Basically, whichever plant you want to get, you have to mutate one of the plants next to it. So if you wanted to get plant #16, you would have to mutate either #15 or #17. Each plant can be mutated into 2 different plants. So if you had a Viola Cactus Ball (#17), the mutation liquid will turn it into either #16 or #18. You don't know which one you will get, but it will definitely be one of those two.

Also, #21 is the Tigerlily Willow Tree, #22 is the Roseceae Fern, and #23 is the Tahitian Ridged Cactus.