What a good question! I don't study plants on land (I study them in the ocean), but I will try my best to answer your question.
When pollen from one species of flowering plant ("Plant A") is transported to a flower on a different species of plant ("Plant B") it is called "cross pollination". There are several different things that can happen, depending on how much the two plants are related.
One possibility is that the pollen from Plant A is so different from the pollen of Plant B that the eggs in Plant B never become fertilized. Another possibility is that the pollen from Plant A can fertilize the eggs of Plant B, but the embryos that develop from the eggs are unhealthy and die. But if the pollen of Plant A reaches the eggs of Plant B and fertilizes them, and the embryos that develop from Plant B are healthy, the new developing plants are called "hybrids". Many of the fruits that you buy in the supermarket are hybrids. If hybrid plants are able to pollinate each other, and grow new hybrid plants, then the hybrids are considered a new species. Often, however, hybrid plants are "sterile", which means that either they cannot produce seeds or their seeds cannot develop into new plants.
1.) Can you find an example of a hybrid that you might find in the supermarket? (If you can, try searching on the web under "hybrid fruit".)
2.) What might be an advantage to creating sterile hybrids? (HINT: Would you buy a seedless watermelon?)
3.) What are some other advantages that you can gain with hybridized crops?
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4.) Why might some people be against creating successful hybrids for large-scale agriculture? (HINT: What if a farmer cross pollinated his crop with a weed and grew this new hybrid in a field next to a meadow with rare native plants growing in it? Or what if his new hybrid crop was pollinated by different insects than his other crops, or caught different diseases?)
Your question shows that you're thinking about all sorts of possible consequences of having pollen floating around.That's great.
Pollen carries sperm to the female part of a flower. The female part could be on the same plant or another plant. Usually, the pollen will only fertilize a female of the same species. The DNA of the two species usually won't work well together, so the female part of the plant doesn't allow the sperm to reach the egg. Remember that the air is full of pollen from all different types of flowers, all of which could land on the female part of a flower. The plant does not want to waste its eggs if they won't end up being a usable seed. (How do some plants get their pollen carried directly from one plant to another of the same species?)
Back to your question, sometimes crosses do work, this produces what we call a "hybrid". This is how wheat was created. First wild grasses happened to fertilize between species naturally. People raised this hybrid for food because it had large nutritious seeds they could grind into flour. About 8000 years ago this ancestor of wheat crossed with another wild grass and its offspring had really large seeds. Our modern wheat varieties came from this double hybrid. Successful hybrids are generally rare. Why?
Sometimes scientists cross crops with wild plant species because the wild plants are more resistant to disease and insects. (Why would they be?) This is a good reason to keep wild populations of plants from going extinct.
If we want to cross one species of plant with another, we can also do it by "grafting". This happens when we take a cutting from one plant and attach it to another. This is where 'navel' or seedless oranges come from.
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