How are the seeds spread throughout out the world?

The seeds were on a plane that either exploded or broke apart when it was attacked by Russian planes. The seeds scattered as a result of the explosion.

What happened to Umberto himself will never be definitely known. It is my guess that over the Pacific Ocean, somewhere high up in the stratosphere, he found himself attacked by Russian planes. It may be that the first he knew of it was when cannon shells from Russian fighters started to break up his craft.

Perhaps Umberto's plane exploded, perhaps it just fell to pieces. Whichever it was, I am sure that when the fragments began their long, long fall toward the sea they left behind them something which looked at first like a white vapor.

It was not vapor. It was a cloud of seeds, floating, so infinitely light they were, even in the rarefied air. Millions of gossamer-slung triffid seeds, free now to drift wherever the winds of the world should take them.