A Fertility Elixir is meant to be used for breeding one or two already existing Plotties. These Plotties will act as "parent plants" and the resulting offspring will bear a combination of traits of the two parents. In contrast, a Lab Kit allows you to create a chimera between any two plants of your choice, regardless if you have existing Plotties with these plants. The chimeras will be artificially engineered in the lab and thus, you won't need parent Plotties.