Answer: Unfortunately I am not sure exactly the purpose of the experiment, based on your question. However, it would be interesting to use seeds with some easily discernable difference between size, possibly of both seed and resulting typical plant. You could use a bush bean variety and a pole bean variety for instance, or dwarf and larger varieties of marigold, or you could use seeds of spinach and Swiss chard. Your results will probably vary depending on which plants you select, however.
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