246 Flora Malesiana [ser. I, vol. 9^ occurs following a minor flowering. Closely related species differ in flowering periodicity. Dryobalanops aromatica and Shorea leprosula populations, for instance, flower on average once every two years, while S. panifolia flowers only half as frequently and the important hill forest species S. curtisii and S. platyclados at intervals not exceeding five years. In Dryobalanops aromatica and Shorea curtisii local flowering can be found somewhere in Malaya almost every year and sometimes occurs in two successive years in one area; this has also been observed in Shorea sect. Pachycarpae in West Sarawak (e.g. Smythies, 1958). Burgess observed that individual trees of Shorea curtisii do not, as a rule however, flower in two successive flowerings. He also observed that in S. curtisii the flowering in 1 968 could be subdivided into two almost distinct periods during which two separate sets of individuals within a single gregarious population flowered. Planted dipterocarp trees, and trees in forests previously selectively felled, flower more frequently, and generalisations about dipterocarp flowering in natural conditions which are derived from observations on such trees can be misleading (e.g. Sasah et ai, 1979). Vatica rassak and Shorea macrophylla flower almost annually in the arboretum of the Forest Research Institute at Kepong (Malaya) as apparently do several species at Bogor and a Shorea stenoptera provenance in a remarkable trial plantation nearby in W. Java, in which all plants are said to have been derived from a single tree near Pontianak that was well known for its annual flowering. Emergent species probably take many years to reach flowering age under forest conditions. At Kepong cultivated saplings of Dipterocarpus oblongifolius have flowered after only 7 months (KocHUMMEN, 1 96 1 ), but fruit was not set and cultivated trees there normally started flowering after 15-30 years (Ng, 1966). Tang (1978) has recorded young trees of Shorea leprosula, growing following selective felling, to set viable fruit after 7 years. The cause of flowering remains obscure. It can occur between March and July, with a peak in May. in Malaya and East Borneo; but a month or two later on average in N.W. Borneo. Both Wood and Burgess have shown that the commonly held belief that general flowerings follow a period of abnormal water stress is not consistently supported by data from rainfall stations; nevertheless the fact that many other families (e.g. Burseraceae and Sapotaceae) also flower unusually heavily in a good dipterocarp year (e.g. Med way. 1972). indicates that an undefined climatic factor must be involved. Pal.mer (1979) rightly pointed out that the climatic trigger must be easily observable outside the forest as its effects are regional. Ng ( 1978) has suggested that flowering may be initiated by a period of high irradiation. Wycherley (1973) demonstrated highly significant statistical correlation between gregarious flowering of dipterocarps and preceding periods with both large diurnal temperature ranges and high maximum temperature indicating high insolation; he deduced that the latter is probably the main inductive factor, and this view is also supported by Ng (1978). Heavy flowerings never occur in successive >ears. It appears possible therefore that accumulation of assimilates, including carbohydrates within the trees, takes place gradually following a flowering, so that in each subsequent year the threshold, over which the climatically induced trigger is effective, becomes lower until the combination of an adequate assimilate level and adequate climatic stimulation induces formation of inflorescence primordia; the intensity of flowering may thus be related to the degree to which the threshold is exceeded. Differential rates of inflorescence and flower development among the species leads to spacing in the periods of anthesis of individual species. Pollination. No reference occurs in the literature. Meliponid bees ( Trigona spp.) are. according to Burgess, abundant in the crowns of flowering Shorea curtisii and other Malayan dipterocarps; Chan & Appanah (1980) have found them to be principal pollinators of Dryobalanops and Neobalanocarpus: as they are short distance foragers that tend to keep to a single crown, and as they are glabrous or sparsely hairy and very efficient at cleaning themselves, they are unlikely to be effective pollinators of trees that present many flowers at a time (D. H. Janzen. comm.). Smitinand reported to me that honey bees are abundant round Dipterocarpus crowns, but they have not been observed on the flowers of other genera. Thrips ( Thysanoptera) frequent the flowers of many Shorea and Hopea (personal obs.; Chan & Appanah, 1980) and are undoubtedly pollinators. Geometrid moths and beetles visit some dipterocarp flowers at night, but have not been demonstrated to effect pollination. The very large numbers of flowers on each tree, and the spatial isolation of the understorey species, may be expected to lead to rarity of outcrossing; the infrequency of flowering and its intensity would tend to prevent vector numbers from reaching adequate levels for effective pollination even were they polylectic (promiscuous).