Malesian Euphorbiaceae Newsletter 8

March 1998

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Analytical Key to the Genera of the Euphorbiaceae

Revised Genera

The present identification key is not the first table to identify the genera of Thai Euphorbiaceae with. Airy Shaw already constructed one in 1972. Unfortunately, one needs complete material, with especially staminate flowers and fruits, to be able to find the correct name with his key. The first half of the key presented here roughly follows the structure of Airy Shaw's key. Whenever possible, vegetative characters are used in the questions, but the use of (difficult) generative questions could not be avoided. The present key has become much longer, 154 couplets of questions, compared to roughly 85 in Airy Shaw's key (1972). This is partly caused by the fact that more exceptions are keyed out (e.g., Croton colobocarpus Airy Shaw, which lacks stellate hairs) and because questions are split into ones for flowering material and others for fruiting material. The questions concerning flowers comprise questions on staminate and pistillate flowers, at least, as far as possible. This should enable users to identify single specimens.

The key is based on the taxa treated by Airy Shaw (1972), generic descriptions as found in Backer & Bakhuizen van den Brink f. (1963), Whitmore (1973), and the material present in the Bangkok Forest Herbarium (BKF). The key in Whitmore (1973) has the same structure as the one constructed by Airy Shaw (1972).

The present key is meant to be tested, because I expect that it is still incomplete. Probably not all specimens will key out correctly. Users are, therefore, kindly invited to send me notes on problems encountered while using the key (see address above, or e-mail: peter.vanwelzen@naturalis.nl). These notes will be used to improve the key. A final and completely functional key will be published in the account on the Euphorbiaceae in the Flora of Thailand.

Some characters are difficult to use. One should carefully examine the following characters:

- Leaves opposite or in whorls versus alternate: the majority of the leaves should be opposite or in whorls. Take care with Mallotus section Hancea: these species have opposite leaves, but one leaf is reduced to an (early caducous) bract.

- Glands. Questions about glands or black glandular areas concern glands or areas present in the basal part of the leaves, often at or near the apex of the petiole on upper or lower surface. Always check several leaves and use a microscope if one is available, otherwise a handlens is a prerequisite.

- Stellate hairs and glandular scales are usually very small and they sometimes appear very infrequently. Again, check several parts, especially the young parts of the branchlets and the lower surface of the leaves close to the basal part of the midrib. Use a microscope or a handlens.

The proper key has been updated and can be viewed as a separete file in these webpages: Key to the genera of the Euphorbiaceae in Thailand.

The help of Stefan Dressler (Bridelia, Cleistanthus), Hans-Joachim Esser (tribe Hippomaneae), Raoul Haegens (Baccaurea), and Anne Schot (Aporosa) in checking several genera is highly appreciated.

This key was constructed during a sabbatical leave, which was granted by the University of Leiden. The following organisations are kindly thanked for their financial support: the Netherlands Foundation for the Advancement of Tropical Research (WOTRO, travel grant no. WR 84-402), the Society for the Advancement of Research in the Tropics (Treub Foundation), and the University of Leiden.

The neighbours and my brother in law are thanked for providing early morning transport to the BKF herbarium and my wife for picking me up in the afternoon.

Airy Shaw, H.K. 1972. The Euphorbiaceae of Siam. Kew Bull. 26: 191-363.

Backer, C.A. & Bakhuizen van den Brink f., R.C. 1963. Flora of Java 1: 441-505. N.V. P. Noordhoff, Groningen.

Whitmore, T.C. 1973. Euphorbiaceae. In: T.C. Whitmore (Ed.), Tree Flora of Malaya 2: 34-136. Longman, London.

Revised genera

The following genera have been revised: