Transplanting seedlings to a hydroponic unit...?

I read that you should transfer seedlings to a hydroponic unit after the second leaf appeared. Wouldn't the plant's roots be too short to reach through the aggregate and touch the nutrient solution?
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With rockwool as a media, you can start small plants in 1" cube pieces, which fit unto bigger 4" cubes, which then sit on large slabs.
Each is moved on to the next stage when it outgrows the current one.
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