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Trees or shrubs, often buttressed and with spreading and drooping branches, monoecious. Leaves penninerved; Inflorescence axillary, paniculate or thyrsoid, many-flowered, condensed or lax at anthesis, ♂, ♀, ♂♀, densely and variously pubescent; Seed with a rather scanty or copious endosperm;


Africa: present America: present Argentina: present Asia: present Asia-Temperate:, Hainan (Hainanpresent) Asia-Tropical:, Indiapresent; Thailand (Thailandpresent) Australasia: Bermuda: present Burma: present Central & S. Florida: present Greater Antilles: present Hongkong: present Madagascar: present Northern America: Pacific Islands: present S. Africa: present S. Japan: present South America: present Southern America:, Argentina Northeast (Formosapresent); Bahamas (Bahamaspresent) Tahiti: present throughout the tropics and subtropics: present warmer parts of the Himalayas: present
About 10-15 spp., widely distributed throughout the tropics and subtropics. In Asia (with 6-7 spp.) from the warmer parts of the Himalayas, extending north-eastwards to China (incl. Hainan, Hongkong, Formosa) and S. Japan and south and south-eastwards through India, Burma, Thailand, Indo-China, and Malesia to the tropical and subtropical parts of Australia and the Pacific islands as far east as Tahiti (31° N-37° S). In Africa (with 3-4 spp.) it occurs south of the Sahara to S. Africa and Madagascar (22° N-28° S). In America (with 4-5 spp.) the genus is known from Central & S. Florida and Mexico, extending south-eastwards through Central America, Bermuda, and the Bahamas, the Greater Antilles and southwards to South America as far south as the northern parts of Argentina (26° N-25° S). In Malesia: 4 spp., widely spread. , .


The genus is homogeneous and closely related to Parasponia and Celtis. This is corroborated by the anatomy of the wood and leaves. Reports on the cytology are, however, suggesting that the number of chromosomes is not constant. Specific delimitation has proved to be difficult and has led to more than 50 names in the genus. This was partly due to the various interpretations of the early described species. There is still no unanimity of opinion about the number of good species in the continents. In Africa, for example, ENGLER () estimated the number for Africa at 5-7, following BLUME (1856), but RENDLE (1917) and POLHILL () accept only one, either under the specific name T. guineensis or T. orientalis.

In absence of a critical, reliable world monograph there is a similar uncertainty about the number of species in the neotropics and in Indo-Malesia. For Malesia out of 20-25 published names of species and varieties, only 4 spp. are recognized here.

The proliferation of name giving in Malesia is mainly due to the fact that Trema spp. have a growth habit of continuously producing lateral and terminal new shoots on which flowers and fruits are borne. Many specimens collected were from these young shoots in which the indumentum and leaf-shape is often different from that of mature leaves. For accurate identification leaves, inflorescences, and fruits of mature specimens are essential. Besides, the indumentum was in earlier descriptions mostly derived from low magnification observations, but to differentiate sterile material of T. orientalis and T. tomentosa the difference in the indumentum becomes only clear under at least 40 × magnification. It is impossible to name young sterile specimens.


A few counts on the chromosome number which have been reported by various cytolo-gists suggest that cytogenetically the genus is rather variable. In Trema politoria from India n = 10+B (); in T. orientalis n = 18 (), or n = 20 (), or n = 10 (; ); and in T. tomentosa (cited as T. amboinensis) n = 10 or 80 ().


Very little is known about the sporogenesis and embryogenesis of the genus. A preliminary study carried out recently on Trema cannabina and T. tomentosa in the Malay Peninsula indicates that the development of the anther and microspores follow the so-called dicotyledon-type, and that of the embryo-sac conforms with the Polygonum-type.


J. J. SMITH - in K. & V., Bijdr. 12. 1910: 649
SOEPADMO - in Whitmore, Tree FL Mai. 2. 1973: 420
ENDL. - in Gen. Pl. 1837: 276
PLANCH. - in Ann. Sc. Nat. 1848: 264
BERNARD - in Bull. Herb. Boiss. 1906: maps 19-21
LAMK - in DC., Prod. 17. 1873: 195
ENGL - in E. & P., Nat. Pfl. Fam., ed. 3, 1. 1888: 65
ELIAS - in J. Am. Arb. 1970: f. 2
HUTCH. - in Gen. Fl. Pl. 1967: 148
BTH. - in Fl. Austr. 1873: 157
B. & H. - in Gen. Pl. 1880: 355
RENDLE - in Fl. Trop. Afr. 1917: 10
DE WIT - in Bull. Bot. Gard. Btzg. 1949: 184
Bl. - in Mus. Bot. 1856: 58