Like us, they can get bigger by environmental manipulation but will ultimately only grow to their genetically-determined potential. 6-foot tall people can be stunted by poor living conditions, stress, and diet. But fabulous living conditions, zero stress, and and a perfect diet will not allow a midget to grow to 6 feet tall. You need seeds that grow big pods. Only save seeds from the biggest pods from your best plants. Then nurture them to reach their genetically-determined potential.
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