Somebody plants the plant and lets it grow until it produces seeds at the end of the season, then collects the seeds.
For your carrots example, this is what the [above-ground part of a carrot plant](https://i.ytimg.com/vi/Q233zFbFrYU/maxresdefault.jpg) looks like. From there [you get the seeds.](http://suburbantomato.com/wp-content/uploads/2011/05/Carrot-seeds.jpg)
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