Nature Diary
Secrets of the Banyan Tree (Ficus benghalensis)

The banyan tree must be one of the most well-known trees in India. I am sure all of you know that it is a huge tree that provides shade, that it has aerial roots that can grow down, take root, and become trunks, and that its bark, seeds, leaves and roots have been used for medicinal purposes from ancient times.
But there are some amazing facts about the Banyan that you may not know. Here are some of them.

The Strangler Fig
The banyan is a type of fig: yes, it is one of more than 750 species of fig trees. Banyans are also called strangler figs. This is because they all start life as a seed that germinates on another tree. How does this happen?

The Banyan tree starts off as a seed which is laid in the crevices of a host tree by a bird which expels it there after eating the fruit of another banyan tree. The seed germinates on the host tree and slowly grows around it. It first grows as a vine which is dependent on the other tree for support. Eventually, the roots it sends down strangle its hosts! Later, roots grow from outward-extending branches and reach the ground, becoming trunk-like. This expands the footprint of the tree, so it is sometimes called a 'walking tree'.

How does the seed form?
Well, how does the seed form in the first place? By pollination. This is also a peculiar feature of the tree. Banyan trees are pollinated by fig wasps through a process called obligate mutualism. This means that the banyan tree and the wasp species are so dependent on each other that without the wasp, the tree would go extinct. 

This coevolution has gone on for millions of years. The banyan tree has figs (not the Mediterranean fruit) growing on them. These figs are biologically part of a structure called a syconium. This syconium consists of a fleshy inner wall and a hollow interior. This interior consists of both male and female flowers. The male flowers produce pollen and the female flowers can get pollinated and produce their own individual fruits.

How does pollination occur?
The banyan tree releases a scent to attract female wasps. A pregnant female wasp follows the scent to the tree. The syconium has a special opening called the ostiole. When the syconium is ready for pollination, the ostiole loosens to let fig-wasps enter inside the syconium.

The wasp squeezes through the ostiole of the fig (see figure), often losing her wings and antennae. She lays her eggs in some of the flowers. She disperses the pollen covering her body. The wasp dies, and the fig digests her remains. 

But there is even more. The eggs of the wasp inside the fig hatch and the male wasps become active after 4-6 weeks, mate with the female wasps and cut through the fig to create openings. The female wasps then squeeze out of the fig, collecting pollen from the male flowers while doing so, and then escape the fig to pollinate other trees. Thus the wasps reproduce with the help of the banyan fig.

Interestingly, the shape of the ostiolar opening of the fig is extremely specific for specific species of fig-wasps. Hence, specific species of fig trees are codependent on specific species of fig-wasps for survival. And vice-versa. And hence, the banyan and the wasps help each other reproduce and thrive.

BOX Miscellaneous Facts on Banyan Trees
Banyans all over the world are essentially hemi-epiphytic in nature. In biology, epiphytic means trees which grow out of seeds germinating in the nooks and crevices of other trees.

Banyans are ecological linchpins. They produce vast crops of figs that sustain many species of birds, fruit bats, primates and other creatures, which in turn disperse the seeds of hundreds of other plant species.

The first Europeans to encounter banyan trees were Alexander the Great and his army, who reached India in 326 BCE. The notes they took back to Greece informed Theophrastus, the founder of modern botany, and — ultimately — led 17th-century English poet John Milton to write in Paradise Lost that Adam and Eve made the first clothes from banyan leaves.

The banyan tree is the national tree of India and Pakistan. 
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Sources: Wikipedia, How Stuff Works, https://www.rachnakar.com/the-banyan-tree-a-world-of-its-own/ by Parul Jauhari