Calcutta, Dec. 14: Satyajit Ray's short story Hunger of the Septopus introduced us to the carnivorous plant Septopus that swallowed Badshah, a Rampur Hound.

Like Ray's Septopus, the AJC Bose Indian Botanic Garden abounds with large trees that have been strangulating and suffocating rare and endangered plants growing across its 273 acres.

The Great Banyan Tree is no exception. With its 486-metre canopy and 3,772 prop roots, it grew on a date palm tree before killing it and spreading its span. Called a monster plant, the genus Ficus includes species such as the banyan and the ashwattha or peepul tree, all of which grow very deep roots even in concrete.

What is worrying scientists at the botanic garden is that these trees are killing off many endangered plants, threatening the very purpose of conserving plants outside their endemic habitats.

"There used to be a rudraksha tree in the garden that had been planted some 50 years ago. It had grown into a healthy tree and there are photographic records of it in the library. Today, there is no sign of the tree. In its place stands a fig," said senior scientist Basant Singh, who along with director Arabinda Pramanik and C.M. Sabapathy of the Botanical Survey of India has authored a paper titled Primary Hemi-epiphytes, a challenge for ex-situ conservation in IBG.

The paper states that there are "164 (such) cases in the garden ranging from newcomers to well-established primary hemi-epiphyte trees (that grow on host plants and spread deep roots)".

Explaining how these monster plants operate, Singh said: "The seeds of fig trees are very hard. Birds and certain mammals like the Asian Palm Civet, Small Indian Civet, Northern Plain Grey Langur, bats and squirrels feed on the seeds and then shed their excreta on plants. From there, seeds softened by the acidic reaction germinate. At first, these plants look very helpless as they grow on the crown of the phorophytes (a plant on which hemi-epiphytes grow). But they soon send deep roots into the earth and slowly form a cage around the host plant and suffocate and strangle it to death."

The process takes around 20 years or so. Many such monster plants have been identified that are in various stages of growth. Some are at a stage where the primary hemi-epiphyte has just germinated. In these cases, separating the primary hemi-epiphyte from the host plant saves the latter.

Others have already formed a cage over the host plants and will possibly kill them in a few years. Nothing can be done to save these, although they are being identified and labelled for records.

"The occurrence of primary hemi-epiphytes is a natural process and death of phorophytes due to these stranglers is also not very uncommon in the tropical rainforests. But for a botanic garden where plants are conserved, the presence of such primary hemi-epiphytes may not be a healthy sign for the long-term survival of rare and endemic species. Care must be taken to remove the primary hemi-epiphyte at an early stage," said director Pramanik.

Swapna Prabhu, a botanist at the Bombay Natural History Society (BNHS), offered a different perspective. "Primary hemi-epiphytes propagating in this way is absolutely natural. The Ficus is not a villain, for that is how an undisturbed natural forest gets its cover. But 164 cases of primary hemi-epiphytes strangulating phorophytes in a botanic garden needs to be investigated. There must be some extraneous factor that is making such propagation happen. There must be some endemic avian species that is feeding on the seeds and carrying the seeds to the phorophytes."