What would prevent the pollen from one plant species from pollinating flowers of another species?

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What would prevent the pollen from one plant species from pollinating flowers of another species?
Just as in humans, plants have their own form of DNA, the building blocks of life. Species with incompatible DNA cannot reproduce with each other. This is why dogs cannot mate with cats, etc. However there are some instances where cross-pollination IS possible. This is how some botanists can create hybrid plant species.
Reply:The same thing that prevents sperms and eggs from different animal species from combining. Although hybrids do form more commonly in the plant world (species aren't as discrete as in animals), most plants are not capable of combining.
Reply:idk
Reply:Nothing. It happens more than you think. Think of flower hybrids. You can find them if you look.
Reply:There are cases of cross pollination between two species of plants, thats when you see the weird looking fruit, so it isn't entirely impossible. But I suppose things like plant structure matter, or maybe the pollen just isn't able to fertilize the ovule of the plant....sort of why most animal species cannot create progeny