Developing an identification key for the economically important genus Inga

Kew placement student Joe Bishop describes his work developing a quick and reliable tool for identifying species of Inga, an economically important tree genus native to Central and South America.

By Joseph Bishop

Herbarium, RBG Kew

Inga is a genus of about 300 tree species in the Leguminosae (sub-family Mimosoideae), restricted to tropical America. The genus has a long history of utilisation dating back over 2,000 years and has been traditionally cultivated largely for its edible fruits. An understanding of the local economic importance of Inga is well established and developing work on taxonomic resources for the genus is underway at Kew, principally preparation of an interactive identification key to its species. 

Taxonomy and ecology 

The genus Inga is a common feature in lowland and montane rainforest throughout Central and South America, from Mexico to as far south as Uruguay. Its highest species diversity is in Central America, and in the foothills of the Andes in Colombia, Ecuador and Peru. Inga occupies a wide range of habitats and occurs at elevations from sea level up to 3,000 m. Species can be found in rainforest on non-flooded land, and in periodically or permanently flooded and disturbed land, often closely associated with rivers.  

Morphology and evolution 

Inga is notorious for the uniformity of its morphology. All species are small to large trees up to 40 m tall, with smooth bark, and all have paripinnate leaves (compound leaves which terminate in a pair of leaflets) with characteristic foliar nectaries found between each leaflet pair along the leaf rachis. These glands produce nectar day and night and attract mostly ants, which defend the plant against herbivores (Koptur, 1984). The leaf rachis in many species is winged to varying degrees (as in the photo of Inga vera shown below). 

Inga is thought to have diversified relatively recently in evolutionary history, with speciation occurring in the last 10 million years, and many species arising as recently as 2 million years ago (Richardson et al., 2001). 

Utilisation and economic importance 

Inga has a long history of cultivation in tropical America, which began when pre-Colombian inhabitants grew the trees for their edible fruits. The useful part of the fruit is the soft, sweet, white tissue that surrounds the seeds within the pods. Cultivation for the production and sale of fruits continues today across tropical America, and each region has its own selection of preferred local species. 

More recently, the genus has been cultivated for other reasons, for example, as a shade species for tea, coffee and cacao, and, to a lesser extent, as a source of fuelwood and timber. Inga is thus a valuable multi-purpose tree crop of the Neotropics, and a handful of species have also been introduced into the Old World (Africa and Asia). 

There are a number of factors which contribute to the capacity of Inga species to be used as part of a successful, multi-purpose agroforestry crop. It is a fast growing, light-demanding tree that is able to establish itself and compete successfully in weedy secondary vegetation. It also has a rapid germination rate and can be easily grown from seed with a success rate of around 95-100% (Pennington, 1998). 

Species of Inga are able to grow in soils of poor fertility, including the red acidic soils found over much of Amazonia, which many other legumes cannot tolerate. Inga species also produce root nodules containing nitrogen-fixing bacteria; these provide nitrogen to the crops they’re grown in combination with, which improves growth and yield. Additionally, all Inga species studied appear to respond well to coppicing at an early age. 

The Inga Foundation project, recently created in partnership with Kew, has as a main aim to increase the use of Inga species as part of an alley cropping system. This is a more sustainable alternative to the traditional slash and burn agriculture systems which are used across much of the Neotropics today, threatening vast areas of rainforest. 

Previous taxonomic work 

The earliest significant treatment of the genus Inga was by George Bentham at Kew, in 1875. His treatment included 140 species. 

In the 1980's it was realised by the botanical community that, in order to increase the scale of utilisation of Inga, a more up-to-date treatment of the genus, including species descriptions and illustrations, was required. This would allow local and cultivated species to be more easily identified.   

Terry Pennington, an Honorary Research Associate at Kew, and an expert in tropical ecology, published a monograph in 1997: The Genus Inga: Botany. This comprehensive tome describes 258 species, complete with distribution maps and illustrations produced by the prolific botanical artist Rosemary Wise. During the course of the five-year project, Terry spent two years in the field collecting Inga species from wild populations, and he examined nearly 10,000 herbarium specimens. Alongside his monograph, Terry also published books on the utilisation of Inga (1998), and the genus Inga in Ecuador (1997) and Peru (1997). 

Development of an interactive identification key to Inga to be implemented on Kew’s Neotropikey site 

As a sandwich student from the University of Reading working in the Americas team at Kew since August 2014, I have been helping to develop the web-based Neotropikey, Kew’s collection of identification resources for the plants of the Neotropics. 

I have worked with my supervisors, Bente Klitgaard and Anna Haigh, to source images of, and test genus-level interactive keys to the families Plantaginaceae and Lauraceae written by the Brazilian botanist Dr Vinicius C. Souza and the Missouri-based Dutch botanist Dr Henk van der Werff, respectively. Both these treatments are now available on the Neotropikey site. All the identification keys found on Neotropikey were created using the program Lucid 3.5, a software package designed to allow users to create matrix keys, in contrast to 'rule-based' dichotomous keys, which are commonly found in field guides.  

Earlier in 2015, having gained more experience using Lucid, I was presented with the challenging task of creating an identification key to all the species of Inga, using a combination of Terry Pennington’s Inga monograph, and the vast Inga collections held in Kew’s herbarium. This was necessary because the regional keys in Terry's monograph are further divided by taxonomic sections of Inga, but the monograph provides no key to sections, so that only those very well acquainted with the species-level taxonomy will know which sub-key to use. When completed, my new key aims to provide a relatively quick and reliable tool for identifying Inga species in the future. 

In discussion with Terry Pennington, a list of morphological characters important for species level identification was created, and I'm now well advanced with the task of inputting data for all species into Lucid. Once I have worked through the 258 species described in Terry’s monograph, the next step will be to gather data for about 50 species which have been described since the monograph was published. Later, images for each species and their taxonomically significant characters will be sourced; these will include herbarium specimen scans, field photographs and line drawings. The final key is to be ready for incorporation into Neotropikey by the end of my 12-month sandwich placement, in August 2015. 


Koptur, S. (1984). Alternative defences against herbivores in Inga (Fabaceae: Mimosoideae) over an elevational gradient. Ecology 66 (5):1639–1650. Available online        

Pennington, T. D. (1997). The Genus Inga: Botany. Royal Botanic Gardens, Kew, 844 pp. 

Pennington, T. D. & Fernandes, E. C. M. (eds) (1998). The Genus Inga: Utilization. Royal Botanic Gardens, Kew., 167 pp. 

Pennington, T. D. & Revelo, N. (1997). El género Inga en el Ecuador: morfología, distribución y usos. Royal Botanic Gardens, Kew., 177 pp. 

Pennington, T. D. & Reynel, C. (1997). El género Inga en el Peru: morfología, distribución y usos. Royal Botanic Gardens, Kew., 244 pp. 

Richardson, J. E., Pennington, T. R., Pennington, T. D. & Hollingsworth, P. M. (2001). Rapid diversification of a species-rich genus of neotropical rainforest trees. Science 293: 2242–2245. Available online