Just brainstorming, but here goes:
Get the number of empires, the number of asteroids, and the number of seedlings on each asteroid.
At EVERY roid, run a FOR loop for each empire, for the number of their seedlings on that roid. This loop would pick up a specific seedling, there is a way of doing this, but I'm not sure how just atm. Add this collected seedling to a variable.
This variable will hold all seedlings.