Here is the table of what happens when you cross any plant with any other plant, color-coded to help you visualize it. I have included the plants that can only be found through mutation liquid or extinct seeds; if you would not like to see those in the table at the moment, click here. (So this table has EVERY plant.)
To use the table, look up one plant on the left column and the other in the top row. The place where they meet is the result of cross-breeding those two plants. This table is useful for finding out the different ways to create a certain plant, as it shows every possible way to create those plants.
One thing that surprised me in my research is that there is no way to create a reptans, and yet it is a common species -- so, if you have a reptans plant, make sure to self-pollinate it so that you will have reptans to mix later on in the game. You can only get reptans by buying them from the store (common seeds) or from your original stash.