Pollen of SoutheaSt aSian alchornea (euPhorbiaceae), with an overview of the Pollen foSSil record

In order to evaluate pollen morphological descriptions of Alchornea in the literature, which are almost completely based on African and American species, the pollen of eight Southeast Asian species of Alchornea was investigated, using light and scanning electron microscopy. very little variation appeared to be present in the Asian material. Slightly deviating from the scabrate ornamentation type are A. kelungensis (psilate) and A. rugosa (striate-rugulate). The scabrate type is also found in A. castaneaefolia (Brazil), A. hirtella (Liberia) and A. obovata (colombia). The operculate Alchornea pollen type, which can be easily recognised using light microscopy, seems to represent a diagnostic character for the tribe Alchornieae (pollen of Bossera unknown). Its characteristic appearance resulted in a relatively extensive fossil record. The earliest records are from the Middle Eocene of venezuela and Nigeria, while records for Australia and Borneo date from the mid-Tertiary and the Neogene (Miocene–Pliocene), respectively. These records suggest that the tribe Alchornieae has an African– American Gondwanic origin, and reached its pantropic distribution at least in the mid-Tertiary.

The Alchornieae are a tribe of nine genera (c.96 spp.), Alchornea, Aparisthmium, Bocquillonia, Bossera, Caelebogyne, Conceveiba, Gavarretia, Orfilea and Polyandra, belonging to subfamily Acalyphoideae of the Euphorbiaceae (Radcliffe-Smith, 2001). of these genera, Alchornea is the largest and most widespread, containing c. 60 species and having a pantropical distribution.The other genera are much smaller, containing from 1 to 14 species.They are restricted to America, Madagascar, Australia or New caledonia.
Pollen morphologically, subfamily Acalyphoideae is the most heterogeneous of the five subfamilies of the Euphorbiaceae (Punt, 1987;Takahashi et al., 1995Takahashi et al., , 2000;;Nowicke et al., 1998Nowicke et al., , 1999;;Nowicke & Takahashi, 2002).Pollen of tribe Alchornieae was described by Punt (1962: 7 genera, 14 species, illustrated with drawings), Taka- hashi et al. (1995: 5 genera, 15 species, illustrated with SEM and TEM images) and Takahashi et al. (2000: 7 genera, 29 species, illustrated with SEM and TEM images).No material of Bossera and Polyandra was available.The most exhaustive of these studies, Takahashi et al. (2000), includes 13 Alchornea species of which only two come from SE Asia: A. kelungensis (= A. trewioides; Ishigaki Island, east of Taiwan) and A. rugosa (New Guinea).Few pollen floras are available for Southeast Asian regions.None treat more than three Alchornea species: Ikuse (1956; no illustrations), Ting (1949;with drawings) and yu & Long (1982;with LM images).To complement the data in these works, a special study was made of the pollen of a number of Southeast Asian Alchornea species, concurrently with a taxonomic revision of Alchornea in Malesia and Thailand (Bulalacao & van Welzen, in prep.).

MATERIAL ANd METHodS
Mature male flower buds were removed from the following herbarium material (all in L): All samples were prepared for light microscopy (LM) and scanning electron microscopy (SEM), according to the techniques described by van der Ham (1990).Ten pollen grains per sample were measured.The terminology used follows Punt et al. (1994).
Pollen data of the individual species are provided in Table 1.dIScUSSIoN

Genus alchornea
Within the group of Southeast Asian species studied, very little pollen morphological variation occurs.Slightly deviating from the scabrate species are A. kelungensis (psilate) and A. rugosa (all three collections striate-rugulate).In comparing with the species studied by Takahashi et al. (1995Takahashi et al. ( , 2000)), the scabrate ornamentation type occurring in most Southeast Asian species is found in A. castaneaefolia (Brazil), A. hirtella (Liberia) and A. obovata (colombia); in the pollen of the other African and American species illustrated, the ornamentation elements are much more discrete.The occurrence in the Southeast Asian species studied of relatively long columellae in fastigium-like cavities at both lateral sides of each endoaperture, as described for the first time by Takahashi et al. (1995Takahashi et al. ( , 2000)), is confirmed in the present study.The presence of a double columellate infratectum could not be demonstrated, as no transmission electron microscopy (TEM) examination was carried out.

tribe alchornieae
In the introduction, the paper by Takahashi et al. (2000) was cited as the most exhaustive of the studies dealing with Alchornieae, describing 29 species from seven of the nine genera, while no material of the monotypic genera Bossera and Polyandra was available.However, Punt (1962, p. 82) referred to a photograph of a pollen grain of Polyandra bracteosa (only known from the type) in the original description of the genus (Léal, 1951).However, the photograph is actually an informative drawing of a pollen grain.It shows a 3-colporate, operculate pollen grain of c. 33 μm.

fossil record of the alchornea pollen type
The characteristic appearance of the operculate pollen grains of the tribe Alchornieae, using light microscopy, has resulted in a relatively extensive fossil record of the Alchornea pollen type.The fossil pollen has usually been attributed to the genus Alchornea, but actually it should be compared with the pollen of the entire tribe Alchornieae, as the genera are hard to separate.The earliest records (as Psilatricolporites operculatus) are those from the lower and upper part of the Middle Eocene of venezuela and Nigeria, respectively (Germeraad et al., 1968;Muller, 1970).Slightly younger are the Late Eocene records from Brazil (Muller, 1981).Graham (1976Graham ( , 1988aGraham ( , b, 1989Graham ( , 1991a, b), b), Graham & dilcher (1998) and Graham & Jarzen (1969) gave records for the oligocene, Miocene and Pliocene of costa Rica, Mexico, Panama and Puerto Rico.Martin (1974Martin ( , 1978) ) found the Alchornea pollen type in the mid-Tertiary of Australia, comparing it with the extant Australian genus Caelebogyne, while Germeraad et al. (1968) recognised it in unspecified Neogene (Miocene-Pliocene) sediments in Borneo.These records suggest that the tribe Alchornieae has an African-American Gondwanic origin, and reached its pantropic distribution at least in the mid-Tertiary.

AcKNoWLEdGEMENTS
Thanks are due to Bertie Joan van Heuven and vernie Sagun for assistance, Ben Kieft for the plates, Peter van Welzen for taxonomic guidance, and the Nederlandse organisatie voor Wetenschappelijk Onderzoek (NWO) for a visitor grant (B85-345) to the first author.

Table 1 .
Pollen data of Southeast Asian Alchornea species.P = length of polar axis, E = equatorial diameter.All sizes are in µm.