I was here today, along with about one third of the population of London. We were all there for one thing, a gigantic deformed penis with a stench of rotting corpse.

Where is the greatest biological diversity on the planet Earth? Madagascar? New Guinea? Probably not. It's probably here, in this pleasant and unassuming corner of south-west London. The Royal Botanical Gardens claim some completely unbelievably impossible figure like one-fifth of all plants. Someone challenged me on this when I told them, and I agreed, it sounds absurd. Then I considered the Order Beds.

At the Kew Gardens they have, in one small corner, these things called the Order Beds. They are beds containing every plant species they can manage to grow in the open air, arranged by order. They just go on and on and on. A little later I came across a reference work that gave numbers: yea many species in order Compositae, and so on and so forth. I strode over those beds, in my head, I added them up, how many I'd seen, and decided, yes, it was just possible they did have one-fifth of the world's flowering plants.

Today we went to see the titan arum, Latin name Amorphophallus titanum. This is probably the most stunning plant anyone will ever see. It's dominated by a single giant yellow inflorescence, a "spadix", that grows upward at almost a centimetre an hour, for the few days when it's active. It hits 3 m high, it's a native of the Sumatra rainforest, and to attract pollinators it stinks to high heaven, like rotting corpse and eggs and... but only for a few days. I queued up today to see and smell it, and though it's still huge, it's starting to close up, and there was no great smell left. Drat!

That's what Kew does so well: the preservation of rare plants. The cultivation of plants that barely survive in their native habitat. They're trying to restore plants that have virtually disappeared from their native Mauritius, Nepal, or whatever.

There are some trees dating back to its foundation in the mid 1700s. For the most part they're showing their age, very bulbous and bowed, propped up by metal bars, unwell. But a huge summer-house covered in ancient wisteria is breathtakingly beautiful.

It's an immense park. You can wander it for ages just enjoying the scenery. They are big gardens full of azaleas. There are woodlands with different species of pine, cedar, oak... There's an ornamental pagoda. There are these greenhouses where you learn about the world's plants.

In passing, I saw a family of baby ducklings. I saw piranhas, covered in what looked like gold dust. I saw stingrays, living coral, clown fish, mangrove roots, golden pheasants ...

At one point I looked at a map in the grounds to see what we'd missed. As with last time I was there, I found we'd seen only one little quarter. It's one of these places Britain has (others are the British Museum and National Gallery and Victoria and Albert Museum) where it's completely impossible to cover any significant proportion of it in a single day.

So I'm glad this time I bought an annual ticket. If we're anywhere near it, Kew should be part of our lives. The River Thames nearby is beautiful too. The Gardens were designated a UNESCO World Heritage site in July 2003.