The bank, in a deep basement 55 kilometers south of London, already holds seeds of more than 18,000 wild plant species from 126 countries.
The billionth seed is from an African bamboo and was collected in Mali, West Africa.
"Plant diversity is a vital part of the system upon which we depend. The need for the kind of insurance policy the Millennium Seed Bank provides has never been greater," said bank chief Paul Smith.
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