Kilayim, Chapter One, Mishnah Seven
This mishnah discusses the prohibition of grafting.
1) They may not graft from one tree to another, or from one vegetable to another, or from a tree to a vegetable, or from a vegetable to a tree.
2) Rabbi Judah permits it from a vegetable to a tree.
Section one: This section is in some ways a continuation of mishnayot four and five. The prohibition of grafting is also a prohibition of mixing different kinds of trees or vegetablestwo different kinds are prohibited. Assumedly, the mishnah relies on the lists already found abovetrees and vegetables that can be grown together, can also be grafted.
Section two: Rabbi Judah allows one to graft from a vegetable onto a tree (I dont really know if this works). According to the Tosefta, he also allows from a tree onto a vegetable. The commentators explain that when a vegetable and tree are grafted together, a new species is not formed, even though they the plant grafted onto the mother plant benefits. According to Rabbi Judah, the Torah prohibits the formation of a new species, but does not prohibit grafting when new species will not be formed. The rabbis are stricter and rule that any grafting of different species is prohibited.