Therefore, the Burseraceae are not the only family with this characteristic. The synapomorphies of the Sapindales include pinnately compound, alternate and spiral leaves that may be palmately compound, trifoliate, or unifoliate, and small four- or five-merous flowers having a characteristic nectar disk and imbricate petals and sepals. Some of these characteristics also occur in the Rosales. However, the Sapindales and Rutales may actually form a complex, since many families jump between them. Indeed, "rbc"L sequence studies seem to indicate that a sapindalean/rutalean complex exists and may better represent the relationships of the families than the separate orders would. A study based on chloroplast-encoded gene "rbc"L reconstructed cladograms that include families within both the Sapindales and Rutales. One such cladogram indicated that the Sapindales are robust and that the Burseraceae (and Anacardiaceae) are within a single clade. This grouping seems to make sense as both the Burseraceae and Anacardiaceae have secretory canals in the phloem and resin canals in the leaves, and are unique in the Sapindales for having biflavones in the leaf tissue. However, the two families have several distinguishing characteristics. The resin of the Burseraceae is nonallergenic and two ovules per carpel occur, whereas the resin of the Anacardiaceae can be allergenic or poisonous and one ovule per carpel is found. The Burseraceae-Anacardiaceae clade is sister to a robust cluster of three other families, the Sapindaceae-Aceraceae-Hippocastanaceae clade. The Rutaceae-Meliaceae-Simaroubaceae clade is sister to the Burseraceae-Anacardiaceae and Sapindaceae-Aceraceae-Hippocastanaceae clade. The "rbc"L technique is supported and considered acceptable until other methods become better developed for the analysis. The Burseraceae are distributed throughout the world and primarily in the tropics, especially Malaysia, Africa, and Central and South America. The three tribes can be linked to a specific region of the world, although this is not obligatory. For example, members of the tribe Protieae are generally found in South America, those of the Bursereae are found in Africa and Mesoamerica, while members of the Canarieae are found in Malaysia.