Sometimes I feel so privileged in my job. The other day I got to poke around in the back rooms of the Millennium Seed Bank at Wakehurst Place in Sussex – home to the seeds of 25,000 of the world’s plant species, some of which are the rarest plants in the world. Six of them are entirely extinct.

The place is like a nuclear bunker inside – in fact if World War Three ever does break out, head on down there as the walls are built of reinforced concrete so thick they’re entirely radiation proof and would survive a direct hit by a crashing aeroplane. Don’t ask me how they worked that out.

The seeds themselves are kept in huge walk-in freezers about the size of my kitchen, but the really amazing thing is that for now – they want to add to their number over time – there are just two of them. For 25,000 plant species. The concentrated biodiversity in those two freezers is just mind-boggling.

If you want a look at the kind of thing they have there, here’s a selection:

Aren’t seeds beautiful? And yes, that is a coco de mer – largest seed on earth. When you compare it to the dust-like antirrhinum seeds I put in last week, you realise just how much variety there is in the world.

The seeds come into the seedbank in big breatheable sacks from every continent – Kew has teams of botanists working with local experts in the field finding and collecting more and more plant samples to add to the bank. This is a sample of huge sycamore-like seeds from Madagascar. I think they’re called Gyrocarpus, at least that’s what it sounded like – in an environment like this even if you think you know quite a lot about plants, it’s not long before you’re out of your depth.

They test all the seeds for viability before they store them, using digital x-ray machines and all sorts of things. Then there’s the problem of working out exactly how to germinate them. Here’s one way of finding out – a mat with varying temperatures in bands across it. You put the seeds in little packets in rows along the heat bands, and wait. Then those that germinate on a certain heat band reveal the temperature the seed must be at in order to sprout. If that doesn’t work, there’s always cold stratification, chipping, bashing, soaking in various chemicals and even setting on fire (mainly reserved for the flora of the South African Cape, I believe).