Pollination from one species of plant to another is possible, if the plants are similar enough. That’s how gardeners come up with exotic plants. However, it does not happen very often.

There are two main reasons.

1. Plants have infection-fighting mechanisms. A cell from a plant that is not recognized will be treated like an invader, and the infection-fighting mechanisms can kill it. A flower recognizes gametes from the same (or very similar) species. If the cell is too different, however, it is attacked.

2. Even if the sperm cell makes it to the ovule, fertilization cannot happen unless the genetics allow it. If the sperm has a genetic code that is too different, the zygote will not be viable and will die.