Keys in Flora of Bhutan by AJC Grierson and D G Long, Vol.2 Part 2:
1. Inflorescence very dense, subcapitate even in fruit; occasionally somewhat lax but then leaves subsessile; corolla lobes 2-3mm wide; calyx persistent on ripening fruit  . .. 2
+ Inflorescence lax or very lax, distinctly branched even in flower; leaves petiolate; corolla lobes 2—5 mm wide; calyx deciduous soon after corolla falls . .. 3
2. Corolla lobes terminating in hair-point 1—2 mm; leaves petiolate, typically elliptic; berry glabrous; shrub 1—4.5m . .. 1. M. roxburghii
+ Corolla lobes acute or acuminate, lacking a hair-point; leaves subsessile, typically ovate; berry pilose; small undershrub less than 1 m .. 2. M. incana
3. Calyx lobes oblong or oblong-elliptic, tapering from middle only; berry pilose, distinctly longer than broad . … 3. M. macrophylla
+ Calyx lobes lanceolate or filiform, tapering from base; berry glabrous, globose . .. . ..4
4. Calyx lobes 4—8(—10) mm; corolla lobes 3.5-5mm wide; leaves 4—13cm wide …. 4. M. treutleri
+ Calyx lobes 1—4 mm; corolla lobes 2-3mm wide; leaves up to 4.5(—5.5cm) wide . . . . .. 5. M. glabra


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Mussaenda erythrophylla Schumach. & Thonn. (Introduced) (Tropical Africa: Angola, Burundi, Cabinda, Cameroon, Central African Repu, Congo, Equatorial Guinea, Gabon, Ghana, Guinea, Gulf of Guinea Is., Ivory Coast, Kenya, Liberia, Nigeria, Sierra Leone, Sudan, Tanzania, Togo, Uganda, Zaïre; Introduced into: Society Is., Trinidad-Tobago as per POWO)
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Mussaenda frondosa L. (Indian Subcontinent to Caroline Islands: Andaman Is., Assam, Bangladesh, Borneo, Cambodia, Caroline Is., India, Jawa, Lesser Sunda Is., Malaya, Nepal, New Guinea, Nicobar Is., Sri Lanka, Sulawesi, Sumatera, Thailand, Vietnam as per POWO)

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Mussaenda glabra Vahl (Sikkim to Malaya, Jawa: Assam, Bangladesh, East Himalaya, Jawa, Malaya, Myanmar, Thailand, Vietnam as per POWO)

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Mussaenda glabrata (Hook.f.) Hutch. ex Gamble (Karnataka, Kerala, Tamil Nadu as per BSI Flora)
(As per efi thread: The berries of M. glabrata are glabrous. Another key difference is that the seeds are spiny in M. glabrata but not so in M. frondosa)

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Mussaenda hirsutissima (Hook.f.) Hutch. ex Gamble (Karnataka, Kerala, Tamil Nadu as per BSI Flora of India Checklist)
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Mussaenda laxa (Hook.f.) Hutch. ex Gamble (Karnataka, Kerala, Tamil Nadu as per BSI Flora of India Checklist)


Mussaenda macrophylla Wall. (Nepal to Taiwan (Lan Yü) and Pen. Malaysia: Andaman Is., Assam, Bangladesh, Cambodia, China Southeast, East Himalaya, Malaya, Myanmar, Nepal, Nicobar Is., Philippines, Taiwan, Thailand (from POWO))

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Mussaenda ‘Marmalade’

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Mussaenda parryorum C.E.C.Fisch. (Assam (Lushai Hills) as per POWO)
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Mussaenda philippica A.Rich. (Introduced) (Philippines; Introduced: Bangladesh, Caroline Is., Solomon Is., Thailand as per POWO)

Mussaenda philippica ‘Aurorae’:

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Musseanda philippica ‘Queen Sirikit’:
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Mussaenda roxburghii Hook.f. (C. Himalaya to N. Myanmar as per WCSP)

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Mussaenda treutleri Stapf (N. India, Nepal to S. Central China: Assam, China South-Central, East Himalaya, India, Nepal as per POWO)

  


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BSI Flora of India Checklist Vol. 2 (Distribution):

Name Distribution
Mussaenda corymbosa Arunachal Pradesh, Assam, Manipur, Meghalaya, Sikkim, West Bengal (India (near Calcutta) as per POWO and IBIS Flora (FBI))
Mussaenda frondosa Andaman & Nicobar Islands, Arunachal Pradesh, Karnataka, Maharashtra, Meghalaya
Mussaenda glabra Arunachal Pradesh, Assam, Bihar, Meghalaya, Nagaland, Mizoram, Nicobar Islands, Sikkim, West Bengal
Mussaenda glabrata Karnataka, Kerala, Tamil Nadu
Mussaenda hirsutissima Karnataka, Kerala, Tamil Nadu
Mussaenda incana Assam, Bihar, Meghalaya, Nagaland, Odisha, Sikkim, West Bengal
Mussaenda intuspilosa Assam, Meghalaya
Mussaenda keenanii Assam
Mussaenda laxa Karnataka, Kerala, Tamil Nadu
Mussaenda macrophylla Andaman & Nicobar Islands, Arunachal Pradesh, Manipur, Meghalaya, Mizoram, Nagaland, Sikkim, West Bengal
Mussaenda maingayi Arunachal Pradesh (only Malaya as per POWO and CoL)
Mussaenda nicobarica Andman & Nicobar Islands
Mussaenda parryorum Mizoram
Mussaenda roxburghii Arunachal Pradesh, Assam
Mussaenda tomentosa Kerala
Mussaenda treutleri Meghalaya, Sikkim, Uttarakhand, West Bengal
Mussaenda variolosa Cultivated species (Bangladesh to Peninsula Thailand: Bangladesh, Myanmar, Thailand as per POWO)
Mussaenda wallichii Arunachal Pradesh, Andaman & Nicobar Islands, Meghalaya

Binomial Habit Notes Ref. Distrib.

