Answer: It really all depends on the plant. Some seeds, like tomatoes, need a post-harvest treatment to ensure viability. (Think about it--what keeps those tomato seeds from sprouting inside that warm, moist tomato?) Some seeds need to be left on the plant to mature--often the fruits and vegetables we pick to eat contain immature seeds that won't grow. Finally, hybrid plants often don't grow "true-to-type". These plants have been cross-bred and back-crossed so that plants grown from their fruit's seeds may produce fruits very different from the parent plant's. (A tomato seed from a hybrid will still produce a tomato plant, but the fruit might be hard or bitter, harkening back to the plant's ancestors.) Finally, you have to consider whether the plants cross-pollinate. If you have a squash patch and some pumpkins, bees might cross-pollinate the flowers. If you plant the resulting fruit's seeds, you may get some strange squash-pumpkin cross, resembling neither parent.
Many seeds germinate just fine from collected seed. These are just some examples of what can happen. I hope your son gets bitten by the "botany" bug--as you can see, I'm hooked!
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