With their distinctive smooth, often peeling or flaking, aromatic bark, resins are frequently tapped from the stems of members of the pantropical family Burseraceae. Commiphora, literally 'gum-bearing', is the largest Burseraceae genus and is common in the dry forests and scrub of southern and northwestern Africa and Madagascar. Myrrh is the genus' best known product, and has been used as a perfume, incense, stimulant, antiseptic and funerary unguent in numerous eastern and western cultures, as well as in Jewish, Islamic and Christian religious rituals. Precise botanical sources of Commiphora resins have been the subject of centuries of academic and economic argument. Such arguments are confounded by ecological observations that more than one Commiphora species frequent areas used for resin harvesting.
For millennia, societies have deliberately moved exotic plants so luxuries can be provided for their deities and secular populaces. In 1495 BC, the female Egyptian Pharaoh Hatshepsut sent an expedition to the Horn of Africa; the earliest recorded state-funded plant collection expedition. The expedition's final report is carved on the walls of the Temple of Deir el-Bahari (Egypt) and describes having acquired living incense trees, such as myrrh, for the gardens of the Temple of Amun. Cultivating these trees meant the Egyptian state had a ready, guaranteed, cost-effective source of resins as incense and for mummification. Similar social and economic principles, together with intellectual and conservation justifications, have driven plant collection expeditions into the present century.
Rock corkwood is endemic to a narrow strip of western Namibia scrubland, and is a small shrub or tree with a short, swollen stem and smooth, grey bark. The deciduous, waxy, compound leaves, clustered on the tips of short shoots, appear after the small, yellow flowers. Individual plants are male or female, the latter produce fleshy, animal-dispersed fruits. When the stems are damaged a colourless, aromatic resin, rich in triterpenes and ethereal oils, flows copiously from resin ducts in the bark. The resin dries rapidly, forming a tough scab. Such features are associated with drought tolerance and the plant's ability to survive in rocky outcrops, where there is limited water and great diurnal temperature variation; days are hot and nights are cold. Resins are also likely to deter herbivores. Furthermore, as our abilities to reconstruct evolutionary histories using DNA technologies have increased, researchers have suggested that investigating relationships among African Commiphora species will help us understand plant adaptation and evolution in Old World deserts.
Weeks A, Simpson BB 2007. Molecular phylogenetic analysis of Commiphora (Burseraceae) yields insight on the evolution and historical biogeography of an "impossible" genus. Molecular Phylogenetics and Evolution 42, 62-79.
De Nova A et al 2011. Insights into the historical construction of species-rich Mesoamerican seasonally dry tropical forests: the diversification of Bursera (Burseraceae, Sapindales). New Phytologist 193, 276-287.
Musselman LJ 2007. Figs, dates, laurel and myrrh: plants of the Bible and the Quran. Timber Press.