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Flora of Peninsular India:
Mussaenda erythrophylla (Cultivated)
Mussaenda frondosa
Mussaenda glabrata
Mussaenda hirsutissima
Mussaenda incana
Mussaenda laxa
Mussaenda luteola (syn. of Pentas lanceolata (Forssk.) Deflers as per POWO)
Mussaenda philippica (Cultivated)
Mussaenda roxburghii
Mussaenda tomentosa Wight (syn. of Mussaenda hirsutissima (Hook.f.) Hutch. ex Gamble as per POWO)
Mussaenda treutleri


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From Floral Database of Tamilnadu : 

Mussaenda belilla Buch.-Ham. is a synonym of Mussaenda frondosa L. Shrub Western Ghats,
Evergreen Forests
Flora of Tamil
Nadu, VOL. II, 1987
Coimbatore,
Kanniya kumari, Nilgiri, Theni, Tiruchchira ppalli, Tirunelveli
Mussaenda frondosa L. Shrub Western
Ghats, Moist Deciduous to Evergreen Forests
Flora
of Tamil Nadu, VOL. II, 1987
Coimbatore,
Dindigul, Nilgiri, Salem, Theni,
Tiruchchira ppalli
Mussaenda glabrata (Hook.f.) Hutchinson ex
Gamble
Shrub Western
Ghats Evergreen Forests
Flora
of Tamil Nadu, VOL. II, 1987; Jayaweera 1963
Coimbatore, Dindigul, Nilgiri, Theni, Tirunelveli
Mussaenda hirsutissima (Hook.f.) Hutchinson
ex Gamble
Shrub Western
Ghats & Eastern Ghats, Moist Deciduous to Evergreen Forests
Flora
of Tamil Nadu, VOL. II, 1987
Coimbatore,
Dindigul, Nilgiri, Salem, Theni, Tirunelveli, Tiruchchira ppalli
Mussaenda tomentosa Wight ex Wall. (? Mussaenda tomentosa Wight ex Hook.f. syn. of Mussaenda hirsutissima (Hook.f.) Hutch. ex Gamble as per POWO) Shrub Dry
Evergreen to Dry Deciduous Forests
Flora
of Tamil Nadu, VOL. II, 1987
Tirunelveli,
Viluppuram

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A Review of the Genus Mussaenda (Rubiaceae) from Great Nicobar Island, India, including a New Species Vinod B. Shimpale*, S. R. Yadav and C. R. Babu- Rheedea Vol. 19 (1 & 2) 53-57 2009-
Abstract– Five species of the genus Mussaenda L. are reported from Great Nicobar Island. Out of these M. nicobarica is described as a new species, M. wallichii G. Don is collected after Kurz’s collection, M. frondosa L. and M. macrophylla Wall. are widely distributed species while for the Indian flora M. villosa Wall. ex Hook.f. is confined to Great Nicobar Island. A taxonomic account of Mussaenda species from Great Nicobar Island along with a new species is presented.


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Botanical name Synonyms Family Common name
Mussaenda erythrophylla Rubiaceae Mussaenda
Mussaenda glabrata Mussaenda frondosa var. glabrata Rubiaceae Dhobi Tree
Mussaenda ‘Marmalade’ Rubiaceae Marmalade Mussaenda
Mussaenda philippica Mussaenda grandiflora Rubiaceae White Mussaenda
Mussaenda roxburghii Rubiaceae Himalayan Mussaenda


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Floriculture in India  By Gurcharan Singh Randhawa, Amitabha Mukhopadhyay (1986)- Description & Keys
Mussaenda luteola (syn. of Pentas lanceolata (Forssk.) Deflers as per POWO)
Mussaenda philippica 

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Biodiversity in India, Volume 4  edited by T. Pullaiah (2006)- Description-
Mussaenda glabrata

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Flora of Davanagere District, Karnataka, India By B. K. Manjunatha, V. Krishna, T. Pullaiah (2004)- Description-
Mussaenda laxa (Hook.f.) Hutch. ex Gamble

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Forest Plants of Eastern India By Amal Bhusan Chaudhuri (1993)- Description & Keys

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A Review of the Genus Mussaenda (Rubiaceae) from Great Nicobar Island, India, including a New Species by Vinod B. Shimpale, S. R. Yadav and C. R. Babu (Vol. 19 (1 & 2) 53-57 2009) (Description & Keys)
Mussaenda villosa Wall. ex Hook.f.,

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Journal of the Arnold Arboretum –
 Arnold Arboretum, Harvard University, 1968-
… Further, the number of foveae in the areoles of the testa is 3-10 as against 5-18 in M. parryorum. Mussaenda parryorum may be recognized by its elliptic-lanceolate, pubescent leaves and stems, sessile and diffuse cymes, smaller fruits, and …

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Floristic Diversity and Conservation Strategies in India, Volume 3 by Botanical Survey of India, Ministry of Environment and Forests, 1999-
Mussaenda parryorum

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Mussaenda (Rubiaceae) page with comparative images:
Pl. go through‎ ‎‎Mussaenda (Rubiaceae) page with comparative images of species in efloraofindia. On clicking the link of species, one can check the complete details. Genus pages generally give details of most of the species found in India.
I request you to pl. go through & point out mistakes, if any. I hope this will aid in identification in future. If anybody can send images of other species of this genus (for incorporation in the website), if any, or can identify unidentified images, it will be really nice. Pl. circulate it widely, for the benefit of all concerned.