The world’s cities have always been radically hostile environments for trees – but there’s one variety that’s proved to be remarkably resilient.  In an unremarkable corner of London’s Cheapside district, tucked away behind black wrought-iron fencing, is one of the city’s oldest residents. With a towering frame and slightly stooped posture, capped with a broad thatch of leathery, star-shaped leaves, this venerable giant is thought to have presided over the city since at least the 18th Century.  …It’s been a stoic witness to the infamous cholera outbreak of 1854 – which led to the introduction of modern sanitation – the 1918 flu pandemic, and the horrors of the Blitz.  …But life for this Londoner has not been easy. …It all started in the 17th Century … amid the chaotic meeting of the so-called New World and the Old, two plants from continents thousands of miles apart – an American sycamore and an Oriental plane – met and reproduced.