Keys in Flora of Bhutan:
1. Perianth uniseriate +/- irregularly biseriate with 4— 8 equal undifferentiated segments; branches usually armed with pseudo stipular, straight, +/-compressed spines; petioles and leaf rachis usually winged (except Z. bungeanum)…… 2
+ Perianth biseriate, clearly differentiated into 4 — 5 sepals and 4 — 5 petals: branches armed with scattered, short, straight or hooked spines; petioles and leaf rachises unwinged  ……. 4
2. Branches usually ferruginous pubescent or sometimes glabrous; lateral veins up to 30 pairs: inflorescences axillary, 0.5 – 2cm: carpels 2 — 5 …… 3. Z. acanthopodium
+ Branches mostly completely glabrous; lateral veins up to 15pairs; inflorescences terminal on short lateral branches 3 — 8cm: carpels 1 — 3 …….. 3
3. Leaf rachis winged between leaflets …………… 1. Z. armatum
+ Leaf rachis unwinged between leaflets ………… 2. Z. bungeanum
4. Leaves always palmately 3-foliolate: carpel solitary……… 4. Z. ovalifolium
+ Leaves pinnate, or if 3-foliolate then carpel more than one …………. 5
5. Tall erect trees with septate pith: inflorescence terminal, corymbose: spines on branches with broad stout conical bases ………… 6
+ Scrambling shrubs with non-septate pith: inflorescence tenninal or axillary; spines without conical bases ……….. 7
6. Flowers 4-merous: carpels solitary ……….. 5. Z. rhetsa
+ Flowers 5-merous: carpels 4. abortive ones persisting at base of follicle ……. 6. Z. myriacanthum
7. Leaflets 2 — 3 pairs, 5 — 15 x 2.5 – 6cm ………. 7. Z. nitidum
+ Leaflets 6 -10 pairs. 3.5 — 6 x 1- 3cm …………… 8
8. Branchlets and inflorescence +/- glabrous: flowers to 0.7cm, subumbellate ………..8. Z. oxyphyllum
+ Branchlets and inflorescence brownish tomentose; flowers c0.5cm, in sparsely branched panicles ………… 9. Z. tomentellum

The Genus Zanthoxylum Linn. (Rutaceae) in India (1974) by C R Babu: (Full pdf can be downloaded):  
Abstract: This paper deals with the taxonomic revision of the genus Zanthoxylum Linn. as occurring in India. A total number of 13 species is recognised in the area, of which 3 specics, Z. burkillianum Babu, Z. nepalense Babu and Z. pseudoxyphyllum Babu are new to science. Z. nitidum (Roxb.) DC. is recorded for the first time from Andaman Islands. Z. myriacanthum Wall, ex Hook. f. and Z. tomentellum Hook. f. are new records to Sikkim and Assam H malayas respectively. The description of Z. tomentellum Hook. f. is imended. Z. acanthopodium DC. var. timbor Hook. f. and Z. hamiltonianum Wall, ex Hook. f. var. tomentosum Hook. f. are reduced to synonyms of Z. acanthopodium DC. and Z. nitidum (Roxb.) DC., respectively. Z. nitidim Wall, semu Hook. f. is deleted from the synonymy of Toddalia asiatica (Linn.) Lamk. and ascertained to Z. nitidum (Roxb.) DC. Each species is described in detail, and its synonymy, flowering and fruiting times, distribution, uses, etc. are ascertained. Lectotypes for Z. oxyphyllum Edgew., Z. tetraspermum Wt. and Z. tomentellum Hook. f. are designated for the first time.


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