ANGIOSPERMS / FLOWERING PLANTS 

The angiosperms, or flowering plants, are the largest, highly diversified, and most successful major group forming the dominant vegetation on the planet earth except in the boreal or snow forests, which are dominated by conifers (usually pines, spruces and larches), but the undergrowth of the most coniferous forests composed of many herbaceous and shrubby angiosperms.  The flowering plants occupy almost every habitat on earth, from deserts to high mountain peaks and from freshwater ecosystems to marine estuaries. The term "angiosperm" derived from two Greek words, angeion, meaning "vessel," and sperma, meaning "seed", which collectively mean "enclosed seeds", allude to enclosed condition of the seeds. The enclosed seeds and the presence of usually colourful flowers distinguish angiosperms from their closest living relatives, the gymnosperms, in which the seed is not enclosed within a fruit, but rather naked or exposed.

The angiosperms are comparatively recent group of land plants, and considered to be young, evolved about 130 million years ago (ma), and flourished from the Jurassic Period to early the Cretaceous Period.  The gymnosperms were the dominant flora during the age of Dinosaurs, the Mesozoic Era (65–245 ma). Some lines of evidence indicate that groups of extinct cycad-like gymnosperms known as the Bennettitales and the Gnetophytes, a modern division of the gymnosperms which show up in the fossil record about 225 ma, are the seed plants most closely related to angiosperms. Other lines of evidence suggest that a group of plants called the seed ferns, or pteridosperms, might represent the ancestors of the angiosperms. Although the flower is the central feature of the angiosperms, its origin and subsequent diversification remain major questions.

Angiosperms range in size from eucalyptus trees well over 100 m tall with trunks nearly 20 m in circumference to duckweed, simple floating plants barely 1 mm long. The largest individual flower on earth belongs to the genus Rafflesia, can grow ca 1 m across and weigh up to 7 kg! It has a very strong and unpleasant odour of decaying flesh, earning it the nickname "corpse flower". It is a parasitic plant, with no visible leaves, roots, or stem and endemic to the rainforests of Sumatra and Borneo.  The angiosperms are most valuable crop and forage plants and at the same time the most troublesome weeds are also angiosperms.

The angiosperms have traditionally been divided into two groups: the monocotyledons (monocots) and the dicotyledons (dicots), terms based on number of cotyledons that the plants have upon germination.  The monocots do form an evolutionarily natural or monophyletic group, and include familiar plants such as rice, maize, wheat, palms, irises, lilies and orchids.

Current studies indicate that the dicots do not form an evolutionarily monophyletic group, but instead include several different lineages, some of which are more closely related to the monocots. Two groups that are well supported in contemporary studies are the eudicots (true dicots), and the non-eudicots (magnoliids). The eudicots include most trees, rose, cucurbits, legumes and potato, whilst the non-eudicot, basal angiosperms include the laurels and avocados, the magnolias and tulips, the birthwort herbs, black pepper and water lilies.

Recently, Christenhusz & Byng (2016) counted the currently known, described and accepted number of flowering plant species to 295,383 with 74,273 monocots and 210,008 are eudicots.  In other words, approximately 96 percent of angiosperm species known today are placed either in monocotyledons (monocots) or in eudicotyledons (eudicots) and the remaining 4 percent of angiosperms are of the non-eudicots or magnoliids, a group of plants considered to have primitive features. These are typically divided into the woody magnoliids and paleoherbs.

The classification of angiosperms at the higher level remains a most controversial except the formal subdivision of monocots and dicots (eudicots and magnoliids). The disagreement about the recognition of families is largely due to the fact that a growing number of authors tend to elevate all subfamilies to the rank of family and they contain an estimated 350,000 species, some 12000 to 13000 genera and 250 to more than 500 families (Kubitzki, 1993). In India, about 20,000 flowering plants are recorded so far including cultivated/naturalised ones with approximately 15% endemic species.

In this enumeration, the concept of the historical division of subclass, dicots and monocots are followed and the families are arranged according to Bentham & Hooker’s (1862–1883) system of classification with slight modifications to accommodate especially the newly elevated families rather the Angiosperm Phylogeny Group (APG 2009, 2016) system of flowering plant classification for easy understanding. Total 257 families are listed with currently accepted genera, of which 213 families belong to dicots and 44 to monocots.

References

Angiosperm Phylogeny Group 2009. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society 161(2): 105–121. doi:10.1111/j.1095-8339.2009.00996.x. 

Angiosperm Phylogeny Group 2016. An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society 181(1): 1–20. doi:10.1111/boj.12385

Bentham, G. & Hooker, J.D. 1862–1863. Genera plantarum ad exemplaria imprimis in herbariis kewensibus servata definita, vols. 1–3. London: L Reeve & Co.

Christenhusz, M.J.M. & Byng, J.W. 2016. The number of known plants species in the world and its annual increase. Phytotaxa 261(3): 201–217.

Kubitzki, K. 1993 (Ed.). The Families and Genera of Vascular Plants, vol. 2: Flowering plants: Dicotyledons, Magnoliid, Hamamelid, and Caryophyllid Families (volume Eds., K. Kubitzki, J.G. Rohwer, V. Bittrich) 653pp. Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong, Barcelona, Budapest.

 LIST OF FAMILIES AND GENERA OF ANGIOSPERMS

• DICOTS  

• MONOCOTS