[@The Narrator]
I wrote that in because with refined technique it wouldn't be long before they perfected combining Flesh and Plant together with little problems. With such techniques, it wouldn't be implausible that the two lifeforms' DNA would mix or overlap each other causing some problems at first. After many generations, basically their body has grown a resistance to it by simply becoming it. Passing on the ability to grow plants in their bodies as a genetic trait to 1-2 children in every generation, becoming an Alchemist or Researcher for the family.

Hopefully that explains it, if not then maybe its something specific we're both not understanding and I'll try to clarify it some more