HINT: It’s not the mustard seed!
The world’s smallest seeds, which have no endosperm and contain underdeveloped embryos, are produced by certain epiphytic orchids (family Orchidaceae) in the tropical rainforest. Some seeds are only 1/300th of an inch (85 micrometers) long, which is below the resolving power of the unaided human eye. One seed weighs only 1/35,000,000th of an ounce (0.81 micrograms). Orchid seeds are dispersed into the air like minute dust particles and come to rest in the upper canopy of rainforest trees, where they eventually germinate.
On the other hand, the seed of the begonia plant is about 1/100th of an inch in size, that of the petunia plant about 1/50th of an inch, and that of the mustard plant (family Brassicaceae) about 1/20th of an inch.
By the way, this is what Jesus Christ said about the mustard seed:
With what can we compare the kingdom of God, or what parable will we use for it? It is like a mustard seed, which, when sown upon the ground, is the smallest of all the seeds on earth; yet when it is sown it grows up and becomes the greatest of all shrubs, and puts forth large branches, so that the birds of the air can make nests in its shade. (Mark 4.30-32) [emphasis added]
Some have argued, despite the clear meaning of the quote, that Jesus was only referring to the seeds known to his audience. Well then, was the mustard seed the smallest seed known in Palestine? No, not even in Jesus’ time. There would be numerous plants familiar to his audience with smaller seeds, of which the best example would be the seed of the black orchid. And just for the record, the mustard seed doesn’t grow to be the greatest of all the shrubs on Earth, either.