DIOSCOREACEAE

The Dioscoreaceae or yam family consists of about 700+ species of tropical climbers or herbs. There are two well-known genera, namely, the air potato Dioscorea, and the bat flower, Tacca. Dioscorea are herbaceous climbers with alternate leaves, inconspicuous flowers and large tubers called ‘yam’. They can reproduce asexually via a bulbil or 'air potato' (young plants formed at the base of a leaf), or capsulate fruits containing winged seeds. Some Dioscorea species like D. alata (purple yam), D. bulbifera (air yam), and D. polystacha (Chinese yam, “Huai-Shan”) are edible, others like Dioscorea hispida (intoxicating yam, “Ubi Gadong”) and Dioscorea sansibarensis (Zanzibar yam) are deadly poisonous. This is due to the presence of dioscorine, a neurotoxic alkaloid which can cause fatal convulsion and paralysis. Poisonous tuber or bulbil of Dioscorea spp. are used as arrow, fish, animal (monkey) and ordeal poison. The tubers of poisonous species can be rendered edible after careful processing or repeated rinsing in fast flowing stream.

The bat flower genus Tacca is inedible but relatively innocuous. They are herbs with long petiolate leaves and large peculiar flowers bearing long whiskers. Tacca rhizomes often contain diarylheptanoids, a group of bioactive compounds with medicinal (anti-inflammation, anti-microbial and anti-cancer) properties.  

Zanzibar yam (Dioscorea sansibarensis). Native to Africa, but naturalised as an invasive species in Southeast Asia, where it is highly destructive to local ecosystem.

The curious looking leaf of D. sansibarensis has earned its nickname, the 'batman plant'. The elongated leaf apex harbors symbiotic nitrifying bacteria that 'fertilises' D. sansibarensis with nitrogen in the air!

Dioscorea sansibarensis, close up of bulbils.

 

Dioscorea sansibarensis, foliage.

Intoxicating yam (Dioscorea hispida). Native to South Asia.

The tuber of Dioscorea hispida is a food crop in parts of Southeast Asia. However, it is highly poisonous and requires laborious detoxification process. A raw tuber the size of an apple can kill an adult man in hours. Aboriginal people of Malaysia incorporate the tuber extract of D. hispida with the latex of Antiaris toxicaria to produce potent arrow poison. In Thailand, the raw tubers are used to spike animal corpses to dispatch large carnivores like a tiger. The intoxicating yam is so named because consumption of carelessly prepared yam would result in 'altered consciousness'. Deaths due to D. hispida are frequently reported.

Dioscorea hispida has very prickly stem.

Bat flower (Tacca chantrieri), native to northern Peninsular Malaysia.