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Discussion

Two hypotheses

On the balance of evidence presented in these pages, two competing hypotheses remain:

• either winged bean (Psophocarpus tetragonolobus (L.) DC.) began as an African species, perhaps Psophocarpus grandiflorus Wilczek., and was carried to Southern Asia where it under went domestication;



• or alternatively, the wild progenitor of winged bean, perhaps resembling Psophocarpus grandiflorus, has become extinct or has yet to be located within Asia.

The African hypothesis

In favour of the first hypothesis, is the accumulating evidence from taxonomy, cytology, and plant pathology, of close similarities between African Psophocarpus species and the winged bean. Many other crops currently grown in Asia are known to have originated in Africa. The long bean, a vegetable form of cowpea Vigna unguiculata (L.) Walp., is believed to have reached Asia through Ethiopia in prehistoric times. Similarly, the bambara groundnut, Voandzeia subterranean (L.) Thoars is known to have been taken by Arab traders to Madagascar at an early date. The progenitor of winged bean could have reached Asia in a similar way.

The Asian hypothesis

Against the African hypothesis, and in support of the Asian hypothesis, is the failure of attempts to hybridise winged bean with P. scandens(Endl.) Verdc., the modern day fragility of P. grandiflorus, its absence from lowland coastal sites, and the lack of any clear evidence for its modern-day cultivation or spread by human hands. In contrast to this, the genetic and cultural diversity of winged bean referred to in these pages, points to a long and rich encounter between plant and Man in Asia, especially in its archipelago, that encompasses both lowland and highland environments and a diversity of domestication trajectories.

 

The quest remains

In the literature, there are many examples of discontinuities in plant taxa across the Indian Ocean. The trans-oceanic distribution of Psophocarpus may well turn out to be the result of ancient geological events, rather than of dispersal by human hands. However, there can be little doubt that the modern-day distribution of winged bean is predominantly an artifact of human hands, hands that have molded the plant to their own purposes, and in so doing, extinguished, perhaps completely, its natural origins.

Further field exploration in Africa and Asia, interspecific hybridisation studies, and the application of molecular labeling techniques to representative collections of the genus, will in time provide a test of these ideas. But will it answer the question?

From where did winged bean arise?

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