total land mass. Out of the 25 biodiversity ‘Hotspots’ identified in the world (Myers, 1990), India has two, namely Eastern Himalaya and Western Ghats. These hotspots posses majority of plant diversity in India. In terms of species diversity, approximately 45,000 plant species are found in India (Khoshoo, 1994, 1995; Sharma et al., 1997). The angiosperms are represented by c. 17,500 species out of which 5725 species are endemic to India.

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